Hey 2Stroke-Friends,

i'm Martin from germany and i'm currently restoring an 1975 DT 175.
Can someone give me the correct sqiush-clearance between piston and the cylinder head?

Take Care
Martin

Hi Martin,

From Yamaha there is no published spec...  in practice the squish clearance from the factory was "way too much"...  Most people have never checked it and leave it as it came from Yamaha.

Guidelines from Jennings and Bell (from memory) state that about 1.5mm is the maximum clearance for the squish to have any effect at all.  For a 175, 1mm clearance is safe...  you can get down close to 0.5mm but there is a risk of piston hitting the head. 

One caveat is that the 175 already has high compression so if you close down the clearance it is possible to end up with too much compression on a stock engine...  Usually piston does not come quite up to top of cylinder so machining top of cylinder is generally the best place to start reducing excessive squish clearance if your engine has way too much.   Heads are already not that strong so use caution machining material from head.

Hey Mark

thanks for the valuable information.
A value between 1-1.5mm is also quite common for other engines I know. I was just unsure and thought it would be better to ask rather than destroy the engine.
Unfortunately, the sealing surface of the cylinder base was damaged, and about 0.25mm had to be milled off. I would like to adjust the squish clearance to get the timing correct again. Setting CR as high as possible is not of my interest.


 

Lowering the cylinder will reduce the squish clearance by that 0.25 mm.  Squish clearance is likely still more than it should be. 

Timing will NOT be corrected with squish.  You'll need to either space the cylinder back up (thicker gasket?) OR have all the ports raised the exact amount that was removed from base.  Ports are critical to get even and at correct angles so I'd just use a thicker gasket.

You're right.
My plan was to use the squish clearance to determine the required thickness of the new cylinder gasket to bring the cylinder back to its factory position.
 

Unfortunately, you can't use squish for that as there is no standard dimension. 

With piston at BDC, the edge of the top of the piston should be flush with the lower edge of the transfer ports.  Using that and since you know how about how much was removed, you should be able to get really close if not exact.
 

Hey Mark,
Yes, I understand, unfortunately, my original idea won't work.

 Thank you for your idea. I think i will do it that way.

Have a nice Day!