# sonic testing, minimum wall thickness



## 60sPontiacs (Jul 14, 2016)

What's the minimum cylinder wall thickness for a Pontiac 455? 

After much reading, I saw .100" on non-thrust and .160" on thrust side, but it wasn't specific for Pontiacs. Block is std bore now with .132" on one cylinder's non-thrust side and I want to bore .030". I always heard .060" can be done on all Pontiac blocks, which would be close on this one.


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## PontiacJim (Dec 29, 2012)

I'm no math expert, but boring the block .030" over does not take .030" out of the cylinder walls because you are talking diameter, so it should be half of that or .015", which should be within the specs you provided - and I can't verify those numbers, only your machinist can do that.

That said, if you went .060" over, .030" would be removed from the walls which would still put you right at the maximum safe bore for that block according to your figures.

Again, not an expert in this area, so don't believe me. Ask your machinist who should be able to give you a qualified answer.


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## 60sPontiacs (Jul 14, 2016)

Your math is right. 

If .100 IS okay, then I'm good with .030 removed from the diameter - would leave .117 wall thickness from .132. Allowing for (hopefully no) damage, someday I may need .040, then .060. At 60 years old, I don't think I'll be doing 3 rebuilds on the motor considering it won't be daily driver. I was just curious if .060 could be removed as I'm trying to learn.

While searching, I'd seen .125 and .130 min all around - any boring on my block goes below this. Some Mopar guys say .160 min all around (I don't know where they start). "My" machine shop engine tuner/carb/intake expert (who's learning machining details from owner who's about to retire) said it "should be fine" because thrust sides were .200+ min, but he's not Pontiac. I'd asked on a different forum, and was told to PM Mr. P-body, but never heard back. The info I'll go by came from a guy with screen name CNC BLOCKS NE who seems most experienced. 

Have some bad vibes after reading about Eric's 67 GTO nightmare. This shop is similar - been around 45 years, 70-year old owner got sick a few months, work stacked up, my block's been sitting there 6 weeks (was said they could do it in 2-3). Want it cleaned/magnafluxed, bored/honed, zero-decked, and balanced - so far it's just been sonic tested. They want to test bore a corroded cylinder/water setting before proceeding - said if it's bad, may want to sleeve that one. Still need to get pistons so he can match to cylinders (planning on SRP 149721 lightweight forged +.034). Will see how it goes.

Wasn't sure I was going to keep the GTO (had it 30 years), but after coming here, I got the fever again...


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## BearGFR (Aug 25, 2008)

60sPontiacs said:


> Wasn't sure I was going to keep the GTO (had it 30 years), but after coming here, I got the fever again...


The best one can hope for is occasional periods of remission. There is no cure. (thank goodness)

Please keep us informed on how things go.

Bear


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## Pinion head (Jan 3, 2015)

.060 over overbore can't be done safely on all Pontiac V8's, not all, always sonic test. Have gone through the "thin cyl wall on the thrust side" frustration with one of my original HO cars engines. ended up sleeving one hole and hard blocked to the bottom of the freeze plugs. Block was then bored, slightly offset in two cylinders, and then finish honed with a BHJ plate.. Light weight forged pistons and 6.8" rods will help in the longevity. Heavy old school forged 455 pistons & 6.625 rods experience cyl bore wear much quicker.


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