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A quick search of this forum hasn't turned up much discussion on this specific subject, so I thought I'd start one.
I need to replace both pistons (~3" dia.) on a locomotive I'm helping rebuild. Another locomotive in the club is having its pistons replaced as well, for the second time now. The gentlemen involved in that other project are steadfast proponents of using aluminum for the pistons, in spite of those aluminum pistons having siezed and scored the cylinders once already. (For whatever reason, lubricator failure, improper clearances, ect. - I've never been sure of, or told for sure what happened) Its been suggested that I do the same for the locomotive I am working on. I'm not so easily sold.
I don't see (and haven't been told by these gentlemen) any real benefit to aluminum aside from it being a softer material than the cylinder wall, in the event of piston ring failure, ect. I know bronze pistons are not uncommon either. Since when in operation, and when properly built, the piston rides on the piston rings, and never touches the cylinder wall, the piston material shouldn't mater too much, am I right in that thinking?
Has anyone here decided to use pistons made from aluminum or another metal? And if so, what was your reasoning for that material selection?
I'm of the opinion that if it isn't broke, don't fix it. Aluminum pistons just seems to be the latest fad of reinventing the wheel. A properly constructed piston of more traditional cast iron or steel with correct clearances and rings should preform just as well, as time has tested and shown in numerous locomotives. But, I'd at least like to investigate the idea before I commit to building them.
Thanks for everyone's input and advice.
-Sam
Boilermaker, Pipefitter, former Railroader
"Preserving the Past and Ensuring Our Future"
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Dished vs Flat Top Pistons
What are flat top pistons? The simple answer is a piston with a flat combustion face or crown. You will find flat top piston with valve relief options and without depending upon the engine application. A flat top piston will usually provide the most even ignition for the best combustion efficiency. With the smallest surface area of all piston types, a flat top piston creates more force and the most even flame distribution, as there are little to no obstructions on the face of the piston. On small combustion chambers, however, the flat top piston can create too much compression. The dished piston, on the other hand, has a lowered piston surface (looking like a pie plate if you will) and provides for a lower compression value with all other engine aspects being the same. Usually used in a boosted application, such as a turbo or blower setup, dished pistons vs flat top pistons are preferred where a high-lift camshaft or a high compression ratio is not required.
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