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Old 01-07-2018, 10:44 AM   #1
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Join Date: Nov 2014
Posts: 16
Default Interpreting Compression Test Results

My 1930 model a had no oil leaks but had a water leak due to corrosion in the cylinder head water jacket. So i put a new Snyders 6.0 head on it, and because it is bored .125 over i got the correct copper gasket to handle the big bore. Now when starting the engine i get oil out the rear main cap at the rate of a drop every 2 seconds (yes, seconds). It stops completely when the engine warms up (roughly 5 minutes). The new oil fill cap has the tabs, so I took it off and taped a ziploc sandwich bag over the fill pipe and started the engine. The bag did not flutter; it completely filled the bag with air in about 40 seconds.. think of a ziploc bag balloon. Looking around the engine also showed oil leaking out the new front crankshaft seal, and at a couple places on the oil pan gasket that was installed with hytack on both sides and is tight. Sounds like excessive crankcase pressure.. So I started doing a compression test and then realized i didn't have a squirt oil can, so off to harbor freight. When i got back the engine had cooled some so i repeated the test and included putting oil in the cylinders. The results were interesting:

Cylinder. 1. 2. 3. 4.
Hot engine. 92. 85. 90. 93
Cool engine no oil. 75. 75. 80. 90
Cool engine with oil. 84. 85. 90. 102

Adding oil brought me up about 10 psi per cylinder. Looking for thoughts on what difference is acceptable bettwen a cylinder with no oil, and then adding oil.

I will do a leak down test later today and see what that tells me. Guessing I also have a a couple questionable valves.. This engine is also idling rougher than before, and i am guessing the new head just made it worse.
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