early pistons and cylinders interchangeable?

[DogBunny]

Are 1970-1973 cylinders and pistons interchangeable? And do they interchange with 1974 and later pistons and cylinders?
Running into the usual confusion looking at the parts drawings.

 

[5twins]

'70-'73 pistons should swap, cylinders should with the right cam chain adjuster assembly. Early cylinders up into '71 sometime used a cam chain adjuster housing that mounted with 4 bolts. Mid '71 saw that changed to a 6 bolt housing .....

In '74, with the introduction of the "447" motors, the connecting rods were changed because the piston wrist pin diameter changed. '74 and later pistons could be fitted into the early motor but the rods or the whole crank would need to swapped for later spec items. Later "447" cylinders I'm not sure about. They may swap onto '70-'73 bottom ends but you would have to work out the cam chain tensioner details. It might be as simple as using the early 6 bolt type B tensioner but if it didn't fit on the later cylinder, you'd have to use one of the later tensioners. That would include using the tensioner blade that mounts inside the motor.

Another issue that might crop up is the head gasket. Later cylinders used a wider one, probably implemented to improve sealing .....

 

[TwoManyXS1Bs]

In a way, interchangeability is conditional. As 5twins noted, you must be mindful of the camchain tensioner systems.

The XS1 and XS1B cylinders have a bridge in the camchain tunnel, which can only handle the type "A" (and maybe type "B") tensioner system.


When the camchain stretches too far, the tensioner pushes the camchain into the bridge, abrading it.

The earliest cylinders, up to s/n S650-002486, used different length front guidebar hollow bolts, and had a longer bridge at the bottom, requiring a clearance notch in the center guide stopper.

Early XS1, with clearance notch:


Late XS1 and XS1B:


In mid-71 XS1B, Yamaha fitted 306 XS2 type "B" tensioner system, 6-bolt tensioner on a 4-bolt cylinder. I believe it retained the tunnel bridge, making chain grinding on that bridge worse.

With the 306 XS2/TX650 cylinder, the bridge disappeared, the 6-bolt tensioner base and the wider/grooved headgasket surface appeared.

256 XS1 - Tunnel bridge, narrow headgasket surface:


306 XS2/TX650 - Open tunnel, wider/grooved headgasket surface, almost indistinguishable from the later 447 cylinders. (Thanx, Mailman)


In summary, the 256 XS1 cylinder, with its bridged tunnel, will only work with the type "A" (and maybe "B") tensioner system, and 256 camchain, essentially keeping it in the XS1 world. But, you can put later cylinders, on an XS1, including the 750cc cylinder kit.

Starting with the 306 XS2, having the open tunnel and wider headgasket surface, all those cylinders should be interchangeable.

 

[TwoManyXS1Bs]

Now, the pistons.

XS1 and Early XS1B pistons had a higher dome, higher compression ratio, up until mid-71 XS1B s/n S650-023129.

But, these all shared the same piston rings and wristpins, and should be interchangeable on all 256 crank/conrods, 70-73.

256 piston rings, with 1.5mm 1st and 2nd rings:


Now, here's the funny business.
The first 446/447 '74 TX650A pistons and rings, with 20mm wristpins.
Up to s/n 447-010151, the first TX650A pistons used the thicker 256 rings. This could fool someone into thinking that they're 256 pistons.

 

[DogBunny]

Finally getting back to this...
Thank you for the responses. 5twins' cam chain adjuster picture is excellent, and TwoMany, I think you have written the book on cylinder and piston differences -- thanks for your herculean effort.

306 XS2/TX650 - Open tunnel, wider/grooved headgasket surface, almost indistinguishable from the later 447 cylinders. (Thanx, Mailman)
View attachment 125428
Starting with the 306 XS2, having the open tunnel and wider headgasket surface, all those cylinders should be interchangeable.

I compared a 306 XS2 cylinder to a 447 cylinder and as far as I can see, they are not only "almost indistinguishable," they are identical. If there actually is a difference, I'd like to know what it is. In any case, the important thing to know is that 1972-onwards are interchangeable.
Thanks again.

 

[TwoManyXS1Bs]

Digging deeper, found a subtle 1973 change to the hollow alignment dowels that fit between case & cylinder, and between cylinder & head. Part #2 in this pic.


91800-16016 == Pin, dowel, (12.3 - 15 - 16) XS1, XS1B/F, XS2
91810-16016 == Pin, dowel, TX650 (no dimensions given)

Don't know what the change is, you may want to measure what you find.

Also, later parts manuals show the 306 cylinders (XS2, TX650) to be downward compatible replacements for the XS1-XS1B/F cylinders.

This excerpt shows that there were (3) variants to the front guidebar's (2) hollow bolts. Part #6 in the above pic.

I know that the -00 is supposed to be a different length, can't tell you what the -01 and -02 differences are. To me, this means to closely check that part of the cylinders, where those hollow bolts screw in...