C2 Newly rebuilt 427 - compression low #3 &4

[Georges C.]

Just completed rebuild of 1966 L36 427 and done preliminary start up, running for about 15 minutes at 700-1000 RPM. Checked compression and 2 cylinders are low as I have approximately 150Lb pressure on all cylinders except nos 3 & 4 which are about 90. Block bored and all new .060 over pistons, rings, and both heads were rebuilt: new valves, seals, rockers, studs, springs, lock nuts, valve seats ground.

Any thoughts will be appreciated.


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[Russ S.]

Re: C2 Newly rebuilt 427 - compression low #3 &amp;4

First 20 minutes should be run at 2000-2500 RPMs. I would say run it longer and harder to break in and seat the rings.

[Steve G.]

Re: C2 Newly rebuilt 427 - compression low #3 &amp;4

Slack the valves off on those two cyls to ensure they aren't staying open. And a Russ said, recheck after proper breakin. Still low, back rockers off completely so valves will not open regardless of where cam sits and adapt your compression tester hose to your compressed air supply and load the cyls. Put your hand over the exh and intake to see if that's where the leak is. Listen for air blowing into the crankcase.
Steve