Experience Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.
Organizations Associate member SAE
EAA member
Publications Import Car magazine
Education/Credentials ASE Master Auto with L-1 certification up to 2000
I've spoken to you before about my 75 Midget with a 78 1500 engine, I rebuilt myself.
For the longest time, I thought I installed the rings incorrectly because I keep getting excessive crankcase pressure (oil blowing out the dipstick). Compression seems good so I started to think of other possibilities. I now believe this is a valve guide issue.
I base this on the following;
- good compression
- new Weber carb with Pierce manifold kit.
- Exhaust header installed, but I did NOT upgrade the factory exhaust (part of problem?)
- Cam was reground to "performance" when engine rebuilt.
- I did not check the valve guides when I rebuilt.
- Have oil build-up on spark plugs 3 and 4 cylinders, (minimal on 1 and 2 cylinders.)
- Thick wet looking oil residue on top side of all intake valves.
Do I need new valve guides?
Do you think installing valve seals will solve my problem?
I'm pretty sure I should install a new exhaust system too, what do you think?
Colin
Answer Hi Colin,
Triumphs don't have valve seals plus even if you had worn guides it would not cause as much trouble as you have.
You said you have good compression but you didn't tell me what the readings were and what the wet test was. The dry test first then do a wet test. The wet test should not be any more than 10% to 15% higher than the dry test. It is the difference between the two tests that tells if it is a ring problem not the dry compression test.
An engine could have 135 PSI on a dry test and not smoke nor build crankcase pressure.
A second engine could have 140 PSI on a dry test and smoke and blow out all the seals and the dipstick.
Then do a wet test on each and see the first one with 135 PSI had a wet test of 138 PSI but the second one with 140 PSI dry would get a 180 PSI on a wet test and has failed and will have bad rings.
When you reinspected your rings, did you note if the second ring down was possibly up side down? Most second ring down are oil scraper rings and must not be installed up side down. Most top rings are not scraper rings but I have run across a few that were.
Run a dry test again (engine hot) and right after put about a spoon full (about 4 or 5 squirts from a squirt gun) of engine oil in the plug hole and run a second test right away. This Wet test tells the tail.
Tell me what the readings are. If they are less than 15% higher than the dry test I would bet some compression rings are up side down.
Valve stem clearance can not build crank case pressure because on the intake stroke it has a vacuum on it not pressure and when it is closed it only has outside air available to it. It never has combustion pressure on it. The exhaust guide has exhaust gas pressure applied only when the valve is open and that pressure never exceeds 1 and a half PSI and only just as the valve opens for as it is closing there is a vacuum applied to it, so not even an exhaust valve can cause crank case pressure. Bad guides cane burn oil but that is on the intake stroke when vacuum is applied to the bad guide which pulls excess oil from the crank case so if anything it would cause the dipstick to be forced inward not out.
My guess that you installed some compression rings up side down or you have a stopped up crank case vent system. I found that most engines are not critical on ring end gap as I have pulled down many engines that did not burn oil and they had a ring end gap of a quarter inch.
Let me know,
Howard
