I have a stock 72 455. I am thinking about swapping in 69 or 70 high compression (10.25:1) pistons in if I can find them. I would like to do this at stock bore, with my current crank, and without any machining. I am still in the disassembly phase, so this is assuming the bottom end looks good. So far the cylinder walls look nice (72000 mile engine).

I would be using the stock Ga heads on the engine, with new cam, lifters, valve springs, valve guides, valve seals, timing set, oil pump.

I hope to run it like this for about 2 years as I save up for heads, roller rockers, full rebuild.

Will there be balance issues from swapping my 8.5:1 pistons with used 10.25:1 pistons? From my measurments and calculations, the actual compression ratio will be about 9.2:1 with the high compression pistons (stock bore, 15 cc dish instead of 40 cc dish, .039" thick head gasket, ~0.050" below deck at TDC). Seems to me the weight difference for stock pistons would be minimal, and therefore I would not expect any balance issues. The car will only be street driven, and won't see much more than 5500rpm.

Any other pitfalls here?

Thanks for your advice / experience. Tom

 

I helped a friend with a similiar situation with a buick 455. We used the entire rotating assembly(crank, rods and pistons) out of a '70 455 and put them into a '74 block. This was a low budget rebuild, the '74 motor ran great so we reused the piston rings from the '74 just kept them matched to the cylinders. We did replace all the bearings though because of the crank and cam change. Motor still runs great 4 yrs later and a lot of hard miles and 11 sec ETs.
If you go this route get rods and pistons together, because I think pressing pistons off the rods will hurt the pistons. You shouid be able to pick up a set cheap. JKaz

 

Technically you are supposed to have the assembly balanced if you change anything. I dont think that the difference in weight is what will make it out of balance, it is differences in the weight of each component. The new pistons you get may have variances that are different from the old pistons which may cause it too be unbalanced.

Sorry if I wrote this confusing, hope you understand what I am saying.

 

Not likely they could be any worse than some of the factory Chrysler engines.

Norm

 

No problem, if the "presser" knows what he/she is doing.

Norm

 

If you are intent on doing a budget rebuild at least invest in a cylinder hone and hone the cylinders. Then when you get new rings match them to each cylinder indiviudally by setting the ring gap appropriately.

 

Um, if you don't press them off, how else do you change pistons? I suggest you review the Chassis Service Manual, which shows the use of the Kent Moore fixture for pressing out piston pins.

 

Why go through the hassel of changing pistons. They will probably come hung on rods already and probably are balanced to each other from the factory. Engine builders I have talked to frown against reusing pressed off piston. but what do they know. JKaz

 

The piston is just "floating"on the piston pin. Why would the piston get damaged? If anything, the rod would get damaged because that is what holds the pin in place.

 

The problem is that one must use the proper support on the piston so that it's not damaged by the high force needed to press the pin out of the rod.
DW

 

Good point, I didnt think about that. So it pretty much comes down to if the machine shop decides to do it properly or if they decide to do it half *** with out the proper tools.