OU-72 race DT motor info for those that have not seen this

racerdave

^ Gone not forgotten ^
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This has been around a long time. I just came across it while deleteing stuff off my laptop. It may be of interest to those that have not seen it. I did my best to copy and paste. I do still have it in a PDF. I will have to post this in several post to comply with the 1,000 character rule here.

OU-72/OW72
Yamaha’s Answer To The Harley XR750
(There was a time when the XS650 shed its wimpy reputation and put on Superman’s cape!)
This 90 horsepower hybrid was the final evolutionary step in the XS650’s
development as a race engine. In 1975-1976 a really talented team of Southern
California tuners, fabricators and specialty suppliers engaged in a crash program
to create a version of the engine that could take on the factory Harley XR750s.
The nomenclature is seen as both OW72 and OU-72 in publications from the
time, however a Yamaha International Corporation publication in 1976 refers to it
as OU-72, so that’s what I’ll use.
At the time, Kenny Roberts was Yamaha’s only fully sponsored AMA dirt track
competitor, and Yamaha was eager to do whatever it took to help him retain the
number one plate. Everyone understood that the number one goal was to make
the XS650-based dirt tracker a more effective weapon. Essentially, this meant it
needed more power. In late 1974 Yamaha USA had hired Tim Witham, founder
of S&W Engineering, and a very highly respected ex-dirt track tuner, to manage
an engine improvement program. He was somewhat successful and managed to
squeeze another approximately 10 horses – probably reaching the mid-70s if
tested on a modern dyno - while still using highly modified stock castings.
Reliability was becoming a problem however, and the final versions had
strengthened crankcases, stronger gearboxes, clutches, beefy connecting rods
and upgraded valve trains. Unfortunately, the alloy Harley XR750 was evolving
also and continued to retain a small but very real advantage.
Facing 1976 with few tuning options left, the stock head castings became the
obstacle that couldn’t be overcome. Every trick was tried by the very best air
flow specialists available, but they couldn’t make the stock head flow any more
air and still retain any kind of a manageable, linear power band.
Consulting with C.R. Axtell and a number of other specialists, Witham had a set
of engineering drawings put together that depicted a new head casting that would
have higher flow potential while retaining the necessary interfaces with the stock
barrels and cases. The new design featured revised intake and exhaust valve
angles and straighter, larger ports with optimized shapes. Also included was a
new barrel with some detail changes including “750 cc” replacing the stock
engine’s “650 cc” raised lettering on the casting.
In July 1975 Pete Schick, Yamaha’s Racing Manager in the US, met with
Yamaha engineers in Japan and asked them to cast at least 24 prototype heads
per the drawings he’d brought with him (24 or more heads were needed because
per the racing rules in force at the time the AMA had to see 24 actual, running
units in Yamaha’s Southern California distribution warehouse before permission
could be given for use in the 1976 racing season). The timing was good because
Yamaha was just getting into four strokes and Yamaha’s Engineering
management thought this would be a good exercise for the production team and
also, of course, because it would assist Roberts to retake the number one plate
and sell more Yamahas in the US. They agreed to supply the castings.
It was already October 1975 and there was no time to waste. The team only had
eight months before the engine’s scheduled debut in May 1976 at the San Jose
Mile.
A few months later the first castings showed up. They had no real ports – more
like small tunnels along the port’s centerline, no valve seats or guides, and of
course no camshafts, valves or valve springs. Everything had to be spec’d,
sourced, fabricated, or cut. Vendors were developing and producing improved
pistons, gears, rods and even the crankshafts had to be specially weighted,
balanced, and fitted with larger crankpins. Dozens of pairs of cases were
magnafluxed to find those with the fewest casting flaws.
On April 14th, just a little more than a month before San Jose, the first engine
fired on the dyno at Yamaha’s race shop. Bud Aksland had assembled the
engine himself and after four hours on the dyno he saw larger torque and
horsepower numbers than any prior Yamaha XS650/750 race engine had ever
produced. The 24 complete running bikes with OU-72 engines were frantically
assembled by borrowing mechanics from Yamaha’s snowmobile division. This
was completed just in time for Earl Flanders, an AMA tech inspector, to check out
the bikes at Yamaha International Corporation’s facility at Buena Park.
To make
sure he wasn’t being given a snow job, Earl had them tear apart some of the
engines and also started a few others. Everything passed and the OU-72 would
be recommended for approval at the AMA Professional Rules Committee
meeting on May 4th.
Actually, none of the engines Flanders inspected were the race versions. Six
race engines, including three earmarked for San Jose, were receiving special
cams, pistons and other components as rapidly as they could be fabricated and
delivered. C.R. Axtell and Mike Libby were flowing and finalizing the new head’s
ports and zeroing in on desired camshaft lobe contours. Kel Carruthers in San
Diego had three race bike chassis ready awaiting arrival of the completed
engines. Sig Erson, Forged True, Webster Racing, Pete Smiley and a host of
others worked incredible amounts of overtime to make it all happen.
On May 6th the engines were run on an engine dyno (not a chassis dyno) and
recorded numbers that translate to approximately 90 horsepower. The still hot
engines were immediately trucked to San Diego so Carruthers could install them
in the three race bikes. On May 9th the fully assembled bikes were taken to the
deserted Santa Clara County Fairgrounds (which had been rented for a day of
private testing) so Kenny Roberts could have an initial outing on the bikes. To
everyone’s relief and joy, the test was a complete success.
The race weekend began on May 16th. In practice, Roberts and the OU turned
laps around 38 seconds; only one of the Harleys was as fast. Engine power
3
wasn’t the problem … there was plenty of that. However, once Robert’s fresh
tires began wearing traction became an issue. By the mid point in his heat race
Robert’s bike was slewing dramatically, getting too sideways to maintain
momentum and losing traction on each drive out of a corner. He had to make a
last lap pass to transfer directly to the main event.
Unfortunately, the story doesn’t have a fairy tale ending. As a final exclamation
point on a frustrating day, the bike refused to start for the main. A later
inspection revealed a coil wire coupler had failed and caused a short. The tiny
laps around 38 seconds; only one of the Harleys was as fast. Engine power
wasn’t the problem … there was plenty of that. However, once Robert’s fresh
tires began wearing traction became an issue. By the mid point in his heat race
Robert’s bike was slewing dramatically, getting too sideways to maintain
momentum and losing traction on each drive out of a corner. He had to make a
last lap pass to transfer directly to the main event.
Unfortunately, the story doesn’t have a fairy tale ending. As a final exclamation
point on a frustrating day, the bike refused to start for the main. A later
inspection revealed a coil wire coupler had failed and caused a short. The tiny
part may have cost ten cents. After a few more race weekends the frame
geometry and suspension were tweaked and tuned to provide a better balance,
traction wasn’t a continuing problem and the OU-72 went on to win its share of
AMA national and regional events.
During 1975 - 1976, additional batches of heads – totaling perhaps 50 or 60 -
were sent from Japan to the Yamaha Race Department. Most were sold to
privateers and non-factory tuners. Cutting ports into a head from scratch is a
daunting task and few are up to such a challenge. It is unquestioned that the
best OU-72 heads were those ported by C.R. Axtell and Mike Libby. Axtell’s
shop continued to develop and evolve the OU-72’s porting and component
specifications throughout 1976. Their engines routinely produced 80 – 83 rear
wheel horsepower as we would measure it today, and featured very progressive,
powerful torque and horsepower curves. But, there weren’t very many of those
as a percentage of all the OU-72s being run at flat tracks across the nation. The
average, so-called privateer OU was generally not as effective as a high level,
expertly modified AMA Grand National caliber XS750 with a stock-based head,
and many OU-72s were simply expensive underperformers.
As Yamaha exotica goes, an OU-72 engine in reasonable shape is the cherry on
top of a XS650 sundae. An unmolested unit from Axtell’s shop is the Holy Grail
for serious collectors and vintage competitors.
 

racerdave

^ Gone not forgotten ^
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The following is a retyped copy of the Yamaha International Corporation
Racing Department OU-72 Specifications sheet published in September
1976.

MODEL XS750-DT (OU-72)
MAXIMUM HORSEPOWER (not listed)
MAXIMUM TORQUE (not listed)
BORE X STROKE X CYLINDERS 80.5mm X 74mm X 2
DISPLACEMENT 744.5cc (45.43 c.i.)
COMPRESSION RATIO 11.5

IGNITION Total Loss
CARBURETOR Lectron 36 – 38mm
AIR FILTERS K & N Oiled element
FUEL 98+ Octane
CYLINDER HEAD OU-72 with 29º (IN) valve angle, 27º EX
Flow ported by C.R. Axtell (see enclosed
flow charts)
VALVE GUIDES Aluminum bronze
COMBUSTION CHAMBER VOLUME 36cc with pistons
HEAD GASKET THICKNESS Copper .055”
CAM SHAFT Sig Erson 224 - 104
VALVE SPRINGS S & W Engineering
SPRING RATE 305 lb. per inch
Installed length (closed) 1.600”
Installed pressure (closed) 120 lbs.
Compressed length (open) 1.100”
Wire diameter .160” outer springs, .120” inner spring
Number of windings 6 turns outer, 7 turns inner
VALVE TO PISTON CLEARANCE Figure varies according to pistons and cam
timing
Intake .050” minimum
Exhaust .070” minimum
VALVE – INTAKE Modified high performance
GM valve by
C.R. Axtell
Clearance (cold) .006”
Head diameter 44mm
Length 109.7mm
Seat width .065”
Seat angle(s) 45º and 37º
Margin thickness .6mm
Stem diameter 8.0mm (O.D.)
Stem to guide clearance .0013”
VALVE SEAT – INTAKE
Angles 45º and 53º
VALVE – EXHAUST Modified high performance GM valve by
C.R. Axtell
Clearance (cold) .008”
Head diameter 38mm
Length 116.4”
Seat width .065”
Seat angle 45º
Margin thickness .6mm
Stem diameter 8.0mm
Stem to guide clearance .002”
VALVE SEAT – EXHAUST
Angles 45º and 60º and 67.5
CYLINDER OU-72
Bore diameter 80.5mm
PISTON Forgedtrue (1474-A modified to fit)
Clearance .006”
Pin to crown 25mm
Pin to skirt 32.5mm
Rings Standard TX750 (2nd O.S.)
Ring clearance Standard TX750
WRIST PIN Harley XR-750
Diameter 20mm
Length 60.75mm
CONNECTING ROD Warren Machine (Carrillo
Length 5.670” (8mm longer than standard)
Small end I.D. 20mm
Big end I.D. 39mm
Small end width 22.0mm
Big end width 22.0mm
Clearance(s) Standard XS650
CRANKSHAFT 0.25” band added to O.D. and three
additional weights added in holes
opposite pins.
Metal added “Malory” metal
Crank pin diameter 29.0mm O.D.
Balance factor 67%
Total weight added 9 lbs. (added to flywheels only)
Dimensions Standard XS650 tolerances
CLUTCH Modified hub – hub is deeper and has
more plate and load capacity
Friction plates Standard metal friction 9
Clutch plates Standard clutch plate 8
TRANSMISSION Special close ratio by Webster Racing
Ratios Drive Axle Driven Axle
1st 13 23 1.769
2nd 15 22 1.466
3rd 16 20 1.250
4th 18 20 1.110
5th 19 20 1.050
CARBURETOR Lectron – tract length 7” from head to carb
center
Size 36 or 38mm
Needle 2 – 3 needle; 2 turns down
LUBRICATON SYSTEM Standard
Capacity 2.0 quarts
Recommended Oil Mobil or Valvoline Racing
IGNITION Total loss
Ignition timing 34º
Point gap .012” - .015”
CONDENSER Standard XS650
SPARK PLUGS Champion R84G (12mm)
Gap .022”
Battery 12V
COIL Chrysler Marine
EXHAUST
TT racing 1 ¾” X 42” exhaust pipe
½ mile and mile racing 1 ¾” X 28 ½” exhaust pipe
Megaphone (for mile racing only) 1 ¾” opening with a tapered body that extends
for a length of 17 ¼”; at which point the
megaphone has a width of 3 ½” ; this is followed
by a 1” reverse cone angled so that the exhaust
exit opening has a diameter of 2 ½”.
The following information is current as of 9-11-76. For updates or any questions, contact Y.I.C.
Racing Department (714-522-9397).
LATEST HEAD (#4) AS OF 9-9-76 (C.R. AXTELL)
Inches of valve lift Intake Exhaust
.100” 35 15
.150” 75 40
.200” 128 75
.250” 180 125
.300” 240 175
.350” 295 200
.400” 332 230
.450” 330 265
.500” 335 275
(Figures are relative to flow bench C.R. Axtell)
Best flow (highest numbers) to date. Head not run yet.
What followed here was a list of suppliers. The data isn’t helpful because all of them
have either moved or are no longer in business. The names included well know vendors
such as Axtell, S&W Engineering (now R/D Valve Spring), Forgedtrue pistons, Carrillo,
LA Sleeve, Shell Racing Specialties, etc.
► This note appeared at the bottom of the September 1976 OU-72 Yamaha publication:
NOTE: Before ordering any of the above items determine the exact model and year. Secondly
the purpose. The above items may require extensive reworking (machining) before they
are usable. Others can be used as supplied.
This engine was developed for professional racing purposes only with cost not being a
factor. If you are not an experienced engine builder or have limited funds we urge you to
refrain from attempting an OU-72 type engine.
Should you have any questions please contact Y.I.C. Racing Department before ordering
parts or service. 714-522-9397.
During the racing year of 1976 we have found this engine to be quite reliable. However,
engine life is greatly influenced by maintenance and rider usage. As a result we cannot
be responsible for any failures resulting from following these specifications, services
listed, or race track performance.
 

joe bike

XS650 New Member
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This has been around a long time. I just came across it while deleteing stuff off my laptop. It may be of interest to those that have not seen it. I did my best to copy and paste. I do still have it in a PDF. I will have to post this in several post to comply with the 1,000 character rule here.

OU-72/OW72
Yamaha’s Answer To The Harley XR750
(There was a time when the XS650 shed its wimpy reputation and put on Superman’s cape!)
This 90 horsepower hybrid was the final evolutionary step in the XS650’s
development as a race engine. In 1975-1976 a really talented team of Southern
California tuners, fabricators and specialty suppliers engaged in a crash program
to create a version of the engine that could take on the factory Harley XR750s.
The nomenclature is seen as both OW72 and OU-72 in publications from the
time, however a Yamaha International Corporation publication in 1976 refers to it
as OU-72, so that’s what I’ll use.
At the time, Kenny Roberts was Yamaha’s only fully sponsored AMA dirt track
competitor, and Yamaha was eager to do whatever it took to help him retain the
number one plate. Everyone understood that the number one goal was to make
the XS650-based dirt tracker a more effective weapon. Essentially, this meant it
needed more power. In late 1974 Yamaha USA had hired Tim Witham, founder
of S&W Engineering, and a very highly respected ex-dirt track tuner, to manage
an engine improvement program. He was somewhat successful and managed to
squeeze another approximately 10 horses – probably reaching the mid-70s if
tested on a modern dyno - while still using highly modified stock castings.
Reliability was becoming a problem however, and the final versions had
strengthened crankcases, stronger gearboxes, clutches, beefy connecting rods
and upgraded valve trains. Unfortunately, the alloy Harley XR750 was evolving
also and continued to retain a small but very real advantage.
Facing 1976 with few tuning options left, the stock head castings became the
obstacle that couldn’t be overcome. Every trick was tried by the very best air
flow specialists available, but they couldn’t make the stock head flow any more
air and still retain any kind of a manageable, linear power band.
Consulting with C.R. Axtell and a number of other specialists, Witham had a set
of engineering drawings put together that depicted a new head casting that would
have higher flow potential while retaining the necessary interfaces with the stock
barrels and cases. The new design featured revised intake and exhaust valve
angles and straighter, larger ports with optimized shapes. Also included was a
new barrel with some detail changes including “750 cc” replacing the stock
engine’s “650 cc” raised lettering on the casting.
In July 1975 Pete Schick, Yamaha’s Racing Manager in the US, met with
Yamaha engineers in Japan and asked them to cast at least 24 prototype heads
per the drawings he’d brought with him (24 or more heads were needed because
per the racing rules in force at the time the AMA had to see 24 actual, running
units in Yamaha’s Southern California distribution warehouse before permission
could be given for use in the 1976 racing season). The timing was good because
Yamaha was just getting into four strokes and Yamaha’s Engineering
management thought this would be a good exercise for the production team and
also, of course, because it would assist Roberts to retake the number one plate
and sell more Yamahas in the US. They agreed to supply the castings.
It was already October 1975 and there was no time to waste. The team only had
eight months before the engine’s scheduled debut in May 1976 at the San Jose
Mile.
A few months later the first castings showed up. They had no real ports – more
like small tunnels along the port’s centerline, no valve seats or guides, and of
course no camshafts, valves or valve springs. Everything had to be spec’d,
sourced, fabricated, or cut. Vendors were developing and producing improved
pistons, gears, rods and even the crankshafts had to be specially weighted,
balanced, and fitted with larger crankpins. Dozens of pairs of cases were
magnafluxed to find those with the fewest casting flaws.
On April 14th, just a little more than a month before San Jose, the first engine
fired on the dyno at Yamaha’s race shop. Bud Aksland had assembled the
engine himself and after four hours on the dyno he saw larger torque and
horsepower numbers than any prior Yamaha XS650/750 race engine had ever
produced. The 24 complete running bikes with OU-72 engines were frantically
assembled by borrowing mechanics from Yamaha’s snowmobile division. This
was completed just in time for Earl Flanders, an AMA tech inspector, to check out
the bikes at Yamaha International Corporation’s facility at Buena Park.
To make
sure he wasn’t being given a snow job, Earl had them tear apart some of the
engines and also started a few others. Everything passed and the OU-72 would
be recommended for approval at the AMA Professional Rules Committee
meeting on May 4th.
Actually, none of the engines Flanders inspected were the race versions. Six
race engines, including three earmarked for San Jose, were receiving special
cams, pistons and other components as rapidly as they could be fabricated and
delivered. C.R. Axtell and Mike Libby were flowing and finalizing the new head’s
ports and zeroing in on desired camshaft lobe contours. Kel Carruthers in San
Diego had three race bike chassis ready awaiting arrival of the completed
engines. Sig Erson, Forged True, Webster Racing, Pete Smiley and a host of
others worked incredible amounts of overtime to make it all happen.
On May 6th the engines were run on an engine dyno (not a chassis dyno) and
recorded numbers that translate to approximately 90 horsepower. The still hot
engines were immediately trucked to San Diego so Carruthers could install them
in the three race bikes. On May 9th the fully assembled bikes were taken to the
deserted Santa Clara County Fairgrounds (which had been rented for a day of
private testing) so Kenny Roberts could have an initial outing on the bikes. To
everyone’s relief and joy, the test was a complete success.
The race weekend began on May 16th. In practice, Roberts and the OU turned
laps around 38 seconds; only one of the Harleys was as fast. Engine power
3
wasn’t the problem … there was plenty of that. However, once Robert’s fresh
tires began wearing traction became an issue. By the mid point in his heat race
Robert’s bike was slewing dramatically, getting too sideways to maintain
momentum and losing traction on each drive out of a corner. He had to make a
last lap pass to transfer directly to the main event.
Unfortunately, the story doesn’t have a fairy tale ending. As a final exclamation
point on a frustrating day, the bike refused to start for the main. A later
inspection revealed a coil wire coupler had failed and caused a short. The tiny
laps around 38 seconds; only one of the Harleys was as fast. Engine power
wasn’t the problem … there was plenty of that. However, once Robert’s fresh
tires began wearing traction became an issue. By the mid point in his heat race
Robert’s bike was slewing dramatically, getting too sideways to maintain
momentum and losing traction on each drive out of a corner. He had to make a
last lap pass to transfer directly to the main event.
Unfortunately, the story doesn’t have a fairy tale ending. As a final exclamation
point on a frustrating day, the bike refused to start for the main. A later
inspection revealed a coil wire coupler had failed and caused a short. The tiny
part may have cost ten cents. After a few more race weekends the frame
geometry and suspension were tweaked and tuned to provide a better balance,
traction wasn’t a continuing problem and the OU-72 went on to win its share of
AMA national and regional events.
During 1975 - 1976, additional batches of heads – totaling perhaps 50 or 60 -
were sent from Japan to the Yamaha Race Department. Most were sold to
privateers and non-factory tuners. Cutting ports into a head from scratch is a
daunting task and few are up to such a challenge. It is unquestioned that the
best OU-72 heads were those ported by C.R. Axtell and Mike Libby. Axtell’s
shop continued to develop and evolve the OU-72’s porting and component
specifications throughout 1976. Their engines routinely produced 80 – 83 rear
wheel horsepower as we would measure it today, and featured very progressive,
powerful torque and horsepower curves. But, there weren’t very many of those
as a percentage of all the OU-72s being run at flat tracks across the nation. The
average, so-called privateer OU was generally not as effective as a high level,
expertly modified AMA Grand National caliber XS750 with a stock-based head,
and many OU-72s were simply expensive underperformers.
As Yamaha exotica goes, an OU-72 engine in reasonable shape is the cherry on
top of a XS650 sundae. An unmolested unit from Axtell’s shop is the Holy Grail
for serious collectors and vintage competitors.


Nice write up, this is a pick of my OU72
One of two of Shells 3 personal bikes .
I’m never on the forum but feel free to text me at17054374324 if you ever want to talk shop!
My bike was raced by Roberts ,Lawson, Rainy , Scott and many more.
 

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JAX71224

jax71224
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This has been around a long time. I just came across it while deleteing stuff off my laptop. It may be of interest to those that have not seen it. I did my best to copy and paste. I do still have it in a PDF. I will have to post this in several post to comply with the 1,000 character rule here.

OU-72/OW72
Yamaha’s Answer To The Harley XR750
(There was a time when the XS650 shed its wimpy reputation and put on Superman’s cape!)
This 90 horsepower hybrid was the final evolutionary step in the XS650’s
development as a race engine. In 1975-1976 a really talented team of Southern
California tuners, fabricators and specialty suppliers engaged in a crash program
to create a version of the engine that could take on the factory Harley XR750s.
The nomenclature is seen as both OW72 and OU-72 in publications from the
time, however a Yamaha International Corporation publication in 1976 refers to it
as OU-72, so that’s what I’ll use.
At the time, Kenny Roberts was Yamaha’s only fully sponsored AMA dirt track
competitor, and Yamaha was eager to do whatever it took to help him retain the
number one plate. Everyone understood that the number one goal was to make
the XS650-based dirt tracker a more effective weapon. Essentially, this meant it
needed more power. In late 1974 Yamaha USA had hired Tim Witham, founder
of S&W Engineering, and a very highly respected ex-dirt track tuner, to manage
an engine improvement program. He was somewhat successful and managed to
squeeze another approximately 10 horses – probably reaching the mid-70s if
tested on a modern dyno - while still using highly modified stock castings.
Reliability was becoming a problem however, and the final versions had
strengthened crankcases, stronger gearboxes, clutches, beefy connecting rods
and upgraded valve trains. Unfortunately, the alloy Harley XR750 was evolving
also and continued to retain a small but very real advantage.
Facing 1976 with few tuning options left, the stock head castings became the
obstacle that couldn’t be overcome. Every trick was tried by the very best air
flow specialists available, but they couldn’t make the stock head flow any more
air and still retain any kind of a manageable, linear power band.
Consulting with C.R. Axtell and a number of other specialists, Witham had a set
of engineering drawings put together that depicted a new head casting that would
have higher flow potential while retaining the necessary interfaces with the stock
barrels and cases. The new design featured revised intake and exhaust valve
angles and straighter, larger ports with optimized shapes. Also included was a
new barrel with some detail changes including “750 cc” replacing the stock
engine’s “650 cc” raised lettering on the casting.
In July 1975 Pete Schick, Yamaha’s Racing Manager in the US, met with
Yamaha engineers in Japan and asked them to cast at least 24 prototype heads
per the drawings he’d brought with him (24 or more heads were needed because
per the racing rules in force at the time the AMA had to see 24 actual, running
units in Yamaha’s Southern California distribution warehouse before permission
could be given for use in the 1976 racing season). The timing was good because
Yamaha was just getting into four strokes and Yamaha’s Engineering
management thought this would be a good exercise for the production team and
also, of course, because it would assist Roberts to retake the number one plate
and sell more Yamahas in the US. They agreed to supply the castings.
It was already October 1975 and there was no time to waste. The team only had
eight months before the engine’s scheduled debut in May 1976 at the San Jose
Mile.
A few months later the first castings showed up. They had no real ports – more
like small tunnels along the port’s centerline, no valve seats or guides, and of
course no camshafts, valves or valve springs. Everything had to be spec’d,
sourced, fabricated, or cut. Vendors were developing and producing improved
pistons, gears, rods and even the crankshafts had to be specially weighted,
balanced, and fitted with larger crankpins. Dozens of pairs of cases were
magnafluxed to find those with the fewest casting flaws.
On April 14th, just a little more than a month before San Jose, the first engine
fired on the dyno at Yamaha’s race shop. Bud Aksland had assembled the
engine himself and after four hours on the dyno he saw larger torque and
horsepower numbers than any prior Yamaha XS650/750 race engine had ever
produced. The 24 complete running bikes with OU-72 engines were frantically
assembled by borrowing mechanics from Yamaha’s snowmobile division. This
was completed just in time for Earl Flanders, an AMA tech inspector, to check out
the bikes at Yamaha International Corporation’s facility at Buena Park.
To make
sure he wasn’t being given a snow job, Earl had them tear apart some of the
engines and also started a few others. Everything passed and the OU-72 would
be recommended for approval at the AMA Professional Rules Committee
meeting on May 4th.
Actually, none of the engines Flanders inspected were the race versions. Six
race engines, including three earmarked for San Jose, were receiving special
cams, pistons and other components as rapidly as they could be fabricated and
delivered. C.R. Axtell and Mike Libby were flowing and finalizing the new head’s
ports and zeroing in on desired camshaft lobe contours. Kel Carruthers in San
Diego had three race bike chassis ready awaiting arrival of the completed
engines. Sig Erson, Forged True, Webster Racing, Pete Smiley and a host of
others worked incredible amounts of overtime to make it all happen.
On May 6th the engines were run on an engine dyno (not a chassis dyno) and
recorded numbers that translate to approximately 90 horsepower. The still hot
engines were immediately trucked to San Diego so Carruthers could install them
in the three race bikes. On May 9th the fully assembled bikes were taken to the
deserted Santa Clara County Fairgrounds (which had been rented for a day of
private testing) so Kenny Roberts could have an initial outing on the bikes. To
everyone’s relief and joy, the test was a complete success.
The race weekend began on May 16th. In practice, Roberts and the OU turned
laps around 38 seconds; only one of the Harleys was as fast. Engine power
3
wasn’t the problem … there was plenty of that. However, once Robert’s fresh
tires began wearing traction became an issue. By the mid point in his heat race
Robert’s bike was slewing dramatically, getting too sideways to maintain
momentum and losing traction on each drive out of a corner. He had to make a
last lap pass to transfer directly to the main event.
Unfortunately, the story doesn’t have a fairy tale ending. As a final exclamation
point on a frustrating day, the bike refused to start for the main. A later
inspection revealed a coil wire coupler had failed and caused a short. The tiny
laps around 38 seconds; only one of the Harleys was as fast. Engine power
wasn’t the problem … there was plenty of that. However, once Robert’s fresh
tires began wearing traction became an issue. By the mid point in his heat race
Robert’s bike was slewing dramatically, getting too sideways to maintain
momentum and losing traction on each drive out of a corner. He had to make a
last lap pass to transfer directly to the main event.
Unfortunately, the story doesn’t have a fairy tale ending. As a final exclamation
point on a frustrating day, the bike refused to start for the main. A later
inspection revealed a coil wire coupler had failed and caused a short. The tiny
part may have cost ten cents. After a few more race weekends the frame
geometry and suspension were tweaked and tuned to provide a better balance,
traction wasn’t a continuing problem and the OU-72 went on to win its share of
AMA national and regional events.
During 1975 - 1976, additional batches of heads – totaling perhaps 50 or 60 -
were sent from Japan to the Yamaha Race Department. Most were sold to
privateers and non-factory tuners. Cutting ports into a head from scratch is a
daunting task and few are up to such a challenge. It is unquestioned that the
best OU-72 heads were those ported by C.R. Axtell and Mike Libby. Axtell’s
shop continued to develop and evolve the OU-72’s porting and component
specifications throughout 1976. Their engines routinely produced 80 – 83 rear
wheel horsepower as we would measure it today, and featured very progressive,
powerful torque and horsepower curves. But, there weren’t very many of those
as a percentage of all the OU-72s being run at flat tracks across the nation. The
average, so-called privateer OU was generally not as effective as a high level,
expertly modified AMA Grand National caliber XS750 with a stock-based head,
and many OU-72s were simply expensive underperformers.
As Yamaha exotica goes, an OU-72 engine in reasonable shape is the cherry on
top of a XS650 sundae. An unmolested unit from Axtell’s shop is the Holy Grail
for serious collectors and vintage competitors.
I know where one was af+er claimed from a fac+or bike.
 

JAX71224

jax71224
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I know where one was af+er claimed from a fac+or bike.
A Yamaha dealer in southern Indiana claimed a Roberts OU72 as he was in the race and eligible to claim the engine. Later, said to have gone to Earl Hayden's racing enterprises........Tommy and Nicky and Roger Lee...... I saw it years ago......
 
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