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Originally Posted by eric32
Hello guys I am having a hard time trying decide what piston and rod combo to get. I am looking at a set of Scat forged 5.7 connecting rods and hypereutectic pistons.
The scat rods I was looking at was the Procomp 5.7 rods that are polished with 7/16 rod bolts but are for floating type pistons and they run for around $300.00. Keith black has a set of hyper claimer type pistons that would work that can be floating or pressed fit type but they run around $180 to $190 depending on where I can find them. But summit has some hypereuctic pistons but are only pressed fit for about $90 bucks. I cant seem to find a floating type hypereuctic piston for around that price.
I could save a little and go with the other Scat forged rods with 3/8 capscrew bolts but are not polished just shot peened and are pressed fit type and are around $230 wich is $80 dollars cheaper.
Are the polished type rods that much better then the non polished type.I have the Scat 5.7 4340 forged rods with the 3/8 capscrew bolts in my other 350 build and they have held up great and the motor has seen 7000 rpm several times.
I know the other rods are a little bit stronger and are polished but for the price floating type pistons I have to pay almost twice as much for a good set of pressed fit type pistons. What would be the best combo to get and are floating type pistons safe to use on the street. I have heard that sometimes they can come apart in the bore and destroy your block. Is this a common problem with them or not?
Thanks
Eric
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Floating defines whether the pin can rotate in the rod, where a pressed pin can't. It doesn't have anything to do with the piston, other than the piston may not have retention device grooves machined at the end of the pin bore for a pressed pin type installation, where this would be required for a floating pin.
For competition engines, I prefer a floating pin simply because it provides and additional point of articulation. Articulation could be lost in a competition engine where the piston is overheated and seizes the pin, in which case a pin pressed in the rod would result in a frozen condition where physical forces will make something either articulate or bust in the attempt.
All pistons are floating in that the pin fit allows the piston to rotate about the pin. However some pistons made for pressed pins don't include the outer groove for the pin retaining clip that is required for a floating pin. This can be overcome with Teflon plugs to locate the pin with it's bore of the piston.
Rods determine whether the pin floats or is pressed. Pressed pins do not require a means of retention in the piston as sighted above. Floating pins are preferred to have a bushing in the rod's pin bore, though several racers run steel against steel in an effort to keep from removing rod material on the small end to make space for the bushing. There doesn't seem to be unusual problems with this from a steel against steel basis probably because while the loads are high the rocking motion doesn't generate enough friction to weld the similar materials together. However, the new approach is the use of Casidiam coated pins which pretty much eliminates the chance of galling from over temp and under lubed situations. The reason for running pins directly against the rod material is that most Chevy rods are designed for pressed pins and are not sized on the small end to provide extra material needed to maintain strength when the pin bore is increased to accept a bushing. So to retain strength, the small bore end is only opened up enough to allow the pin rotational clearance, thus retaining most of the original strength of the rod's small end.
Floating pins on the street are not necessary, the engine is unlikely to see the high heat loads for extended times that are the reason for adding the insurance of floating pins. Same can be said for polishing rods, it just isn't necessary for a street engine. But if done the process needs to be followed up with professional shot peening. The reason is that the forging process leaves and compression layer on the surface that helps contain the stresses in the part. Polishing removes this layer and makes the part subject to stress induced fracture unless the compressive layer is replaced. Shot peening replaces this layer and adds greatly to part's fatigue strength, which for rods that are constantly switching between compression and tension loads is very important.
Bogie
01-30-2009, 02:27 PM
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