Another boring thread (cylinder)

Wisely or not I just got in a bunch of piston sets from cruzinimage. I want to have my local engine guy bore a bunch of cylinders to match. Nothing is simple, looking for some opinions experiences on cylinder boring. He has bored three sets already for pistons I bought last year and one set is in an engine and running (rephase and I got to ride it!)
That motor had "some machinists" bore and Mikes pistons for about 500 miles and it didn't go well, there was a lot of scuffing, metal swapping, those bores had distinct wear lined up with the where the studs are ie the 4 stud corners show the scuffing, they got one of my sets and put it in.
OK the questions.

Is it a big deal to do a full cylinder clamp down torque jig set up before boring? Or is "standard boring machine clamping fine? I don't know if anyone is has done a jig build. Building an XS specific jig is not trivial and the exact jig set up may raise more questions than answers.
Cylinder to bore clearance. stock is .002" inch I am planning on staying with that but anyone got any "other" clearance suggestions? Does use matter, say street vs race? I will be doing several 706 78mm big bores and also some 2nd over sets.

the specs I supplied my machinist with last time;

XS650 Cylinder boring and honing

Stock bore 75mm
Wear limits 75.1mm, taper .05mm (.002")

Supplied pistons 75.5mm nominal dimension

Cylinder to piston clearance

.050mm to .055mm
.002" to .0022"

Ring end gaps
1st and 2nd rings .2mm to .4mm limit .8mm
.008" to .016" .030
Oil control rails.
.3mm to .9mm
.012" to .035"
Basically copied from the Yamaha manual.

OK what'da think?

Gary, a torque jig isn't needed for what you're doing. Joe Minton recommended that procedure back in the day when liners in 750 kits were thin and prone to deformation, and it's standard procedure on tall HD cylinders, but there's no need for it on XS650 jugs even boring for 77.5 mm. pistons in OE liners or boring modern 750 kit liners, as long as the job is done right.

Forged pistons expand more than cast pistons do, so more clearance is needed. If the cruzinimage pistons are cast you should be fine with Mamayama's book specs.

BTW, Bill Moeller of Bore Tech does fantastic work, and his silicon carbide liner treatment is worth every penny spent on it.

Ditto what grizld1 sez.

Yes, the cruzinimage pistons are cast.

My oldschool method was to use a Sunnen sizing hone to finish-hone the last 0.0005"-0.0010" of bore diameter, checking with Starret 12" ribbon guages.

If we knew the engine would be experiencing severe heat (not unusual down here), would sometimes add 0.0005"-0.0010" to the final bore. Not necessary in your refrigerator climate.

I'm sure that 40-year ago method has been improved upon, so no worries there.

Edit: Keep track of what piston goes to what bore...

hypereutectic alloys are another case . These are the least expansion seen in cast pistons with silicon content approaching 16% . We learned about the care and feeding of these alloys through work on a common irrigation pump ...

Although YammaMama says .002 to .0022 (and I agree) the hypereutectic alloy will tolerate much closer clearances initially .

The process was to finish the bore within .002" , finish to piston size with #220 and then create necessary clearance with the softer finer finish hone (approx #320) . Finish clearance could be as little as .00075 for smaller bores and a rch over .00125" for 3 5/8" bore (kinda gave it away there) . Initial start up and following heat cycles are a bit scary . I use time to temperature as a guide . Usually and somewhat scary is the engine will heat and slow down noticeably from the usual 2K RPM and shut it off taking infrared temp at shut off . After a full return to cold iron put a stop watch and a timer on the next cycle . After several heat cycles the time to temp will increase noticeably . At this point throw the break in oil away and reload , Go out and put increasing loads on the engine .
If you get it tight you're screwed . If you get it loose it never breaks in . If you get impatient it squeaks . If you tune to the edge of detonation you'll turn a piston into itty bitty pieces . If you get it right you have a build with great longevity .
Just some more virtually useless information


~kkop

kopcicle said:

hypereutectic alloys are another case .

Finish clearance could be as little as .00075 for smaller bores and a rch over .00125" for 3 5/8" bore (kinda gave it away there) .

Click to expand...

Man, that's close. 92mm...????!!!

kopcicle said:

...Just some more virtually useless information...

Click to expand...

Not for this 'ol Rip van Winkle, who just found out about lightbulbs replacing candles.

Thanx, Kop...

I have a Winona boring bar (its done a few XS650's overtime) and never have used anything other than the two toggle clamps to hold the cylinder down. That would include everything from a PW50 cylinder to a inline 4.

if your final passes aren't like 8-10 thou hog passes there isn't an issue .

~kop