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Compression test results

moremonkey

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Today I ran a compression test on the BN1 and was hoping people with more experience in these matters could help me interpret the results (and suggest options to consider).

When I pulled the plugs they were coated with dry black soot. No oil, no gas, no moisture, no damage to the plugs, no unusual wear, just covered with black soot.

I've attached a photo of the test results and a photo of a leak I have in my head gasket right at cylinder number two.

The car starts easily, runs well, does not consume or leak unusual amounts of oil for a 59 year old Austin Healey. It does leak some coolant but not enough to make a puddle, just drops to let me know where I have been. I thought (ok, hoped) it was from a leaky water pump gasket, so I replaced that and it seems dry up in front, but there is still a little bit of oil and coolant coming from the head gasket. I've not run the car more than a few minutes in the garage since replacing the water pump gasket. There is no oil in the coolant or coolant in the oil. There is no significant white smoke from the tail pipe. (See my earlier post about that end of the machine.)

What do these compression numbers tell me? What would bad numbers look like...the kind that indicate an immediate engine rebuild is in order? What do great numbers look like...the kind to expect after a rebuild? And what do typical driver-quality numbers look like?
 

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  • Head gasket leak.jpg
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  • Compression chart.jpg
    Compression chart.jpg
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NutmegCT

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A 69 year old A-H? Built in 1945? Didn't know they were made that long ago.

Dry black plugs often just mean the engine hasn't been run up to hot yet. I wouldn't worry unless they remain like that after a good long run.

I couldn't open the attachment showing the plugs, so don't know the compression readings. Usually, you worry if there's a big variation in the psi readings, like 165, 165, 140, 165. That would show problems. Something like 165, 165, 162, 166 - not a problem.
 
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moremonkey

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Nutmeg,

I fixed that math error just before you posted. Good catch.

The compression results were:


4.....3.....2....1....Cylinder
169 169 160 160 DRY PSI
181 176 165 175 WET PSI
 

NutmegCT

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More experienced AH guys will chime in, but if you've got a 10% (or less) difference in the psi readings, you're most likely ok.

Any traces of coolant in the oil? or oil in the coolant?
 
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moremonkey

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No coolant in the oil, no oil in the coolant.

What PSI (or range of PSI) are generally considered good pressure in a 100-4 engine?
 

NutmegCT

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Sounds ok to me. But wait for more experienced AH guys to comment. I think you're ok, and if the head gasket isn't leaking much, you can probably live with it.

My two cents.

Tom
 
Last edited:

Bob Claffie

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There is nothing wrong with your engine ! Retorque the head and be done with it. Bob
 

BoyRacer

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Well, the compression in not that bad. Cylinder 2 is a bit weak. What you really need to do is a leak down test. That will tell you the condition of the rings and the valves for each cylinder. Plugs that are sooty black says the mixture is too rich. What does the inside of the tail pipe look like? If it's black and sooty, it's too rich.
 

Brinkerhoff

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That coolant leak around the head is normal for this engine. I'm pretty sure of it. Its normally not hard to fix . I've run an MGB with the same type of leaking for 12 yrs. I wouldn't worry about the compression readings but I'd make sure the valve lash was in spec. Road test and lean the carbs a bit. The plugs should brown out.
 

Hot Wings

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Well, the compression in not that bad. Cylinder 2 is a bit weak. What you really need to do is a leak down test. T

Second the leak down test. The crank and pump type compression test is OK to confirm a very dead cylinder but if you want good data use the leak down.

Take the radiator and valve cover cap off and listen for leaks, same at the tail pipe. Block the throttle open and listen there as well. If you hear leaks in the radiator then put the cap back on and leave the pressure there for a bit to see if there are any visible external leaks. I do mine with the car in high gear so I can rock the piston a little to see if the ring lands may be bad. Just make sure that you have enough room front and back that if it gets away from you nothing gets bumped.

A pressure test, cold, of the cooling system will detect any leaks there that may not be into the cylinder.
 

busybrit

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Guys, while your info on leak down testing is correct and is a fine thing to do, if you know what your doing!,,, this thread was started by a Freshman member asking for reassurance that his engine is ok.

At this very moment he is loosing sleep over leak down tests. Based on his compression numbers I wouldn't bother doing any other test other than a gentle a test drive.
 
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moremonkey

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Busybrit, I'm not losing sleep over anything on that car. I've been driving it since the 1980s and responsible for keeping it on the road for the past 7 years. Every winter I work on one major system to improve drivability and was just gathering information for what might be next year's project. I'm thinking I may move into leak-mitigation mode soon. About 20 years ago I owned a 1970 TR-6 that I eventually got to be leak-free. Right now the only fluids the BN1 doesn't leak are shock juice and brake fluid.
 
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