Top End Help

[bdb5454]

I have an 08 Apex 90 that I bought used about a year and a half ago. My son does not race, but does like practicing at the track. I decided to do a compression test and it read 120 psi. After reading up on this, it looks like anything under 130 psi would need a new top end. I purchased a top end kit and installed it. Everything appeared to go well, but when I fired it up, it seemed to have lost power. I ran it at idle and warmed it up and shut it down 4 times. I then performed another compression test and it read 90 psi. Does it need more break in time or have I done something wrong??? My only other thought is maybe this 90 cc has been bored before and I bought a stock piston. I would need to use a micrometer and compare I guess. If it has been bored I thought I would not get much psi at all??

[ontargetracing]

I would have suggested checking the bore but you already thought of that. Poston facing the right direction ?

[Tim kapopoulos]

What's the squish measure? Whats size gaskets came off?, what size did you put on? Port timing could be off. All cylinders , rods, pistons are + or - .02 , so you could be way off if you haven't measured your squish . Lets us know

[bdb5454]

Thanks for the reply Ontarget and Tim! I made sure I had the piston in the right direction and the bore looks ok. The squish test is at .010 or 0.25 mm. As far as the gasket, I am not sure what size came off. I didnt think of checking before throwing it away. The gasket kit I bought from G Force and is stock OEM. It came with two gaskets. One black and one green. I put both on. Looking back seems dumb now, but I'm def not a good mechanic. :) The gasket that came off was black. I assume I should put the black one on only? I will need to order a new gasket kit since I have already had this head off a couple times trying to fix this. Another thing I could have did wrong was not making sure the rings were centered as I put it into the head. I didnt have an extra pair of hands and eyes at the time. Any thoughts? Thanks for the help!

[bdb5454]

Today, I pulled off the head, removed the green gasket and left the black one on. I had a helper make sure both rings were centered. Did another compression test and it came up to 110 psi. So, a little improvement but not over 130 psi yet.

[chunky0071]

How are testing the squish? .010 is very close!

[bdb5454]

How are testing the squish? .010 is very close!

To test the squish, I placed the new ring in the cylinder. I then used a feeler gage and tested the gap. I hope that was right?

[fomospede]

squish is the distance from top of piston to cylinder head
check at the edge inline with the pin on both side
im not sure what the squish is to be set at though

[bdb5454]

Thanks! I appreciate the info. I am assuming I will need to use the kick start to find the highest point of the piston and measure the edge on the cylinder wall? If it is off, how will I adjust this? Thanks for the help!

[fomospede]

take modeling clay and stick on piston at the edge inline with pin both side with it down from top a bit ,about the size of a BB
put just a bit of oil on head where clay will hit so it wont stick
assemble head and role over engine remove head and measure clay thickness and average outcome
to adjust thicker or thinner base gasket are available

[bdb5454]

Okay, I performed the squish test with the modeling clay. One measured 0.060 inches or 1.52 mm and the other 0.062 inches or 1.57 mm. So, if I understand this correctly, I will need a thinner gasket? Any recommendations when I purchase the size?

[fomospede]

Im not sure im assuming .019 is desirable
But im a 70 guy id wait for better info !

[bdb5454]

Thanks fomospede, I appreciate the info. I guess my first step is to measure the gasket that is on there now. If someone happens to know the two sizes the OEM/stock gasket comes with, I can narrow down my current size. I measured the green one that I took off and it is .50 mm

[bdb5454]

Anyone out there that could make a recommendation for a gasket size? Sure could use the help.

[chunky0071]

ok if your squish was .061" average than you need to be in the ball park of .025" to .030" so measure the base gasket you had on say it was .070" you need a gasket that is .040' to .035". That is simply subtracting the amount squish desired from the total squish, gives you the thickness of the base gasket needed.