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Old 02-20-2013, 11:12 AM
oldbogie oldbogie is online now
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Quote:
Originally Posted by Torque454 View Post
I am working on overhauling my engine and I see three choices of piston rings. Standard Iron rings, Moly rings, and Chrome rings.

What are the pros and cons of each? The way I understand it is that the iron rings will seat faster than moly rings, but wont last as long. I dont know anything about the chrome rings.

What are the cons of it taking longer for the moly rings to seat?

I don't know if it matters or not but the engine is a (currently) stock Ford 302 in an 87 F150. It will not exactly be a race/high performance engine but i want increased torque for pulling a trailer, and longevity. The engine uses a speed density EFI system so it will probably not see excessive changes, unless i convert to mass air.

I do plan to use a Comp Cams 31-255-5 cam and lifter kit, possibly gt40 heads (maybe even shaved some for a little extra compression), and a set of headers. Everything else will be stock (intake, pistons, bearings, oil pump, timing set, pushrods, rocker arms, rods, crank, etc)
Ring material depends a lot on machining accuracy and oiling of the engine. The simple cast gray iron ring harkens back to a day when machining was pretty loose both in axis accuracy, straightness and finish. This was also a period where bottom end oiling was excessive by design where often, if not always, the rod was drilled to put an indexed pressure oil stream onto the cylinder wall and bottom of the piston to prevent upper cylinder wear and reduce operating temperature of the piston and ring pack. At that engine life was not long espeically by todays standards. The chrome plated ring came along in the 1960's which needed much less upper cylinder lubrication but also required better machining the latter possible by the OEMs but usually not the corner auto repair shop. In this period the OEMs also eliminated pressure oiling of the cylinder wall and piston bottom, drying up the bottom end somewhat because the chrome ring did not requre near so much lubrication and cooling as simple gray cast iron. The problem with chrome rings occur mostly in rebuilds where the cylinder walls were not aligned well enough with the crankshaft while the bore was not concentric enough in roundness nor straight enough in the lenght of the bore top to bottom while the free hand of the finish hone added to these errors plus often had a finish that was too smooth so the rings had a hard, if not impossible, time seating into the wall. Bon Amy anyone? High oil consumption and blow by was the result.

Moly coatings, filled or plasma spray, provided much the same benefit of chrome in that they live with very little upper end lubrication while not micro-welding to the cylinder wall. This greatly reduces the wear rate so the engine holds better compression for a longer time. They fit well into the scheme of modern low emissions engines by allowing even less lubrication of the cylinder walls and piston bottoms and are tolerant of high operating temperatures. They are not as wall finish sensitive as the chrome ring which makes them very effective for rebuilds that come from shops with older machining equipment and/or questionable capabilities of the machinests.

The base material of the ring has changed a lot; the old cast gray iron ring with or without coatings of chrome or moly are better suited to less well machined and finished cylinder walls with pistons of softer alloys (low silicon) as the softer gray iron doesn't pound on the soft ring lands of these piston types with such intensity as ductile iron or steel rings will do. Ductile iron whether coated with chrome or moly is better suited to modern high silicon cast pistons like the hypereutectic or forged pistons whether low silicon 2618 or the high silicon 4032 alloys as forgings are harder and can tolerate the better ring alloys beating on the ring lands with every direction change of the strokes.

Bogie
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