Compression confusion

[Luckyone]

I’ve finally dragged the AJP out of the back of the garage to have a good check over it.
Before I took the Cerb off the road for a quick freshen up that turned into a complete body off rebuild the engine was running great, always has in fact. Has a full history from Mole Valley then AMP Andy (I know cos I took there myself!).

However as I had the body & exhaust manifolds off with the engine still sat in the chassis it seemed rude not to give it a quick compression test before taking it out. Rather worryingly I got a difference of 2bar between the cylinders, seemed a bit much so I checked it a couple more times but got the same results for each cylinder, so a bit miffed I chucked the engine in the back of the garage & tried to forget about it.

That was two years ago, today I did a leak down test on it but the results of that seemed quite good with the readings between 11% & 16% over all cylinders. The leak down tester instructions say anything more than a 15% difference is bad, so I’m well inside that.

Maybe stating the obvious but the engine was unavoidably cold for both tests, not ideal but I can’t see any way round that.

I did try to poke a 3thou feeler gauge under one of the inlet cam lobes but it wouldn’t go, so it’s safe to say the shims need changing (I’m fairly sure it was due a 12k service).

So I’m wondering if it’s worth setting the valve clearances then doing another compression test. I hoping that maybe the tight inlet valves were holding the valves open longer resulting a lower compression on some?


On a side note – how do the cam & followers (or is it buckets?) get oiled with no obvious oil spray bars or anything?

[ukkid35]

Luckyone said:

That was two years ago, today I did a leak down test on it but the results of that seemed quite good with the readings between 11% & 16% over all cylinders. The leak down tester instructions say anything more than a 15% difference is bad, so I’m well inside that.

...

On a side note – how do the cam & followers (or is it buckets?) get oiled with no obvious oil spray bars or anything?

Mine was about 17% variation, AFAIR, and that was after setting value clearances. Once Joolz had done his magic the car made very good power indeed, so I wouldn't worry unless you want a Red Rose beater.

From memory, the followers are continually bathed in oil, rather than sprayed, as the drains are quite high.

[Luckyone]

ukkid35 said:

Mine was about 17% variation, AFAIR, and that was after setting value clearances. Once Joolz had done his magic the car made very good power indeed, so I wouldn't worry unless you want a Red Rose beater.

From memory, the followers are continually bathed in oil, rather than sprayed, as the drains are quite high.

I was very happy with the leak down results, it's just the fact I got a two bar difference between readings on the compression test that bothers me.

I assume a compression test is a better overall test as you are testing the whole cylinder bore, with the leak down being the next step to find were the problems highlighted form the compression test are?

So I guess the leak down test is telling me there is no real problem (at least a the top of the stroke) & something else is causing the different compression readings? I’m assuming valve clearances & possibly carbon build up?

[ukkid35]

Luckyone said:

So I guess the leak down test is telling me there is no real problem (at least a the top of the stroke) & something else is causing the different compression readings? I’m assuming valve clearances & possibly carbon build up?

Or perhaps cam timing, or corrosion in the bores.

Edited by ukkid35 on Sunday 26th May 10:09