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Sir Paolo

Cold Compression Test

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Gents, with my rebuild hopefully nearing completion, I thought I'd carry out a cold compression test to make sure all was ok before I fitted the sump etc.

With the head on, no carrier's and no starter, I could of course only turn the engine by hand. Even so, I still got readings of 50-60psi.

I would hope this meant all was sealing properly, and I could expect figures around 200psi in ideal test conditions i.e with a hot engine and throttle wide open.

I'd be interested to hear your thoughts on this.

Many thanks

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Ahh, this is my point - I haven't completed the rebuild yet, so the engine hasn't been run at this stage. I just wanted a ballpark figure.

 

What the cold test did show was that the compressed air was being forced past the rings and not the valves, so at least, I hope I've got the valve lapping/seating correct(?)

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A leakdown test will definitely tell you whether the valves or rings are leaking. And you'd be able to get a relative reading between each cylinder as a leakage percentage.

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I'd say the omens are good. If you can get 60 psi just turning it over TDC by hand, you haven't got any crass leaks; the valves would seem to be seating properly, at least. Once you get it started and have run the beast for a bit, do a proper hot test and see how it is then...remember that things ought to steadily improve during the running in period. As Travis says, leak down tests are the way to go, but the equipment for these isn't as easily available as a "standard" compression test gauge.

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Thanks, John. I would indeed expect figures to improve under ideal test conditions i.e. hot engine and throttle wide open, but I wasn't sure if 50-60psi was representative of anything under my test conditions.

 

Even so, it was evident that air was wheezing down through the piston/liner gaps and not out the ports via the valves. I was keen to do this test as I wanted to be doubly sure I hadn't bent any valves when setting up cam timing with the new vernier pulleys! 

 

Travis - yes, you're right, and as John says, a leak down test is the 'way to go', but in my humble garage, a compression tester was all I had to hand.........................

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