How thin can I go on the cylinder wall?
 

I am building a new 400 stroker engine to replace the one I destroyed at Norwalk. It's a 1971 400 block STD bore, I just had it sonic checked and there is one cylinder I am not sure about. I have one cylinder on the right bank thrust side that is .185 and I plan to go .06 over and that would be .155. I do plan to hard block it up to the water pump holes. What is the rule of thumb on this? Thanks for your advice on this.

Will it clean up at .035-.040 ?

Paul Carter and I had a long conversation about cylinder wall thickness a few years ago; Paul’s thinking was the minimum wall thickness on the non-thrust should be not less than .125. This is pretty close to what Grumpy Jenkins used to say: .135. With Hard Block, a fairly light reciprocal mass and careful assembly you should be OK.

Yes. But Bill Jenkins point of view came from a motor with a much shorter stoke and in turn less thrust loading.

I don’t know, maybe at the rpms he was spinning his motors to the loadings would become equal between the two motors in discussion here, but still the Pontiac will be spending more average time with a higher thrust loading.

I want to check the paperwork Butler sent on the engine i have now, but i think the number you posted is around what they listed, and if i am not mistaken i think that was after they bored it. the block is a 68, so not sure if that matters much.
I've been thinking if i were to rebuild it and open it up more, at what point do you start considering the aftermarket blocks? i know it adds costs, but also adds good insurance, especially when factor in lifter bore braces, main caps, and more importantly protection to the other expensive parts, etc. I guess this makes more sense if one is starting from scratch?
some benefits would be strength, larger displacement, safety from a carnage incident, and having little thicker walls which should help sealing and cooling right?

Anybody figure that some day the CYL walls will use the technology of a beer can. Albeit a bit thicker. I think not. Post#3 sans rust.

When we were racing 455's, before aftermarket blocks were available, we just honed them and ran them until they failed. They were pretty prone to failure in short order around .100 wall thickness. As a little test, you have several cylinders from your failed engine with chunks missing. Measure some of those walls where you have access. can probably do that with a caliper. That will give you some idea of what the failed one was. If I had to run a stock block, I would try one with a .150" wall vs installing a sleeve. One repair sleeve weakens the entire block structure. Your car is fast and making allot of power. If possible, I would consider buying a new or used IA block or Kauffman block. The cylinder wall thickness issues go away completely as well as cracked main webs to the cam tunnel, lifter bores falling out. Billet 4-bolt caps hold the crank in nice and secure. Used ones are for sale all the time on this forum. Good luck with your build whatever you decide.

I would think with hard blocking the block you'll be fine JMO.

GT

Bore it +.030 instead if it will clean up at that size.

I’d sacrifice a few cubes and keep a more stable cylinder wall if it was my engine. Chances are the rings will seal better with the stiffer cylinder walls and the engine will make just as much or more power without the extra cubes.

Non-thrust cylinder walls verses thrust face cylinder walls; Grumpy was talking about NON-THRUST cylinder walls, the same cylinder walls Paul and I discussed.

The thrust side is the critical one and if it's too thin you will have too much flexing and your ring seal will be compromised. For a race engine or even a high performance street rod hard block will stiffen up the cylinders and makes the overall block more rigid. All the non-thrust areas are plenty thick I have one cylinder after boring to 4.181 will be .1575 on the thrust side. I believe after adding in the hard block and using 6.8 long rods which reduce side loading I think it will be fine and hopefully last a long time. Four bolt mains, forged crank, Ross pistons, Eagle 6.8 rods, new ballancer I hope this engine will last a long time. If there were a place near by that did cryogenic I would cryo the whole engine prior to machining.

Considering you are filling the block, you should be fine.

Is there a reason this block can not be bored 4.150-4.155 ?
If you are going to go through all the expense of having billet caps installed why bore it to the max if you do not have to ?
Might as well get the most life out of it you can as you could always bore it .060+ later if you have to.

Have the fella "jerk the bore" so much less matl removed from the thin wall concern.

"jerk the bore"?????

Move the bore over a little.

GT

. Yeap.

Rather than aim for crank-Main center, bore concentric to acoustic measurement. It's a thing.

And the question still remains…..why the need to create thin cylinder walls when you really don’t have to take the bore out to the max?

The simple solution is don’t remove metal that’s unnecessary.

I kinda doubt the power difference from a 4.15 bore to a 4.18 bore will amount to much.
Bracket car so why would it matter anyway ?
Way back when I was looking for a 400 block to install billet caps on there was no way I would put the money into it if the block was not a clean std bore so I could run it .030 for the first go around and still have one left in my pocket.

They do this quite a bit in lower level dirt track racing,I first heard guys having it done in the 6 cylinder class