I’m currently rebuilding a fairly low mileage S2 4.2 engine. The liners and pistons show negligible wear but show some marking on the liners where the engine stood still for some time. The ring gaps in the middle of the liner are about 25thou (should be 15), so I’m having the block very lightly honed and will fit new rings. The question is which are the best piston rings to use? I’ve seen some comments regarding not using chromed top rings as they require a specific honing process, but the costs of a set of rings from the usual suppliers seem to vary from about GBP65 to over 140. Any and all opinions will be much appreciated.
I favour moly-faced rings. I’ve seen advice that chrome rings have a longer break-in, which has associated risks. Certainly the chrome rings that I’ve just replaced in mine (an SIII engine) weren’t broken-in, with the original bore finish nicely glazed over.
Your OE pistons might call for a radially narrower ring than aftermarket replacements that can have deeper ring grooves. Not sure if this affects availability (Jag OE only?).
Thanks, difficult to find Moly faced rings in the UK, so I’ll go with the plain rings, and thanks for the tip regarding ring width/depth I need to do some careful measuring!
I personally would stay with chrome faced first ring and handle the brake in process. Moly faced are sensitive to any ign problems and could brake out/off because of detonations … Chrome is handling any “abuse” better.
If you still prefer Moly, please let me know your necessary dimension. Perhaps I have any suitable in stock.
best regards
Thomas
When fitting aftermarket piston rings to old pistons it’s absolutely essential that you check the back clearance of the rings and that it’s within spec. Pistons are usually cam ground - that is they are slightly oval, the smallest diameter being through the bosses in the same direction as the wrist pin. There is more material and more expansion here, and the piston becomes round when it heats up. The grooves however are machined round and then become oval when the piston heats up. This means that the back gap gets smaller through the bosses. Stick the new rings into the grooves over the bosses and measure how much the ring is depressed in the groove. That’s the back clearance you are most concerned with. If the groove expands into the ring it forces the ring into the bore with nasty consequences.
I.m amazed to see Cords are still making piston rings…my father who is long gone swore by them in his cars which dated from the thirties, I fitted a set to a Mk2 3.8 back in 1968 in which the bores had a fair degree of wear, didn’t get a chance to evaluate them long term as I wrote the car off before the run in period, might be worth a look as they have been in business for eighty years, they must be doing something right …
Big THANKS to Thomas and Terry,this is why this site is so valuable, for advice and reassurance when working on our cars.
Something I hadn’t realised or read before was whilst I was stripping and cleaning the engine was that the engine number appears throughout the engine, on the block, the crank, the timing chain cover and on 1 of the pistons, is this what matching numbers means I wonder.