[65madgoat]

Curious if anyone has ever seen a factory 455 Pontiac engine with 10 over bore standard GM pistons originally in an stock block?

This engine builder friend of mine says he has seen them occasionally over the decades.... wondering how uncommon.

[PAUL K]

Never

[Gary H]

Nope. And I've torn down hundreds of Pontiac engines

[steve25]

Maybe in a factory cast piston, but I am not sure and would have to look.

As far as the off the shelf aftermarket goes, never!

[sdbob]

In 1975 tried to order +10 for SD455 block for NHRA STK. nope, +30 yes but took time with parts mans help. NHRA wanted the part no in the piston. The block I was using I found in a scrap yard engine pile,for $35.00 yes!! The block was an SR block with #5 sleeved!! Could never find original owner or dealer block came from. Went +30. Still in use.

[65madgoat]

I may be able to get photos of some that are 10 over apparently 'stock' 455 pistons..... will circle back

[65madgoat]

Ok photos of GM pistons attached. I am no engine assembler, but just mic'ing them I get about 4.156 or so at the skirt and 4.078 at top.

Stock 455 block with these oem pistons had a 10 over bore so around 4.161 or so if I recall.

Does this add up?

[steve25]

Please post a photo of some of the skirts.

[65madgoat]

Here are those pics of the skirts -- and the block a while ago was verified by 2 shops to be 10 overbore with these GM pistons....

[dataway]

I thought I heard somewhere that factories would occasionally do this to save a block that had a minor flaw in a cylinder bore that could be corrected by a .010 overbore.

[Formulas]

I would say this the lathe marks on the piston skirts are hardly worn so there is little wear,.... wouldn't be afraid to reuse those provided no real defects.
You might be able to use file to fit stock bore size rings and be close to a good running ring gap without doing anything .. of course verify

[steve25]

Thanks for the photos because I am wondering if those pistons were knurked during a rebuild and that would then account for the bore size.

If you have the time please put a few top rings in a bore and report back on what your average ring gap checks in at, as this will tell a lot.

PS.

Please disregard what I have posted and what I asked you to do.

I found my catalog , the factory did offer a .010” piston for the 455 motors in both cast and needed forged for the SD motors.

They offered a .010” over for the 428.

They also offered a .010” over piston for all the 400s and even the RA motors that used the TRW forged piston .

The part number for the 70 to 74 455 piston was 485648.

The number for the 75 to 76 piston was 497048.



The only way you could get a new block from Pontiac as a fitted assy with the rings on the pistons and the pistons in the block.

I can see the factory setting up for a production run of these service replacement blocks as they were called and going with a bore size to use up the inventory of .010” over rings and pistons.

The only other way to get a 455 block was in short block form, or as Pontiac called them a PARTIAL.
And this was only for the 76 model year according to the info I have.

[PAUL K]

Fwiw ..... 1974 Factory Service Manual

I'm guessing the .001 & .002 were the ones used on the assembly line.

However I'm doubting the .010 were available for the SD-455 as mentioned by SD Bob

[steve25]

The factory catalog showed them.

[65madgoat]

Awesome stuff. Now Paul K can say he saw them too ;-)


Thanks all!

[PAUL K]

Originally Posted by 65madgoat

Awesome stuff. Now Paul K can say he saw them too ;-)


Thanks all!

Just to be clear I never doubted they made them.... I've never seen any. However forty years ago a shop I dealt with was building a .010 over 455 but using custom pistons. I asked why ten over and he responded "why not".

[steve25]

Also at .010” over your locked into using factory rings on the next freshen up type rebuild.
At least at .030” over you have more ring choices.

[Stan Weiss]

There are choices.

https://www.summitracing.com/search/...d0ETGtBw6-3Jl0

Stan