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Curious about compression check results


Jedi Warrior
Out of curiosity while checking the look of my spark plugs, I decided to run a cylinder compression check on my 18GK engine. I had the head rebuilt and installed a new head gasket just under 1500 miles ago. I retorqued the head bolts not long after re-installing everything.

Test results twice per cylinder. Engine cranked without spark plugs until oil pressure gauge showed a reading. Engine was cold.

#1 145 140
#2 135 130
#3 135 135
#4 145 140

I don't suspect that anything's wrong, but am curious how these numbers stack up against what everyone else sees on their cars.

Thank you,


Great Pumpkin
Country flag
As far as compression goes, you're golden, Will

...a leakdown test is a more ~telling~ check, tho.


Jedi Warrior
I've heard of a leakdown test, but not much detail. How does one do this?

Thank you,


Jedi Hopeful
Use an "aviation" differential compression tester. This shoots compressed air into each cylinder, one at a time, while at TDC. Then check to see the pressure drop. Typically the regulator is set at 80 PSI and you are looking for even readings across the cylinders at 60PSI or above. Below 60 is un-airworthy. Roadworthy? Anyway, if you have a big drop across one or more cylinders, you can listen for the air leak to find the problem. If the hiss is in the exhaust, leaky exhaust valve. Hiss in the carb?...leaky intake valve. Hiss in the oil filler? bad rings. Bubbles in the radiator?? leaky head gasket, cracked head or block.

At home, you can use your el-cheapo compression tester hose as a substitute. Just screw the hose into the suspect cylinder while it is set at TDC and hook your air hose to it and listen or look for the leak. You won't get the leakdown numbers but you can find the problem.

Good Luck


Great Pumpkin
Country flag
Yup! That'll be the way.

As far as what you've got for compression readings, that'd say to me: "Drive the thing and enjoy it!"

You won't get much higher readings after a rebuild and you'll be spending time and money on all the "May as well" stuff too!
Don't fix whut ain't broke. :wink:


Jedi Hopeful
Out of curiosoty: did you re-adjust (or at least check for any changes) valve clearance after retorquing head? Won't affect compression, but still should be done.

While on subject of heads, it is usually recommended when replacing or reinstalling head to not only use new head studs, but also to, using a larger drill bit or tapererd grinder to relief the stud holes. I'm sure there is a better clinical term for this but over time the surface of the block will pull at the stud holes and if severe enough could interfere with gasket, resulting in an uneven fit.

I'm a fan of no-retorque head gaskets as found on most American iron but not sure if any of MG gaskets are such (maybe the non-copper?)

Here's a rundown on how to make your own leakdown tester https://www.mossmotors.com/forum/forums/thread/9148.aspx I would think an air regulator from an old thrown away air compressor would fit the bill.
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