Quote:
Originally Posted by mgarblik
SD-455 pistons do not use an offset pin. Yes, they have a single complete cut for valve clearance. The rods do have a proper facing ID mark on them I am refurbishing a stock engine right now. The cold noise maker were the clunky, heavy TRW forgings, the large piston to wall clearance specified, .006"-.0068" factory spec., and the high growth 2618 alloy used to forge them. Personally, I love the sound. Like a well tuned diesel until they warm to operating temperature. No doubt there is an SD-455 under the hood when one starts up cold.
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Maybe you could check and see if the SD pistons have the holes drilled thru the oil ring instead of the long slots like the other OEM pistons? I could show some pictures if you don’t follow what I mean. If the SD pistons were designed by TRW and run that much piston too wall clearance I bet they are the VMS-75 alloy, which is closer to a 4032, but the design of the skirt expand’s a bunch and needs a lot of piston to wall clearance. If they had that skirt design with the small holes under the oil ring, they will have quite a bit of piston slap cold no matter what the offset is from that .006-.007” piston to wall needed for the extra skirt expansion.
Just a side note, and I really don’t care one way or the other if someone wants to make a different case for piston offset, but it seems like I have been the only one so far that has built some Pontiacs with the stock type pistons in backward. I have not done a Pontiac with reverse hanging pistons for years. But with their long factory pistons skirts, tall decks, and good rod to stroke ratios, the cold piston slap noise was less than what I hear today with newer LS engines. Some have mentioned some problems with SBC’s and reverse pistons, and that could very well be. I know a lot of people that did that in the circle track world, and on stroker SBC to get more rod to block clearance's on some SBC’s. I think the SBC 400 and even a SBC 350 can do some funky stuff with the pistons reverse hung, the 400 only had a 5.565” long rod, with a 3.75” crank, in a short 9.123” block. The strokers or SBC with a 6” rod did better, but the 400 with the 5.656” rod almost pulls the piston pin out of the bottom of the cylinder, and does not take much to unseal the rings or make noise. Joe Sherman use to claim 15 to 30 HP from reversing pistons on some SBC’s. Engine master’s could find any difference on one of their tests. Smokey Yunick supposedly had some equation that maximized what he thought should be the best piston offset for making power at certain RPMs. Kind of what I remember is he was not on board with the keeping the same offset or no offset, he seemed to think it should vary. He claimed that most the nascar teams were running there pistons in backwards in that era were offset was run in factory backed cars. I have no idea what Pontiac or any of the teams did. Tom Hoover, a Chrysler backed drag racer, whom was also was an Engineer for Chrysler ran the piston is backwards, and did some seminars back in the day for direct connection where he talked about piston offset. He more or less said the reverse offset gained some low end torque and added some extra ring wear. Most of the time is does very little and just adds some noise in cold start ups. Like mentioned, nothing a good stereo can’t cure. Lol
12-01-2023, 09:50 AM
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