What is the clearence from the top of piston to top of cylinder supposed to be??

[wexy21]

I have the service manual but can't find it.  I installed a new top-end and the gasket kit came with two base gaskets (one green and one black).  Since I only had the green one installed originally I only used the new green one and left out the black.  Bike seems fine after initial break-in but I want to make sure.  I measured .50mm after the top-end inspection.  All seems well so far but I would like to know the clearence allowences if anyone knows. Thanks!

[wexy21]

Correction...I wasn't at TDC (pulled ignition cover to verify) My clearence is .35mm loosely and .40mm tight.

[riffraff]

What kind of bike?

[wexy21]

That would be important wouldn't it! 2002 CR125

[riffraff]

If you got the old base gasket you can check it's thickness with a caliper or micrometer and compare it to the new one's

[wexy21]

It was the same gasket.  But this bike has been rebuilt before and not by me which is why I wanted to know the correct clearence to see if it was done right.  I'm thinking it's ok, but just want to make sure.

[TMKIWI]

That can be a problem when someone else has rebuilt the engine.
You just don't know if the base gasket used was the correct one YOU should use.
I am sure you can find the info on the net.

[wexy21]

I would think that as long as the piston doesn't come flush with the top of the cylinder it should be fine.  The gap is miniscule and it ran fine.  But still.....why would the gasket kit come with a black AND a green base gasket?

[TMKIWI]

The 2 different thickness gaskets are so you can change your port timing by lifting or lowering the cylinder.
But by doing so you also change the compression ratio.
You need to find out the correct clearence but .4 sounds like it is in the ball park.
Do you have a problem or just wanting to get it right ?

[riffraff]

Have you measured the two that you have?

[wexy21]

I was just wanting to make sure it was right.  the gasket I'm using the exactly the same one that was on there before so I'm just going to leave it the same.  Thanks guys!

[RedFoxCR]

I wish i had your problems...! My base gasket is cut from a magazine cover! )))))

[Friar-Tuck]

Here's a pretty good write up.
http://www.gasgasrider.org/html/measuring_squish.html

 Tuck\o/

[JohnN]

Here's a pretty good write up.
http://www.gasgasrider.org/html/measuring_squish.html

 Tuck\o/

Nice article! Thank you.

[JETZcorp]

Gasket cut from a magazine cover?  You call that a problem?  I heard the story once of a guy whose CZ had the whole barrel come unbolted while he was riding.  He pulled up to the rest of the riders and said the bike was "acting kinda funny."  They looked down and saw the whole cylinder rattling up and down by quite a lot (didn't hear an actual number, but I got the impression it was something like 1/4 to 1/2 of an inch, sounds a bit crazy).  Understandably, everyone panicked and waved wildly for him to shut 'er down.  Imagine that that must've done to porting and compression, no matter how little movement there was.  That was pretty much all the proof that anyone needed that CZs were tough as hell.