750 Big Fin 9:1 CR Kit ==> Machining pistons to lower compression?

[patentgeek]

Does anyone know if the pistons in the 750 Big Fin 9:1 CR kits from Heiden and Mikes XS can be machined to achieve lower compression on the order of 8:1 to 8.5:1?

http://www.xs650.com/forums/the-garage.2/create-thread

https://www.mikesxs.net/yamaha-xs650-447-big-bore-kit-80mm-750cc.html

Or alternatively, are low compression pistons available?

Wossner manufactures 8.4:1 CR overbore pistons, but I believe they are only available for the Euro 533 crank and not the 256 or 447 cranks. I'm in the U.S. and would prefer to stick with 256 or 447 parts.

 

[grizld1]

You might consider doubling the base gasket instead of modifying the pistons. That would raise the deck .020" and get you close to the desired CR. That would also have the effect of advancing valve timing, so you might need to move the sprocket on the camshaft to compensate. 2Many wrote a fine thread on the topic; a quick search should turn it up.

 

[patentgeek]

You might consider doubling the base gasket instead of modifying the pistons.

I read that thread earlier, but doubling up head or base gaskets on an overhead cam engine seems like a less than ideal solution to lowering compression. My engine is stuck and will need to be gone through, so I'd prefer finding a set of pistons that will get me in the range I want to be.

 

[TwoManyXS1Bs]

For an 80mm bore:
Wouldn't take much to drop from 9:1 to 8.5:1.

About 3.1 cc's reduction in the piston dome.
(Visualize a 1" x 1" piece of 3/16" aluminum plate)
(Or, a 2" diameter piece of 1/16" aluminum plate)

Or, add 0.617mm (0.024") to the head/base gaskets.

http://www.xs650.com/threads/shaving-wiseco-big-bore-pistons.37553/

http://www.xs650.com/threads/when-to-rebuilt-engine.46862/#post-469432

http://www.xs650.com/threads/whats-the-ideal-compression-ratio.44147/

 

[Signal]

I agree its not ideal to use two head gaskets however I have done so to lower compression.
I bought a road bike XS 650 (750) it had been rebuilt using high compression pistons I measured over 11:1.
I lowered the compression using a thicker base gasket (custom made) and two Head gaskets using 5 twins guide on where to put sealer. The compression is now around 10:1 still high.
The cam timing was reset using an adjustable sprocket.
So far as reliability I did some long trips on that bike over a few years, then took the motor out and put it in a race sidecar for around 2 seasons. No leaks but I do from time check the head bolts and it was heat cycled and retorqued a few times initially. They are torqued to 29 ft/lbs max.
I would do it this way again without hesitation given this experience and you can always bump the compression up again if you change your mind.

 

[patentgeek]

Thanks for the replies suggesting thicker head or base gaskets, but I'm really focused on finding lower compression pistons, or modifying pistons to achieve lower compression.

If nobody makes low compression pistons, any recommendations for custom piston manufacturers?

 

[hooser]

My JE pistons can be cut, but I must ask why, email me at gary@hoosracing.com

 

[jay760]

Smedspeed in the UK shave the tops on wiseco pistons not sure what ratio they end up at.

 

[patentgeek]

Update on my Big Fin cylinder + low compression piston research for my XS1 . . .

Hoos Racing indicates that their 256-type JE 700cc 9.5:1 CR pistons can probably be modified to achieve ~8.5:1 CR.

Heiden Tuning says their 256-type 650cc pistons are 8.7:1 CR, which is equivalent to original XS1 pistons but a little higher than the 8.1:1 CR that was introduced in the XS1B model. I'm guessing the Heiden pistons are cast, so potentially more risk in machining the domes for lower CR, but 8.7:1 might be good enough.

Mikes XS appears to sell a 650 Big Fin cylinder without pistons at this link:

https://www.mikesxs.net/yamaha-xs650-big-cylinder-block-for-650cc-standard-bore.html​

So at this point I'm looking at buying 256-type Carillo rods, Hoos Racing or Heiden Tuning pistons, and a 650 Big Fin cylinder. Am I missing any compatibility issues using that combination on an XS1 July 1970 production engine?

For those of you who are wondering why I started this thread, I'm building up this XS1 for my girlfriend, so I want to make it as easy as possible to kickstart. A later head with a compression release will also be used. In addition to the easy starting aspect, street gas octane levels aren't what they used to be, and temperatures where we ride often exceed 100F. So it seems like a good idea to set up the bike close to the original 8.1:1 or 8.7:1 CR of the XS1/XS2 models.

 

[xjwmx]

Sometimes the low rent way is the best way. Not often, but sometimes.

 

[wannabridin]

There are folks on e-bay who can make custom thickness gaskets. I asked a guy for a super thin copper gasket for a build one, he had no problem making it. His waterjet didn't care about .050" extra thickness. I had to anneal the gasket after receiving it, but no biggie.

 

[ihaxs650]

I had a shop install the XS650 Big Fin 650 Kit on my 1980 special in lieu of the Big Fin 750 kit. One of the potential issues that I did not want to deal with was the higher compression pistons of the Big Fin 750 kit.

 

[TwoManyXS1Bs]

Just an FYI, here's the bulletin showing the compression ratio change:

 

[TwoManyXS1Bs]

... Am I missing any compatibility issues using that combination on an XS1 July 1970 production engine?

Watch the ringset and piston groove fitments. OEM top rings have a 1.5mm thickness. There's some alternate offerings using the later 1.2mm thickness.

I'd recommend sticking with Gary Hoos for 100% of your parts orders, he'll ensure that everything is compatible...