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Unexpected Compression Readings

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SteveHall64Healey

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I've just finished re-installing the cylinder head on my BJ8, following a head rebuild by a reputable local shop. I've set the cold clearance to 12 thou and have gone through them twice. The carbs are off the engine, so nothing is restricting intake. Before re-installing the head, I measured the stroke and all 6 were highly consistent at 3.5" +/- .007". Head bolts were installed at 75 ft-lbs with the correct sequencing observed. The head was minimally decked by the machine shop. I measured compression (twice):

1- 115, 115 psi.
2- 100, 100 psi.
3- 130, 130 psi.
4- 122, 125 psi.
5- 122, 126 psi.
6 - 100,100 psi.

Much lower and with more variance than I would have expected. Each cylinder is holding pressure for at least 15 seconds. Has anyone encountered this before?

Prior to rebuild, the combustion chamber and piston tops were significantly caked in carbon. Compression readings were:

1 - 155 psi
2 - 140 psi.
3 - 155 psi.
4 - 158 psi.
5 - 155 psi.
6 - 150 psi.

Strange .... any ideas?

Steve
 
SteveHall64Healey said:
I've just finished re-installing the cylinder head on my BJ8, following a head rebuild by a reputable local shop. I've set the cold clearance to 12 thou and have gone through them twice. The carbs are off the engine, so nothing is restricting intake. Before re-installing the head, I measured the stroke and all 6 were highly consistent at 3.5" +/- .007". Head bolts were installed at 75 ft-lbs with the correct sequencing observed. The head was minimally decked by the machine shop. I measured compression (twice):

1- 115, 115 psi.
2- 100, 100 psi.
3- 130, 130 psi.
4- 122, 125 psi.
5- 122, 126 psi.
6 - 100,100 psi.

Much lower and with more variance than I would have expected. Each cylinder is holding pressure for at least 15 seconds. Has anyone encountered this before?

Prior to rebuild, the combustion chamber and piston tops were significantly caked in carbon. Compression readings were:

1 - 155 psi
2 - 140 psi.
3 - 155 psi.
4 - 158 psi.
5 - 155 psi.
6 - 150 psi.

Strange .... any ideas?

Steve
Click to expand...

Put the carbon back in?

Sorry, couldn't help myself. My BJ8's compression readings are similar to your original readings--with rebuilt head--but it seems to run fine. Assuming head work was done right it's gotta be bottom end issues--have you tried a leakdown test?
 
Do you mean your BJ8 is about 150 psi per cylinder and it runs fine?

I have not done a leakdown test as I don't have the equipment. If the issue is rings or a bad head gasket, why would the pressure not fall off quickly? I am watching mine for about 15 seconds and there is very little loss. Further I would wonder why I wouldn't have had lower compression prior to the head work if the bottom end was leaky. When I had the head off, I inspected the cylinder walls and all were excellent: smooth with no scoring.

Does anyone know how long a cylinder should hold pressure with no visible loss of pressure on the gauge?
 
Off the top of my head I believe that 125 to 130 psi readings are in the right ballpark for good strong Healey engine. Higher or lower compression engines will obviously have slightly different readings. The variance in readings is astounding given the relative consistency with the first set of readings before the valve job. Try it again, but this time quirt some oil into the low reading cylinders to make sure the worn rings have less effect on your readings. Also, your readings cannot hold steady for 15 seconds...it doesn't work that way. Compression testers record the highest pressure attained, and then you push the release button. That's why a leak-down test is the best way to analyze the condition of an engine.... the leak down will show you the condition of the rings (and valves...but your valves are new). Another thought...although this is a stretch.... did your reputable local shop install hard seats and somehow dramaticaly change the volume of the cylinder head in certain cylinders? Check the height of all the valves when each is fully seated, that will answer that question. Can't wait to hear your results.
Richard
 
SteveHall64Healey said:
Do you mean your BJ8 is about 150 psi per cylinder and it runs fine? ...
Click to expand...

Sorry, I wasn't clear; my engine gets about the same readings you got before you had your head rebuilt--the lower readings--and that was after I had my head rebuilt. For contrast, our 100M engine is giving 170-175psi on all four.
 
Hi Steve and I can appreciate your frustration,

Have you replaced your piston rings? I would suggest following Bob's and Rich's suggestion and doing a Leak Down Test to identify the condition of your rings.

A while back a friend pulled his head for rebuilding and decide to also pull the pistons. He found that the compression ring of 4 of his pistons were missing. Having purchased his '67 Healey in the mid '70s, he had always had difficulty getting a good idle and initial start off the line and often wondered if the PO had started an engine job and decided to sell when realizing the extent of the effort. After asking the Forum if anyone had experienced the disappearance of the top most ring, I learned that a number had opened their engines to find the disappearance of the top ring along with the securing ridge lip and found the cylinder walls with no mars or marks.

So where did the rings go. A few suggested the rings and upper lip of the retaining groove broke apart and were pulverized and blown through the exhaust valve. Others suggested they were never present. I tend to believe that the rings were initially present and you may have lost them long ago. With your head rebuild, I would expect that you also cleaned the carbon from the top of the pistons before installing the head and inadvertently eliminated the carbon acting as a replacement seal for the lost rings.

As mentioned, the leak down test suggested by Bob and Rich would give you some indication of your ring condition.

Yes, this is a wild thought but so is the unexpected results of your head rebuild.

Good luck,
Ray (64BJ8P1)
 
Steve:

I would put the engine back together and drive for at least 100 miles before testing again. During the rebuild, your engine sat probably for weeks with the head off. You also scraped off carbon that will lower compression and possibly let some debris into the bore. No reason for all the cylinders would be in the same state of lubrication. Not sure what the effect of testing with the carbs or manifolds off, but that could also impact the readings. I'll bet the readings smooth out after driving it a bit.

They say the absolute readings vary a lot due to temperature, altitude, etc, the difference between cylinders is the important thing. I think that except for 1 and 6, your readings look reasonable to me. I might add that my Healey four has compression readings that vary quite a bit more cylinder to cylinder than yours yet it runs rather well. I solved that problem by reading the compression less and driving it more! Seriously, mine has various cracks in the head that cause erosion of the valve seats so it tends to need valve jobs more often than usual.
 
Thanks for the advice everyone.

I tried injecting several pumps of oil into number 2. After allowing 15 minutes, compression went to 125 psi, up from 100 psi. and this reading was reproduced several times.

I've ordered a leak down tester and will report results by the end of the week.

The rebuilder did replace the exhaust seats with hardened inserts but they are indistinguishable from the original, and there appears (visually) to be no increase in the combustion chamber volume. When the head was off, the height of all of the valve springs with the valves fully seated (ie not in contact with the rockers) were identical by eye.

Thanks
steve
 
Last edited:
Steve--

This usually indicates bad ring seal but you will be able to verify this when you get your leakdown tester.
 
First, I noticed some socks started missing. Some say they get chewed up by the dryer drum and motor. NOW, rings are missing?
 
No Richard, the missing socks become transformed into additional hangars in your closet.
 
When you cleaned up the pistons did you leave about 1/4 inch around the edges untouched or did you clean them right off? - I was brought up on leaving a margin untouched to help keep the seal.

:cheers:

Bob
 
Bob Hughes said:
When you cleaned up the pistons did you leave about 1/4 inch around the edges untouched or did you clean them right off? - I was brought up on leaving a margin untouched to help keep the seal.

:cheers:

Bob
Click to expand...

I cleaned them completely Bob, and now I think I understand why you were 'raised' that way ....

steve
 
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