CU engine Rebuild / Piston rings

[Rockhead_Rumple]

Bit of a decision on my hands at the moment...but first a bit of background:

Around a year ago I brought a van off Merc trader with a partially seized CU engine (only rotate part way) as a bit of a learning project. I dropped the engine, and stripped it down to the stage of heads off. The engine had only covered 80k, and heads looked good. Managed to un-seize the engine which seemed to just be a sticking piston. Never took it any further once un-seized, and didn't remove the barrels from the pistons (as it was a learning experience and I was a bit scared!). What I think had happened was that one of the barrel gaskets had gone, leading to the engine peeing oil. Instead of fixing it, the previous owners had just kept putting more oil in, eventually parking it up for a couple of years. When they came to start it again it had partially seized due to low/crappy oil(my theory).
Anyway I put it all back together and after many trials and tribulations to my surprise the engine ran, well enough in fact to cover the last 3000 miles.

A year on I took the van to a rolling road for a carb tune, and the knowledgeable beardy bloke told me the engine is breathing heavy and down on power (evident by a fair bit of oil from crankcase breather) and he guessed the engine had been slightly seized in past without me telling him. He reckoned the piston rings were worn and allowing pressure into the crankcase. It also happens this was at the same time I was considering LPG and he said not to bother before the engine is sorted as it will just lead to its demise quicker.

At this point I am wishing I had changed the piston rings to start with, and I have priced up new rings and a gasket set for the engine which I can just about justify (£100 ish). However never having changed piston rings before, can I get away with just changing the existing ones for new ones? I am more than happy to strip the engine down and change them, but I am worried that I might do more bad than good!

Im still learning so advice appreciated!

[Rockhead_Rumple]

Did a compression test today, with warmed engine and all plugs out. Repeated it three times to get good average. Results:

cylinder 3 = left front, cylinder 4 = left rear, cylinder 2 = right front

Cylinder 3: 8.2 bar ~ 122 psi
Cylinder 4: 9.8 bar ~ 140 psi
Cylinder 2: 9.7 bar ~ 138 psi

Did a further wet/drop of oil test on cylinder 3:

Cylinder 3 Wet: 9.9 bar ~ 141 psi

However I was unable to get the tester to catch on the cylinder 1 thread. No matter what angle or direction the damm thing wouldn't tighten up. Obviously I didn't want to force it. What makes it even wierder is that all 4 plugs are identical and they have no problem screwing into cylinder 1. There is nothing blocking the tester (as far as I can see). I am going to try some PTFE tape on the thread of the tester.

Anyway regardless of cylinder one, is the 1.7 bar difference in cylinder 3 enough to warrant new rings? (I bet cylinder 1 is similar if not worse...)

[horizontal kipper]

Hope you dont mind me adding to your post as it looks like we after the same answers, changing the rings on my cu motor over easter, but after talking to stateside tunning at bug freeze, not sure whether to go for total seal rings (less blow by) or standard rings. Mine runs lpg. Will the barrels need to be honned?

[VWCamperfan]

Barrels will always need to be honed with new rings. I would fit standard rings. Total seal rings won't 'total seal' for long when they begin to wear a bit!

[horizontal kipper]

Thanks mark.

[VWCamperfan]

This honing thing... In America they leave the barrels outside for a while to develop a thin layer of rust on the inside. Does the same job as honing!
Just can't decide if I would do it but I can't see why not!

[Rockhead_Rumple]

How much is barrel honing typically?

Realistically if the rings do need replacing would I be better off paying £230 for some new pistons and barrels and be done with it?

Also anyone got thoughts on my cylinder compressions?

[kevtherev]

How much is barrel honing typically?

Realistically if the rings do need replacing would I be better off paying £230 for some new pistons and barrels and be done with it?

Also anyone got thoughts on my cylinder compressions?

a honing set like this...



costs £15
and is used with an air drill or electric

you could sell it after wards even
but if money is no object then replacing new would be my choice

[steve8090]

And do not run those rings in on LPG, you will need to do at least 500 miles on petrol to get the best seal possible and longevity from your new rings

[horizontal kipper]

And do not run those rings in on LPG, you will need to do at least 500 miles on petrol to get the best seal possible and longevity from your new rings

Thanks steve, was going to run it on just petrol till i can get it back up to you for a gas service.

[Rockhead_Rumple]

Just tried to PTFE tape on the compression tester to get it to screw into cylinder one, but still no luck. Its peeing me off, why the hell wouldn't it fit into just one cylinder when all the plugs are identical??

E D I T: Also Kev, you have got me thinking now but slightly scared. I thought honing was a precision job, can I really do a proper job at home with the barrels in a vice and a suitable powertool?

[camper]

Honing your barrels shouldnt be a problem .It can be done at home using a drill stand & in the old days when fitting new rings into bores we used to just bust the glaze with fine emery cloth.

[Rockhead_Rumple]

So just for final clarification:

If I decide to change rings, I can buy standard rings, hone the existing old barrels with a tool, and fit the new rings without issue? ie the tolerances in the new rings will take up the slack from 80k miles of barrel wear?

[Damien]

Maybe check the wear, normally can do this by feeling for a lip at top of bore where the black ring is, this will show the original size compared to the worn size of barrels. If thats not like a cliff edge you should be alright with new rings and a honing.