compression test

[Busguy71]

I picked up a '73 472 with 80k miles a few months back and I am just now going to be able to work on it. I'm hoping for a spray can rebuild. I'm going to compression test it first, so what readings should I look for? Whats the lowest a cylinder can be and still be considered good enough? I was taught all cylinders should be within 10% of each other, just need to know the high and low for this specific motor. Thanks!

[Clay]

Busguy71
Cranking compression should be between 125 and 135 lbs. The reason it varies is because the atmosphere pressure varies and altitude also.
You are correct in that it is more important that they be within 10% . A better check for soundness is a leakdown test which tests valve seal as well as ring seal. The cylinder should leak down only so many lbs. in 30 or 60 seconds. I have to check my manual to be exact but it is a better test. Also if the plugs are very oily the oil rings or valve seals are gone.
A simple rule of thumb is take atmosphere pressure (14.7) average and times it by the known static compression 8.5 and times it. Generally this will be ballpark for compression. 14.7 X 8.5 = 125.
If compression is low put a little oil in the cylinder and retry. If compression comes up rings are bad. If it remains low valves are bad and also maybe the rings.
Clay

[Busguy71]

thanks for info! I might pull the heads, get a valve job done and new head gaskets. I just dont want to touch the bottom end. I'll cross my fingers and pray the rings are good!

[zenish]

sometimes a valve job will cause the rings to go because of the added pressure.

[shakedog]

sometimes a valve job will cause the rings to go because of the added pressure.
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I've heard this so many times, but never seen the evidence of it. I've done lots of valve jobs with no bad results to the rings. If its simply a valve job that the engine is needing (and not rings) you will only benefit from it and a timing chain.
I once had an engine that smoked so bad thru the valve guides I got complaints, seriously! I had the heads done completely, new valves, seats springs and seals. I put another 5 years of use on it and sold it still running great! Lets face it, if the bottom end isnt worn out its still a servicable piece._______________

[STDog]

I picked up a '73 472 with 80k miles a few months back and I am just now going to be able to work on it. I'm hoping for a spray can rebuild.

I'd have to ask why. You shoud use plastiguage and check the bearing clearances on the rods and mains. I'd recommend new gears on the oilpump, and check the pump drive for wear ( cused a failure on my 95k mile engine), replace if needed. New timing chain at least, probably the cam gear too (if it the original style nylon gear for sure).

With 80k, I'd do the bearings and rings anyway. Hone the cylinders too. Then a valve job. If money is an issue, start with a kit. Al will probably give you a good deal on a rering/freshen up kit. Shouldn't be much to have the heads done, as long as everything is good.

Then again, you can just run it as is untill it breaks. Do nothing to it at all. Start saving money, buying part to get it done right later. But I wouldn't do it half way.