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Old 08-19-2008, 05:23   #1 (permalink)
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Is .060 overbore too much for a 428?

I have the opportunity to buy a freshly rebuilt 428 (mostly stock parts) for a really good price, but I learned that the block was bored .060 over. Some have told me this is way too much for a 428 and unless the block was sonic tested, magnafluxed, microscoped, and psychoanalyzed to ensure it was solid and stable, that overheating and/or catastrophic failure is likely to occur. This will go into a street car that is taken on long road trips, so it has to be reliable.

Do you guys concur that this is risky?

Thanks!
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Old 08-19-2008, 15:56   #2 (permalink)
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Re: Is .060 overbore too much for a 428?

You should be OK at .060 over as long as you have an 1/8 inch of cylinder wall, any less than that may be critical. Pop out one of the freeze plugs from the side of the block and measure the wall thickness. Stock bore is 4.130, that's your starting point, then subtract .060 from the wall thickness and see what you have.
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Old 08-19-2008, 18:42   #3 (permalink)
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I forgot to add: This is a late '65 C6ME block

This is an early 1966 C6ME block with the "A" brazed on the back (Date code is late '65). So its not as beefy as a CJ block, etc. (Dunno if that makes a different in the cylinder wall area)
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Old 08-20-2008, 08:42   #4 (permalink)
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confused Re: Is .060 overbore too much for a 428?

It depends on the price, power level, use, and how much risk you want to take. .060 is a lot for a 428 but not necessarily "too much".

I gather from your post that it will see moderate use, so if you keep the engine temp down (below 200F, pref 180F-190F) you should be fine down to a wall of .090", maybe even .085". That doesn't mean your risk is zero, since a flaw in the cast iron could cause a wall to crack, there is always a small chance of that even in a standard bore.

Turning the engine higher (6000-7000) or making lots of torque (500-600 ftlb) would raise your risk too.

Because of core shift, I don't know of any sure way to get the wall thickness without a carefully calibrated sonic test.

If the engine had run like this for many years I'd say your risk is lower. But, you don't know its history. If it was just bored again, it might be just about to crack, or it might be just fine.

If it's a high price tag, can you make a written deal that you will buy it, sonic test it, and if any of the wall thicknesses come back less than say .090" you will get a refund?

Well that is a messy answer but it's a messy situation too.
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Old 09-10-2008, 13:18   #5 (permalink)
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Re: Is .060 overbore too much for a 428?

Quick note....for a 0.060 overbore, you only subtract 0.030 from the wall thickness to calculate your new wall thickness. Also, most cylinders don't split vertically...they split "around" the cylinder....generally near the bottom, and from stress between the block deck and the crank bearing supports. Uselss info, I know....but anyway, I have seen one 428 overbored 0.060 and it ran fine for years.....no sonic checking, so probably a fair bit of luck.
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