Compression pressure AJS 500cc twin

condor · Post by **condor** » Thu Dec 02, 2004 11:27 am

I have a 1955 AJS Model 20 500cc twin which shows a compression test pressure with the throttle open of 135psi in each cylinder with the throttle open.

Can anyone tell me what the minimum acceptable pressure should be for this model?

Thanks Mike Hilton

m knight. · Post by **m knight.** » Sat Dec 04, 2004 9:41 pm

A friends Harley was running a little rough and we decided to do a compression check just to see if both cylinders where the same, and they showed 150p.s.i. this was higher than s.t.d as it had been tuned. I would not think that 135 would be too bad and the fact that both where the same shows that the cylinders are in good condition, as usually compression will drop on one cylinder more than the other as both sets of components rarely wear at the same rate.[;)][8D]

Ralph Whitaker. · Post by **Ralph Whitaker.** » Tue Dec 07, 2004 11:32 pm

When I was in the motortrade I was tought that anything over 100psi at cranking speed was acceptable as long as there was no more than 10% variation between cylinders.