Squish ?

[Lester Gillett]

I have read a few post here and some talk about checking the squash. Why do you need to check it and can you not do it at all. I am replacing
the cylinder and piston on my ms460. I am putting a new Stihl unit so do i need to check the squash ?

 

[dando127]

I have read a few post here and some talk about checking the squash. Why do you need to check it and can you not do it at all. I am replacing
the cylinder and piston on my ms460. I am putting a new Stihl unit so do i need to check the squash ?

I would think as long as you are using the stock gaskets and cylinder/piston there should be no change from stock, so you wouldn't have to check. When people delete the gaskets or change gasket thickness, that is when it needs to be checked.

 

[ericm979]

It's called squish clearance. It's the space between the piston crown and the squish band of the combustion chamber at TDC. The piston comes close to the squish band and squishes out the air between the head and piston, which increases turbulence and mixing in the chamber. That makes for better combustion.

(from https://dragonfly75.com/moto/SquishVelocity.html)

It's a good idea to check it when replacing major engine parts. Cylinders, pistons and rods aren't produced to a perfectly exact length. There is a production tolerance for each. If you have a long rod, tall piston and short cylinder you could have too little squish clearance. The piston might hit the cylinder (maybe only at high rpm when the rod stretches at TDC). If you have a short rod and piston and tall cylinder you'll have too much clearance. That will not cause damage but will reduce performance and increase the chance of detonation.

Production tolerances are selected so that the piston hitting the cylinder will be very rare, which means that there is often too much clearance. You can run it like that and a lot of people do but it's not optimum.

It's easy to check by putting rosin core solder through the plug hole and turning the crank, then measuring the smashed solder. Use two pieces and put them where the piston pin is, so the measurement is not affected by the piston rocking in the bore.

For small saw sized two strokes, 0.5mm (0.020") is about the minimum clearance that's generally safe. More than .75mm (.030") and I'd think about making a thinner base gasket. I have only seen one manufacturer offer a range of base gaskets and it was for a motorcycle not a saw.

 

[Bob Hedgecutter]

For small saw sized two strokes, 0.5mm (0.020") is about the minimum clearance that's generally safe. More than .75mm (.030") and I'd think about making a thinner base gasket. I have only seen one manufacturer offer a range of base gaskets and it was for a motorcycle not a saw.

Stihl offered cylinder base gaskets for some models in either 0.5 or 1mm depending on where in the World the saws were being used, so the owners could drop compression levels for lower grade/ lower octane fuels.

 

[Bob Hedgecutter]

Squish is as @ericm979 explains above.
Squash is what happens when a log, tree, skidder, tractor, loader, truck or something else real heavy lands on your saw.

Who else clicked on this post expecting to see pictures of a squashed saw?

 

[grizz55chev]

Squish is as @ericm979 explains above.
Squash is what happens when a log, tree, skidder, tractor, loader, truck or something else real heavy lands on your saw.

Who else clicked on this post expecting to see pictures of a squashed saw?

I thought he was looking for the gardening thread.

 

[TRTermite]

Squish is as @ericm979 explains above.
Squash is what happens when a log, tree, skidder, tractor, loader, truck or something else real heavy lands on your saw.

Who else clicked on this post expecting to see pictures of a squashed saw?

I like the word squaRshed...Much more betterer..

 

[TRTermite]

Squish is as @ericm979 explains above.
Squash is what happens when a log, tree, skidder, tractor, loader, truck or something else real heavy lands on your saw.

Who else clicked on this post expecting to see pictures of a squashed saw?

Has anyone started a thread on squished/squarshed saws?

 

[Bob Hedgecutter]

Has anyone started a thread on squished/squarshed saws?

Well yes, the most recent:
https://www.arboristsite.com/threads/set-a-200-year-old-oak-on-my-462cm.358887/#post-7707773
At the end of the day, when all said and done, you have to squash something to measure your squish.

 

[pioneerguy600]

I thought he was looking for the gardening thread.

Fixed.

 

[TRTermite]

Well yes, the most recent:
https://www.arboristsite.com/threads/set-a-200-year-old-oak-on-my-462cm.358887/#post-7707773
At the end of the day, when all said and done, you have to squash something to measure your squish.

Nice closing sentence.

 

[Captain Bruce]

Stihl offered cylinder base gaskets for some models in either 0.5 or 1mm depending on where in the World the saws were being used, so the owners could drop compression levels for lower grade/ lower octane fuels.

Looks like this thread is doomed to the rabbit hole. Forget squash. Assemble as per the manual. Its not rocket science........go find the dirko thread.

If you are building hot rod saws......I'm in the wrong forum. I thought this was a guy trying to re-invent the wheel....