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Joined: Feb 2004
Posts: 27,000
Kettle Custodian (pot stirrer)
A leakdown tester doesn't need to be complicated, although the kit in the link is handy and inexpensive. I just use an adjustable air pressure regulator from Harbor Freight and a spark plug shell with an air hose quick connect nipple soldered into it. Adjust the regulator to 100 PSI on the built-in gauge with no air flowing, and plug the air hose onto the adapter screwed into the spark plug hole. If the pressurized cylinder holds 90 PSI, for instance, there's a 10% leakdown rate. A cylinder with good piston rings, valve seats, and a head gasket that's not leaking should hold pressure in the 90%+ range. Listen at the carburetor, tailpipe, or crankcase filler to determine where the leakage is. A leaky head gasket or a cracked cylinder head will cause bubbles in the radiator.

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Last time I did a compression test on the green turd was in 2015. Been wanting to do another test but remembered something about doing a leakdown test instead. Thanks to all for the reminder comments and suggestions.


My '50 Chevy 3100 5 window, 235cu
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If I'm not working on my truck or VW camper, I'm fishing with the wife or smoking Salmon.
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Posts: 526
Do a compression test in conjunction with other assessments, such as evidence of blow-by, oil consumption, power on the road, etc.

For example, on my trip south winter of '22 the engine began using oil which got worse as the trip progressed. Downwind of the road-draft tube was a mess of mixed oil and dust. Power wasn't down much but some. By the time I got home it was using a quart in 300 miles. I did a compression test and compared it to previous tests. No. 6 cylinder was down to 110 pounds from its previous 135, while the highest was 140. Previously, no. 5 was the lowest at 120 from a scratch in the cylinder wall that occurred somewhere in the past. The plugs showed no signs of oil burning. Given 200,000 + miles on the shortblock I decided to pull it apart. No. 6 piston was burned back about 1/2 inch from the edge and about an inch long yet it still showed 110lbs but there was a lot of blowby blowing oil with it out the vents.

Oil in the cylinders from leaking oil rings can mask worn rings by still showing normal cylinder pressures on the compression test. Likewise leaking valve-stem seals.

1951 3800 1-ton
'62 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.
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Originally Posted by Hotrod Lincoln
Try testing compression on cylinder #1 when the #5 piston has a big hole in it. All the cylinders must be tested, and a percentage variation between them needs to be calculated. Compression tests in general are a very primitive way of monitoring the condition of the cylinders. A leakdown test where air pressure is introduced into the cylinder with the crankshaft locked is a much better indicator of the mechanical condition of the piston rings, valves, head gasket, etc. Depending on the skill of the operator and the testing procedure he uses, a compression test is sort of a "measure with a micrometer, mark with chalk, and cut with an ax" process. It's better than nothing, but not much.

Just to add to Jerry's statement. I see it as a generic comp test tells you if there is a problem. The leak down test tells you where the problem is. I knew nothing about the leak down tester until I borrowed one and watched some videos how to do it.

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