Piston Rings

[Gardner]

I've just recommissioned a G12 engine. (1960 alternator). The bores and pistons looked OK except there was a broken oil ring on one piston. I purchased a set of oil control rings for both pistons. The original rings were the double type (two rings in the same groove). The replacement oil control rings were the single type.

Ring gaps were: .006" on the new oil rings and .008" to .009" on the existing compression rings. The ridge at the top of the bore was barely detectable. The bores are at + .060"

On starting I noticed a smoky and oily exhaust. Having covered about 50 mls the exhaust is very smoky and black oil is being blown from the exhaust pipe / cylinder head joint.

I've a Siamese exhaust but I'm pretty certain that both cylinders are problematic.

Ideas please.

[1608]

IMHO 6 thou is a bit tight for the oil ring. If it was me I would open the gap up to the same as the old oil rings at least
and ensure the oil rings are the right way round ie chamfer up if there is one. See what others say though.

[Groily]

I agree 6 thou is a fraction tight, but not I think likely on its own to be the cause of the oiliness (unless they've broken already). Could it be that the new oil rings aren't a snug fit in the grooves - in terms of up-and-down clearance? If there is play, the rings will act as pumps and shove oil up past the compression rings and out into the exhaust.
Many years ago I foolishly didn't check the dimensions of some replacement rings on my Mod 20 (didn't know there were variants to be honest, callow youth that I was in the 1970s!) and had exactly the same problem.
Just a thought . . .

[Gardner]

Thanks for your observations. I'm coming round to the conclusion I should dismantle and investigate further.
The new rings were not marked top or bottom (the first I'd seen like that). I assumed these oil rings were symmetrical.
I was wondering if there was any incompatibility between the pistons and one piece oil rings. The pistons were initially supplied and fitted with the double oil ring spec.

[Stan Palmer]

I had a problem once when the gap behind the rings was excessive and caused smoking. Pattern rings in a standard piston. I guess there must have been some bore wear to cause the pumping effect. A surprising plume of smoke.
Stan

[g5wqian]

the engine will smoke for a bit before the rings are bedded in , perhaps give it a few hundred miles and see how it is .
use a N5c plug as it will run hotter and is better suited to the engine , i use such on my ajs mod30 .

only thing i would check is your head gasket , i had issues on mine using solid copper gasket which was too thin and realistically i had the barrel spigot forced against head and that was the only thing sealing it .

i use the clubs composite head gaskets which measure around 1.7mm thick and they are fine .

ian

[1608]

Unfortunately I was supplied composite rings in a set from the club and they were too thin and copper ones. The response I got was "they are standard"!! I had to source thicker ones from elsewhere!!