Piston rings

[Thowdfella]

My car is a 1966 122s and I have been concerned because compression on all cylinders has been measured at 125 psi on a hot engine. So, I took the head off, couldn't see much wrong but had hardened valve seats, new valve guides and new exhaust valves fitted and vacuum test showed it was very good.
Checked the compression again - this time on a cold engine. - and this time they were all 100psi.
I can only presume the problem is now with the piston rings as the bores looked smooth with no lip at the top of the bores.
Is it possible to take out the pistons with the engine in situ or does it have to come out? I remember doing an Austin 10 from underneath!
Any thoughts would be appreciated!

[Derek UK]

Have you done a comp test after putt a few squirts of oil down through the plug holes? Try and put the same amount down each one. Wide open throttle for the tests. (Take the coil wire off for safety)
Pistons out through the top I'm afraid. No lip is good but it's desirable to be able to see honing cross hatching. If there are no signs of that the bore maybe glazed and not allowing the rings to seal properly.

[Thowdfella]

I have now done thecompression test after squirting oil in the cylinders and it is still coming out at 100-105 psi so not making any difference. Don't understand why it is so low as the car appears to be running well, pulling well and ticks over well. When I looked in the bores when the head was off the cylinders looked smooth but couldn't see any cross hatching marks. I also tried three different compression gauges! Any thoughts

[c1800]

Is that 100-105 warm? Did you test compression with all plugs out at once, and throttle held wide open while cranking?

Smooth bores might indicate wear, as no cross hatching present. Bores could be glazed, tapered and out of round from wear. AFAIK the only way to check is disassembly and checking bores with the proper tools/gauges.

I don’t think you can disassemble in situ, difficult to just get the oil pan off in situ. Probably could if you dropped the cross member and supported the engine from above, and that’s a lot of extra work. But others may have further advice.

Taking the engine out, disassembly, cleaning, boring etc is the proper way to do it for a long lasting rebuild.

On the other hand if you’re happy with the current performance, you could just keep driving it.

[Thowdfella]

The latest test was done on cold engine, all plugs out, and foot to the floor. I think you are right about the bore as I have this morning gone through all the many documents I have about the car from new and the bore has never been touched throughout its 103000 miles. So it's engine out I think and take it from there. Although it's very usable, a compression drop of that amount bothers me so it needs to be fixed.

[Underdrive]

If the compression figures are still fairly equal, the difference will be something in common with all four cylinders associated with the work done to the head that has lowered the compression ratio. Do you know if the head gasket was the same thickness as before?

[Clan]

My car is a 1966 122s and I have been concerned because compression on all cylinders has been measured at 125 psi on a hot engine. So, I took the head off, couldn't see much wrong but had hardened valve seats, new valve guides and new exhaust valves fitted and vacuum test showed it was very good.
Checked the compression again - this time on a cold engine. - and this time they were all 100psi.
I can only presume the problem is now with the piston rings as the bores looked smooth with no lip at the top of the bores.
Is it possible to take out the pistons with the engine in situ or does it have to come out? I remember doing an Austin 10 from underneath!
Any thoughts would be appreciated!

It might interest you that the B21 engine in the 240 volvo compression test figures state Minimum 129 psi .

I hope you sleep better tonight , don't worry about it .. You could get the car tested on a rolling road for about £50 to determine the power it is producing .. It might well be more than specified , as volvo engines always seem to go better the older they get :-)

[Derek UK]

It's certainly rare that there is only 5psi difference between cylinders at this mileage. 10% difference is acceptable. If there is blow by at the rings it will be a bit fumy when you run it with the oil filler cap off. It will be noticeable and if you put your hand over the hole you shouldn't feel more than a very minor pulse. The head job with new hardened seats has included guides and seals so that would mean that very little oil should get into the combustion chambers and cause exhaust smoke.
As mentioned, dropping the rods and pistons means removing the pan and that gets complicated. It may seem a major task but much easier to remove the engine and work on it with an engine stand. Actually quicker and less frustrating than working partially blind under the car.