squish clearance on YZ250

[D36-108A]

What's the optimum squish clearance for a latest-generation YZ250 engine?

Is simply milling the head a good fix?

I prefer to burn race fuel or race fuel mixed with pump gas.

 

[2 strokes for life]

I thinl you can get different thickness head and base gaskets to tune your poer without taking off material.

 

[D36-108A]

This machine uses o-rings, not head gaskets.

 

[2 strokes for life]

D36-108A, sorry, as you can see none of my bikes use a o-ring.

 

[Pete Payne]

What squish clearance do you have now and what type of riding do you do as well as the type of fuel you want to runn all the time?
All of these things factor in .

 

[funfactor1]

Do you not just change the base gasket

 

[Pete Payne]

If you change the base gasket alot of things change.
All port timing gets retarded (they open later)
Bottom end compression increases
the squish clearance tightens up
and the compression ratio goes up .
If you mill the head gasket surface (where the orings seal on the head) only the squish clearance tightens up and the compression ratio goes up .

 

[flyinzuki]

I believe it's worthwhile on any motor (checking squish).
Here's a pic of a ktm head, I took .9 mm off.
Also added a smaller (.2mm) base gasket.
Squish is .85mm tapers to 1mm
100MON

 

[Jasle]

If you change the base gasket alot of things change.
All port timing gets retarded (they open later)
Bottom end compression increases
the squish clearance tightens up
and the compression ratio goes up .
If you mill the head gasket surface (where the orings seal on the head) only the squish clearance tightens up and the compression ratio goes up .

This is all true but mfg's make different thickness base gaskets that are readily avail. if your in doing a topend and you find your squish clearance is not where you want it you can easily adjust squish with the base gaskets. Not nearly as easy to mill the cylinder/head at home.
Quite easy to change basegaskets at home.

 

[Pete Payne]

Just offering up some advice . Just didn't want this guy to take .020" off and then go , "man , now I have a lot of detonation and why is this hole in my piston "

Yes , the base gasket is easy to change , cometic , among others makes a lot of different thicknesses. But , It changes more than compression alone .

Yes , It can yield a good dividend if executed properly .

 

[D36-108A]

FWIW, my local machine shop is charging me about what a high-compression Cometic gasket kit costs. I don't have to remove the cylinder (and pipe and carb, etc.) and I don't have to put up with port timing changes or deck height changes. More important, I'm fixing something that's just plain wrong.

 

[Pete Payne]

Do you know how much higher the compression will be ?
Or is this a cut and try thing for ya ?

 

[D36-108A]

I don't know how much higher the compression ratio will be, or the cranking compression. I never even measured the combustion chamber volume and I don't plan to now.

If it tries to detonate itself to death, or if it measures over 250 pounds of compression, I'll get another head. But I seriously doubt either will happen. I took a conservative amount from the head, maybe not enough for the ideal squish clearance.

 

[euro gasgas]

There is a good article at www.rb-designs.com on measuring squish. You will want to get a good compression reading and have the combustion chamber reshaped for optimal compression as well (this is done for the altitude range which you ride). FWIW - Ron Black at RB-Designs (portland, oregon) does fantastic work and is very reasonably priced as well.

jeff
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