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Compression Test Results

66 Sat

66 Sat

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I did a compression test today on my poly. Nothing much wrong with the engine in terms of driving, runs fine, but one plug at the back of the engine always fouls up, crusty oil deposits (cylinder 7). It's done it for ages, I just take the plug out and clean it up periodically and put it back in. I've tried a colder plug (NGK 6 instead of 5), made no difference. Test was done with engine warm. It's 10.8:1 compression, forged pistons.
Anyway back to the results.
Compression Test 23-4-2025.jpg


So everything looked good but the last cylinder I checked was the one I was concerned with. It had the highest reading which I'm thinking is due to oil leaking in there, but I'm unsure. Would this be a sign of an valve stem seal problem on this cylinder?

A couple of times when pulling this plug there has been oil on the shell of the plug - I thought it might have run down from the back of the valve cover but it seems dry there. Could it wick up the threads internally and pass the gasket, or maybe getted dragged onto there as I'm pulling the plug out?

Anyway, any advice appreciated.

20240205_152432.jpg
 
You sure it is firing? Easy test idle it and pull the plug wire and see if the RPM drops.
 
I’d look at the intake being sealed up and valve seals. Always a slight possibility of an oil ring issue but unlikely.
Doug
 
Other than the colder one, did you ever change the plug?

Yes intake gasket leak.

Yes hotter plug.
 
Ok sounds like it's nothing too serious. I'll start with the valve seals and see how I go. I've got a compressor, just need to buy a fitting and a valve spring compressor. I watched a few videos on youtube last night, seems straightforward.to change them.

I've thought about the intake before as it's hard to get the composite gaskets for the poly, most sell the steel shim type which are currently in there and they don't seal as well.

I found some composite ones a few months back and bought them so they are sitting in the garage already if the valve seals don't fix the issue.

The plugs have been changed a few times over the last 4 years, and I've swapped that plug around with others to see if it was an issue with the plug itself. I remember I went to a colder plug to see what effect it had on detonation (not much) - there was no more or less fouling with the 5 or the 6 heat range.

I've checked inside the distributor cap as well, all looked OK.

Thanks for all the replies everyone, much appreciated.
 
On some of the old polys you had to pull the rocker shaft out the end of the head I believe.. that true?
 
The seals are new as are the guides, valves, pistons, crank, rods etc (4 years old). I'm assuming a defective part.

Rockers slide to the side once the adjuster is backed off, the shaft is held in by the head bolts so that stays in place.

20240115_152642.jpg
 
You need a hotter plug, NGK 4 [ You cannot get a BP4S, only BP 4]. Colder plug will build up deposits quicker, will not burn off. I have been helping a mate with a 403 Olds, he got some last week. I use 4s in my engine & I also 4s in a drag race engine that I tune, it ran 119 mph.
You have to think like the manufacturer, who has to provide a warranty that covers both ends of the spectrum:
- grandma just drives to church on Sunday
- family with boot full of luggage & towing a trailer in the high summer heat.

These cars today get babied/pampered by loving enthusiasts....
 
66 Sat said:
The seals are new as are the guides, valves, pistons, crank, rods etc (4 years old). I'm assuming a defective part.

Rockers slide to the side once the adjuster is backed off, the shaft is held in by the head bolts so that stays in place.

View attachment 1841621
Click to expand...
It looks like the rocker shaft hold downs are cast into the heads , is that the case?
 
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