MadaboutKitcars

Hi all. I thought I'd test the compression on the M50TUB25 engine I'm fitting to my Cabrio before starting to strip the donor - on the basis it might indicate any problems that could be investigated while the engine's on a stand. The numbers are consistent but seem ridiculously high. To the nearest 5psi:

#1 : 225
#2 : 225
#3 : 230
#4 : 220
#5 : 225
#6 : 220

Max values were reached after about 6 strokes in each case. Bentley gives a minimum value range of 142-156psi.

Does this just indicate the tester I bought is poorly calibrated (or crap...)? If so then I suppose the fact the figures don't vary too much is the main thing?

I noticed a little oil had run down into the plug recesses on nos. 3, 4 & 5 cylinders - not enough to form a pool, but certainly enough to need wiping off the bottom of the plug socket. Could this be anything more serious than the cylinder head cover gasket needing replacing?

Mark.

Looks fine, it's the comparison between the 6 that's more important, if you have one way lower than the others it would be an indication of a problem. Numbers seem familiar to the one on my M20 although its been a while now

Does the M50 have those round rubber gaskets around the bolts that hold the rocker on? If so those can be a source if the rocker gasket look's OK. I'd replace the gasket anyway while you're working on it

Thanks for the reassurance, Patrick. Looks like the M50 gasket's in three parts:



I'll take a closer look when the engine's out but I guess there's poor sealing around the tubes for cylinders 3, 4 & 5. I also just read on wikipedia (yeah, yeah...) that cylinder pressure in psi can be roughly estimated at between 15 and 20 times compression ratio. 20 x 10.5 = 210 so my figures seem reasonable after all.