Facebook Import: Compression Information

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xing6666
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Joined: Fri Feb 07, 2020 4:35 am

Facebook Import: Compression Information

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Original Post - imported from "Chang Jiang Owners" facebook group with permission, for technical reference

Harry Lok
February 16 at 6:17 PM
Just dropped off my CJ 750 M1M for servicing and asked the mechanic to do a compression test on each cylinder. He responded with “what is norm for this bike”? Blank look from me 😐. Any one know ....over 85 psi but up to 135psi?

Useful responses:

Gregory Stewart
If the bike is a sidevalve bike, the compression pressure should be between 60 and 80psig, depending on how fast it's turned over and how much carbon is in the chamber, etc, etc ..... Ek Paul Prengel is EXACTLY right here, however - the cylinders need to be EQUAL, no matter what the reading. If they are any more than about 5% off of each other, there is a problem of some kind. It's tantamount that the cylinders are equal in compression AND running (carb balance, etc) so the crank is not stressed. They WILL twist out of alignment with an imbalance of fourth order harmonics in the engine.
Also, if you do a leakdown test and come up with anything LESS than about 12% you are golden. The rings are NOT great at sealing these engines when cold .......

George Ferris
When I bought mine one side was 90 psi the other side was 120psi. After I tried a number of things, it was a variation in the heads. Put a new set on and they are now equal.

Michael von Zelewski
the SideValve has 6 bar (87 PSI) , and the OHV has 8 bar ( 116 PSI), when new.

Norman Zidek
The running play of pistons and cylinders is more important than compression. Here in the forum in Germany (where the motors have to Work hard😁) a running clearance of 0.08 - 0.1mm is recommended. Up to 0.12mm is also OK. When compressing, I only have 65psi.

Gregory Stewart
Norman Zidek is ABSOLUTELY correct here!! I run mine at the maximum clearance because the pistons are NOT alloyed with silicone and they will expand an insane amount when hot.
-Kai
-1970 CJ750

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