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My first tour through Flint was 1985 (30th anniversary of the small block) -- and they balanced the rotating assembly then and in the subsequent visits I made there. They put pins into slots in the vibration damper, and drilled to remove weight from the flywheel/flex plate. All this at the station where the engine was run on natural gas, and timed.
The selection of pistons and rods was for size -- to fit cylinder bore diameter, but I don't recall seeing a station that weighed the pistons. Not to say there wasn't one, just no one mentioned it. If the pistons were supplied by others the weighing might have been done at the supplier. Same with balancing the crankshaft and weighing the connecting rods. Exactly what John said.
FWIW: Wixom Performance Build Center (where the LS7 is assembled) does no balancing either -- balancing of the complete assembly is contracted out. All the rotating and reciprocating parts arrive already balanced. What's more every piston can fit in any hole. No more selective assembly. It's not exactly your father's (or even your) small block.
I toured the LT5 plant, Mercury Marine in Stillwater, Ok two or three times and watched them assemble and test engines. It was as Terry described, the rotating assembly was not balanced near the assembly line, possibly some other place. They told us both ends of the rods and the pistons were carefully matched. The crank, rods and pistons arrived together so I assumed at that time the crank had been balanced with those weights. After the engine was assembled it was prepped for the engine dyno and while warming up they checked balance at low RPM and inserted pins in the balancer. I did not see them do anything to the flywheel end but may have missed it.
Those engines were something on their power runs, 7200 RPM and none produced less than 420 HP and those were the 375 HP engines. The 405's were about 450 with all accssories and stock exhaust manifolds.
thank you, this has concerned me for a while, i just have to finish it up and have the nerve to fire it up, i have more in this engine than many cars i have owned, thanks,bill
I only got through Stillwater once, but I am still impressed with their care in assembling the LT5. Even now some 17 years later, their methods are still almost current.
I remember the pistons arriving matched with the cylinder sleeves from Germany. An amazing operation. I don't remember anything about balancing, but do recall that they measured the cylinder cases with automated equipment and kept the measurements on file.
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