[Bmcu] TR3 compression test

[match at ece.utah.edu]

On 22 Jan 2008 at 23:25, Jeremiah Curry wrote:

> howdy all,
> 
> I just put the head back on my TR3 the other day and wanted to make sure I
> did everything right, and that the parts were all good.  We did a leak down
> test and 2 of the cylinders where "yellow" and two were "green".  To be
> doubly sure, I borrowed a compression tester from autozone and did that
> test.  My starter has decided not to cooperate since then, so I had to turn
> the engine over with the hand-crank.  Due the manual nature, I only went
> through about 2-3 compression cycles, but the PSI rating for the cylenders
> were from 30-60 psi.  I understand I should be looking for 120 psi.  Is this
> an indication that something is horribly wrong?

How long has this engine been sitting? I ask because the rings need a tiny 
splash of oil to seal, and if the engine has sat for some months the oil has all 
drained away, the rings are dry, and they're not sealing much. You could 
squirt a little oil in each cylinder and run the compression test again. Note 
that this will give you an artificially high reading, but since you're only 
concerned about confirming that you assembled the engine correctly, this 
would get you a little peace-of-mind. 

Also, the numbers we toss around (120 psi) are somewhat relative, and are 
what you'd expect to see when the starter spins the engine with no spark 
plugs in. If you're not spinning the engine at least this fast by hand, the 
compression readings will be low.

The fact that you see any compression at all tells me that the engine will 
probably run. I'd concentrate on that, then after you've run it up to 
temperature and maybe taken it around the block once, do the compression 
test again.

Marvin