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Old 01-07-2018, 10:44 AM   #1
Stage1gs
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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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Old 01-07-2018, 11:49 AM   #2
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Default Re: Interpreting Compression Test Results

Your hot number is more important than the cold number with oil. I wouldn't worry about my cyls if those were my numbers. Have you given it a good tune up. are you sure your intake is sealed perfectly? I had a valve that was causing a slight problem and its top is white where the good vavles were black.


edit: Is it noisey when you rev it? Are you thinking you have blow by from worn cylinders? I dont know how my car would have performed on the bag test I hadn't heard of it till now but makes sense. My engine is out right now due to worn cylinders. But it probably would have continued to run. The model a rings are large and heavily sprung so they will follow a egged cylinder.

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Old 01-07-2018, 11:52 AM   #3
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Default Re: Interpreting Compression Test Results

I differ with your conclusions.A rise in readings with a 'wet' compression test indicates piston ring wear.An increase of crankcase pressure from using a higher compression head indicates compression pressure blowing by the rings,verified by the 'bag' test.The increase in rear main leakage may not be a rear main bearing leak,but an oil leak from the pan/bearing area aggravated by higher crankcase pressure caused by compression leaking past the rings..in short,given what you've stated Id say you have bad rings.


Raising compression does not cause accelerated bearing wear..it does show any weakness in rings,valves,deck condition and head stud retention ability.
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Old 01-07-2018, 12:02 PM   #4
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Default Re: Interpreting Compression Test Results

I am in agreement with the bad rings, will do the leakdown test today to check the valves. Does the engine have to be hot to do a leakdown test, or can you do it cold?
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Old 01-07-2018, 12:16 PM   #5
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Default Re: Interpreting Compression Test Results

Welcome to the family!

In order to compare apples to apples, I recommend you re-do the wet test again with a warm engine.

Interpreting results after adding oil:
+10psi increase good engine
+20psi increase normal engine
+30psi increase worn engine
NO INCREASE burned or stuck exhaust valve.
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Old 01-07-2018, 12:18 PM   #6
Purdy Swoft
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Default Re: Interpreting Compression Test Results

I agree with others that there is a problem with blow by . If the engine is a recent rebuild , the rings may not yet be seated. If its not a recent rebuilt engine or possible gasket or rear pan seal leaks It may be time for a rebuild .
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Old 01-07-2018, 12:47 PM   #7
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Default Re: Interpreting Compression Test Results

The numbers look pretty normal to me, except the 102 on #4 cold. That's odd. Would retest. Leak down test should confirm it or not. I do not think a 6:1 head would generate 102 psi.
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Old 01-07-2018, 01:01 PM   #8
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Default Re: Interpreting Compression Test Results

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I am in agreement with the bad rings, will do the leakdown test today to check the valves. Does the engine have to be hot to do a leakdown test, or can you do it cold?

Wet/dry and leak down tell you the same thing,whether a potential issue is valve or ring related. any increase on the wet side of the wet dry test preclude a burnt valve being the issue,generally valve issues dont respond to the oil.When you installed the new head did you inspect the cylinder bores? piston clearance in bore? ridge on bore?Now might be a good time to freshen it up,check the rod and main bearings,hone and install new rings and lap the valves..you can do it with the engine in frame.Its a good time to set the rod and main bearings,when the valves and piston assemblies are out.makes getting a good bearing 'feel' easy.
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Old 01-07-2018, 01:28 PM   #9
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Default Re: Interpreting Compression Test Results

I was taught not to worry about compression readings unless there was a difference of 15 psi or more between cylinders. Your hot engine readings are great, and since that is the way the engine normally runs, be happy. As for the valves, if you can take your A out and coast down a steep hill in second gear and not have any back-firing be happy.

I’ve learned over the years that the best way to find out if the bottom end needs work is to install a new or rebuilt head. In your case with the higher compression head, you’ve possibly magnified any lower end problems.

My suggestion is to get the A running smoothly and drive it awhile to get used to the new head and the change in performance. You might find out that you are reacting to the change caused by the new head and not actually something being wrong. Good luck.

Mike
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Old 01-07-2018, 08:36 PM   #10
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Default Re: Interpreting Compression Test Results

I started up the ol' model A today to let it warm up before re-doing the compression and doing the leakdown test. It didn't leak a drop of oil out the rear main.. Yesterday it leaked a drop every couple seconds until it warmed up. I got it up to 195 degrees and then did the compression test followed by adding oil to the bore. I then did the leak down test. Here are the results:

Cylinder. 1. 2. 3. 4.
Compression. 90. 86. 87. 86.
Added oil. 96. 90. 92. 92.
Leak down %. 58%. 20%. 70%. 20%

On the 3 cylinders below 60% the sound was coming out the oil fill pipe, and on the 2 with 20% i could hear air coming out the plug hole next to that cylinder. I verified each of the cylinders were on TDC..

Based on this I am thinking valve related issues.. Bad seats, bad guides, burnt valves, etc. Thoughts?
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Old 01-07-2018, 09:14 PM   #11
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Default Re: Interpreting Compression Test Results

Simple...run it..or if your anal,lap the valves re ring it and shim the bearings..
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Old 01-08-2018, 08:57 AM   #12
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Default Re: Interpreting Compression Test Results

I say rings are good. Recommend just doing a valve job.
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Old 01-08-2018, 11:25 AM   #13
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Default Re: Interpreting Compression Test Results

Listen to Railcarmover
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Old 01-08-2018, 02:07 PM   #14
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Default Re: Interpreting Compression Test Results

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Originally Posted by PC/SR View Post
The numbers look pretty normal to me, except the 102 on #4 cold. That's odd. Would retest. Leak down test should confirm it or not. I do not think a 6:1 head would generate 102 psi.
A 6:1 head will generate 120psi, ask me how I know.
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Old 01-10-2018, 09:20 PM   #15
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Default Re: Interpreting Compression Test Results

I pulled the head tonight and have a valve with a small chunk missing out the side of it. So a valve job is next on the list. I also noticed that all of the exhaust ports were dripping wet with oil. I would have expected that after letting it idle for 30 minutes before draining the oil that the exhaust ports would have been drier.

The bores in all cylinders look good and even, no unusual shiny spots and no rough spots. None of the cylinders have a ridge at the top that can be detected with a finger nail. I measured the clearance between the top of the piston and the cylinder, and pushed the feeler gauge down until it hit the top ring. All 4 cylinders have a piston to cylinder between .011" and .012". My model a repair manuals don't say much about this particular dimension. Any thoughts on if this is an issue..?
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Old 01-11-2018, 02:21 AM   #16
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Default Re: Interpreting Compression Test Results

Post a picture of the piston tops and the tops of the valves.
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Old 01-11-2018, 04:01 AM   #17
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Default Re: Interpreting Compression Test Results

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I also noticed that all of the exhaust ports were dripping wet with oil. I would have expected that after letting it idle for 30 minutes before draining the oil that the exhaust ports would have been drier.
At idle, there isn't enough heat or velocity to completely burn off all the oil. Take it for a drive and that will clean them up.
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Old 01-12-2018, 07:36 PM   #18
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Default Re: Interpreting Compression Test Results

See attached picture of cylinder 1
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Old 01-12-2018, 07:40 PM   #19
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Default Re: Interpreting Compression Test Results

Cylinder 2
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Old 01-12-2018, 07:42 PM   #20
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Default Re: Interpreting Compression Test Results

Cylinder 3
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