| Piston rod stretch in the 455 [message #279727] |
Fri, 12 June 2015 12:08  |
cbryan
Messages: 451
Registered: May 2012
Location: Ennis, Texas
Karma:
|
Senior Member |
|
|
After all the talk about lugging the engines being destructive to the well-being of the powerplant, resultant focus on the pistons, etc. I am still wondering about one more thing.
Note that Dick Paterson rebuilds his 455s with loose rod bearing clearances and with high volume oil pumps.
I gathered from some other sources that weak rods will go oval, egg shaped at the big end due not to compression pressures, not to detonation, but from rpm. The rod has to make sure the piston stops at TDC, obviously. It is stressed in the elongation direction there, especially at the exhaust stroke. If the rod isn't very strong at the sides of the big end, it might be possible for the sides to move toward the journal enough to cause metal to metal contact, heating, and knock, knock, knock. So, Paterson checks for ovality, and then makes sure there is a lot of area for the rod to collapse inwards before untoward noises spring forth. The Oldsmobile rods are supposedly not too strong. I do believe there are aftermarket rods that are much stronger. If we are to wind out our engines, are we not 'asking for it' with stock rods?
I write this not because I have conviction either way, only to broach another factor that may bear on the "normal" failure mode of the 455. It is rod knock. Presumably, if the oil pressure alone was the culprit, there would be an equal number of main bearing failures, and I am not positive about most of the failures being those of rod bearings. There is the problem of contaminated oil galleries, oil coolers in radiators with debris from the last fiasco still in them, oil pumps damaged and not checked, all can contribute to catastrophic failure. Lighter pistons might help with stock rods if a hedge of bets is called for.
Here's to a meaningful dialogue. Is this effect where the term "hot rod" comes from?
Carey
Carey from Ennis, Texas
78 Royale, 500 Cadillac, Rance Baxter EFI.
|
|
|
|
GMCforum
Search
Help
Register
Login
Home
