Originally Posted by NC Cams  Manley, Childs and Albert (C&A) and Bill Miller Engineering (BME) ALL used 7075 for car engine con rods for YEARS. Some of them still are and some have moved onto a new 70 series trick "aircraft" alloy.
The dragster engines did/do NOT annodize the rod and they do NOT run pin bushings (steel pins run right on the aluminum forging). They DO run con rod bearings.
Briggs and Stratton made their conrods for their lawn mower engines from die cast material and those ran forever and they used NEITHER a con rod bearing nor a pin bushing.
When it comes to making a rod for an engine, it is the stress levels in concert with the material choice that determines if a particular comination will work/live or not.
As annodizing is effectively aluminum oxide and Al O2 is an abrasive, I'd be disinclined to use it for a bearing surface unless you can superfinish it in some way.
Regarding surface finish: The BME rods are beautifully polished forgings free of any machine marks in the beams. The Manley's and C&A's are fully machined as I recall. The old Howard's con con rods were aluminum (I dunno the material) but they were definitely shot peeded.
QUITE a few people have cut con rods out of aircraft quality plate for drag race engines for YEARS. There is even a go cart rod maker who's making them out of plate stock.
Might want to do an "aluminum con rod" search on Google to see what else is being done by the pros.... |
What about 2618? They make most aftermarket pistons out of this material, and I know it is actually more dimensionally stable at high temperatures than a 7000 series metal would be.
I would be hesitant to make a rod out of plate in a high stress application, as you can't really be sure what the grain structure is. Maybe if it is a forged plate?
The current set of rods I have are a 7000 series aluminum made from extruded bar stock, in which the grain of the metal is in the direction the rod is being stressed (lengthwise). I personally like this better than a forged rod, since the grain runs perpendicular to the direction of the forces in the rod. The down side however is that when machining the profile of a rod made of extruded bar stock, you will have exposed grain boundaries, creating possible stress risers. This can be avoided by shot peening and/or polishing, but nevertheless interesting food for thought. A forged rod will likely have less exposed grain boundary ends, fwiw. BME's are obviously working for the fuel cars, and my rods are obviously working for my alcohol car.