I have a standard compression tester, i.e., pressure gauge on top of rubber hose with threaded end. The issue I'm having is securely threading the device into the spark plug hole at the bottom of the spark plug tube. I'm pretty sure it's not a tight fit since I can turn only the rubber hose and it's most likely affecting my compression readings. Does anyone have a clever trick to seat the standard tester, or alternately, is there such a thing as a threaded metal tube that fits inside the spark plug tube (and sticks out the top) to which I could attach the compression tester? Anyone? Thanks.

 

I searched for a few minutes for 14mm threaded nipples but didn't find anything long enough. Just a thought: do you really need highly accurate readings or is it enough to know if all of the cylinders' values are close?

 

Well, I'm measuring 100-110psi across the cylinders which is pretty low. The car runs really, really well and certainly seems to have the power and acceleration of a higher compression car. I dynoed at 193.5hp at the crank so I'm thinking I'm not missing compression but the low numbers are throwing me off. I believe they should be in the 140-150psi range and the lower numbers are telling me either bad rings or bad valves/seals though it doesn't seem to underperform. The threaded nipple at the bottom of the tester hose has an o-ring and I'm not sure I'm getting a proper tight seal just by hand-turning the rubber hose and that may be under-reading the compression. I don't use oil so the rings appear to be OK and the car is fast so I doubt there's valve/seal leakage. It just bothers me that I can't pin down a correct compression reading because of the silly way MINI uses that long metal tube to house the spark plugs. You would think because of the design that MINI would have a hard tool that would thread down into the spark plug tube and facilitate a compression gauge on the top but evidently not. At least I haven't been able to find one. I'm not familiar with BMWs. Do those engines also use the spark plug tubes?

 

I agree -- I'm surprised that compression tester rigid extension tubes (for lack of a better name) aren't common.

Was your compression test done with the engine warm or cold? Cold readings will be lower.

 

I did a cold test. After searching around I did find the following extension at Summit Racing that fits both 14mm and 18mm. Just ordered one.
https://www.summitracing.com/parts/ino-3619?rrec=true

 

Good find.

Here's a NAM thread that is related. The OP was getting readings in the range 102-106 with a cheap tester. He then bought a used Snap-On tester and the readings were significantly higher: https://www.northamericanmotoring.co...n-results.html