Hello to everyone,

I have recently purchased a P38 2001 4.6 Thor at a reduced rate due to three cylinders misfiring.
The vehicle has been LPG converted and not serviced for a few years. I tried a leak-down test on the missing cylinders and found a very large leak via the exhaust valves on all three.
I pulled off the heads and found that indeed the exhaust valves all were burnt out on the edge between 3 and 5 mm wide. I have now replaced all the exhaust valves on both heads and cleaned up and re ground the inlet valves and refitted the head with new valve stem seals all round.
After start up the miss-fire has now gone however both banks have started to show blue smoke when at idle and vast plumes on acceleration. I think this was like it when i first got the vehicle but with the miss-fire i never ran the engine for more than a few seconds so as not to damage the cat with raw fuel. With the valves now leak free i ran another leak-down test and was maybe not surprised when even at a low 10 PSI i could here leaking air.

I have now removed the entire engine as well as the rocker cams so that all valves are 100% closed and repeated leak-down test with sump removed and no surprise air is leaking past piston rings.
I have measured the ring end gap and found it to be almost twice the maximum spec. I do not want to spend lots of money fitting top hat liners with seals and having to find someone able to work the heads to fit said liners. I have a known working engine from another P38 same year size etc but would like to see if i can fix the one leaking without spending to much money, time i have as we are on lock down.

I have ordered another head gasket set and new piston rings as these were not to expensive. Today i test fitted a new ring and the end gap is now within spec so i am guessing that the actual cylinder has not worn. I know that the best next step would be to measure the cylinder for both size and roundness but do not have the tool or possible the skill as i have heard it is a little tricky. I would be interested in your thoughts on this. I could have heads skimmed this time just to rule that out but the heads appeared fine and the gasket i have just removed again was like the original one not broken and no signs of blow by, wear or oil.

If it helps the exhaust manifolds are heavily oiled all fresh and wet. I am aware that sending it off to a shop and having them check everything would be best but as i have time and some money to spare i would like to see if just rebuilding with new rings may fix the problem.

Thanks in advance

 

According to the Rave service manual, it says Not to hone the cylinders.
It also gives a piston o.d, and bore I.D of 4 decimal places.
That is bs, period.
No way, no how, is every person world wide that ever measured a bore or piston going to get the same reading.
Reason being, even if you know how to properly use a micrometer and bore gage, they won't all be calibrated the same, held the same, or have the same amount of pressure applied...so no way a .0002 tolerance is realiastic...especially on a low rpm small block, it's not an Indy car.
I used a bore gage on my rebuild, mic'd the pistons, gapped the rings, and called it good.

 

Also, do you have a ring grinder?
Rings are purposely made on the small side, and odds are you'll have to gap/grind several of them.
Always only grind one end too, never both, and always use a ring grinder, nothing else.
Ring end gap is very important in terms of engine life.