455 - Thoughts on max over-bore?

[MalteseFalcon]

I have a line on a 455 Service Replacement block cast in 1973 that could be used in my 1972 Trans Am (currently missing its original engine); it's the right casting number and has been cleaned and magnafluxed and appears not to be cracked. However, the seller told me that, while the bores are in good shape and probably just need a hone, the current bore is + 0.040, so I think I should expect that it may be necessary to go to a 0.060 overbore when rebuilding it. I seem to remember that most Pontiac engine builders won't go over a 0.060 overbore on a 455 block, not to mention the difficulty in getting pistons to fit. Is that right?

Ideally, I'd find a block with standard bores, but this one is a pretty reasonable price. Obviously, the price should be lower with non-standard bores, but I guess the question is woudl you be reluctant to put a significant amount of money and work into a 455 block with a .040 overbore to start with?

Thanks,

[71455formula]

If I recall correctly when you start getting that much bore; you need to make sure your cooling system is good. If you're just gonna be cruising around you should be fine, but maybe someone with more experience might have a thought.

[5th T/A]

Honestly this is a question probably best answered by your engine builder. You could also try calling Butler Performance, a Pontiac engine builder with a great reputation and ask them about rebuilding your block.

[nas t eh]

You can get custom pistons for reasonable prices. If honing it to 0.045 over will clean it up, that is what I would do and order pistons to fit.

I would also not worry about 0.060 over. If you want to go bigger than that you need to have the wall thickness tested to make sure there is no unreasonable core shift in that casting.

[firebirdparts]

Brian Morelock had one bored 60 over.  We were talking about it on the way to the Nationals.  He said it ran hot.  obviously being overbored didn't help that, but then again it might have run hot anyway.

[DynoLee]

My old 67 Firebird had a 455 bored 0.060" over.  I drove it from Houston to Dallas twice for the Pontiac Southern Nationals, and another time to Corpus Christi for a car club function.  It would get to 190 on the highway, doing 70 or so (3.42 gears, 275/60-15 drag radials used all the time), but would actually run rather cool in traffic.  It had a BeCool radiator, a Black Magic shrouded electric fan, and a CSR electric water pump.

At one time it had a 4-row copper, regular pump and a flex fan, and still never overheated.  But it ran cooler (and a tenth quicker) with the electric fan & pump.

As Nas stated, just get custom pistons.  NO NEED to bore from 0.040" to 0.060" if all you need is 0.047" (or whatever).  I've bought a couple sets of custom pistons from RaceTec this year, and they assured me that they can provide the pistons at ANY diameter I desire.  Their price (to me, at least, I have an account) is actually a bit less than some of the "off the shelf" pistons on the market.

Have your machinist determine the minimum over-bore needed to clean up the cylinders, and get pistons to match.

Even better with getting the custom pistons, they can tweak them to give you the EXACT compression ratio that you want, as well.

[MalteseFalcon]

Thanks guys! I decided to take the plunge.

[tajoe]

4 bolt caps to boot. "Sweet". 73, huh? I have a 73 T/A that doesn't have the original motor. I have a std bore 455 with a 72  casting date, and you have a 72 T/A, with a 73 cast. Hmmmm.  

[MalteseFalcon]

4 bolt caps to boot. "Sweet". 73, huh? I have a 73 T/A that doesn't have the original motor. I have a std bore 455 with a 72  casting date, and you have a 72 T/A, with a 73 cast. Hmmmm.  

Not bad thinking - But the reason for getting the SR block is that I would otherwise need a 1972 455 HO block cast before January 15, 1972 with the correct engine code - which would be WM. By using an SR block, it's considered "correct" without an engine code.

[tajoe]

4 bolt caps to boot. "Sweet". 73, huh? I have a 73 T/A that doesn't have the original motor. I have a std bore 455 with a 72  casting date, and you have a 72 T/A, with a 73 cast. Hmmmm.  

Not bad thinking - But the reason for getting the SR block is that I would otherwise need a 1972 455 HO block cast before January 15, 1972 with the correct engine code - which would be WM. By using an SR block, it's considered "correct" without an engine code.

"Picky, picky, picky"...

[MalteseFalcon]

"Picky, picky, picky"...

Yup.