Can a thin base gasket make a huge compression change?

[turbowrenchhead]

When I had OEM base gaskets in my compression was 130psi. I was looking to go up a little more to 140psi or less. I tried a thin paper style gasket with yamahabond on both sides. Ran a compression check. Got 170psi. Way to much. I'd have to run race fuel then.

[mike0chek]

i was told the same thing you are saying. different thickness's will change your final compression numbers. someone that knows these shee motors better than me should be able to give you the resons why better than i could lol iv'e always wondered if putting sealer on the base gasket is a bad idea ( as in: say you have a .012 base gasket and you put sealer on, it would then be thicker then .012??)

Edited August 16, 2013 by mike0chek

[mike0chek]

btw i also put sealer on that base gasket so im thinkin the bad part is changing the thickness, of course is good to get a good seal in that area

[turbowrenchhead]

I understand that running a thinner base will bring your piston closer to your dome therefore increases compression. I didn't realize that it could change that much. This test was done after a set of fresh cast piston just after re assembly with a little 2 stroke oil dropped down through the plug hole.

[turbowrenchhead]

Is there a gasket that is slightly thinner then an OEM base?

[mike0chek]

jeff at fast strongly encouraged me to use a snap on compression tester also. i didnt think there would be much of a difference between a snap on and my craftsman but i was wrong. i was getting 125 at best with craftsman and 140 every time with snap on.

[mike0chek]

ive always used .012 on my cub with 24 cc domes and get 140psi. the thinnest ive seen is .010 but there could be thinner, im not sure how thick oem is.

[Nieskes]

What is your squish and dome volume?

[turbowrenchhead]

Dome volume is 20cc, haven't checked the squish. However 170psi is way to much for pump gas

[pickerg1]

jeff at fast strongly encouraged me to use a snap on compression tester also. i didnt think there would be much of a difference between a snap on and my craftsman but i was wrong. i was getting 125 at best with craftsman and 140 every time with snap on.

 

I agree with this guy... there are variances amongst compression testers...

Know anyone in the area who might let you borrow their snap on ? Or a shop you can pay a couple bucks to that will test it for you?

[J-Madd]

If you want more compression, shave the head if it is stock, or change domes if aftermarket. Don't change the base gasket solely to change compression.

[turbowrenchhead]

I thought of that but I used the same compression tester on the original build. I will need get some spark plug washers and make some measurements to get an accurate reading. I'm also going to have mine checked against a snap on to see if its accurate.

[Zillaguy]

Stock base gasket thickness= .030" I just measured a set of OEM Yamaha gaskets.

[J-Madd]

Unless I'm doing math wrong (and I may be, been a very long time since calculus, much less basic trig), you'll have 2.4 ccs less in the dome assuming a 64 mm bore.

[J-Madd]

Unless I'm doing math wrong (and I may be, been a very long time since calculus, much less basic trig), you'll have 2.4 ccs less in the dome assuming a 64 mm bore.

as opposed to no gasket.