392 Hemi maximum safe overbore- cylinder wall thickness

Discussion of the 331-354-392 HEMIs.

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oldngood
Posts: 121
Joined: Thu Mar 13, 2008 5:14 pm

392 Hemi maximum safe overbore- cylinder wall thickness

Postby oldngood » Sat May 19, 2012 7:22 am

I've seen this topic knocked around from time to time. How much can you bore out a 392 early Hemi Chrysler ? Having invested in a new sonic tester years ago, I can check blocks for myself and see what the cylinder wall thickness really is on various engines. I have checked many 331 and 354 Hemis and Polys. The 331 can go to a 354 no problem, and both can almost go to a standard bore 392 4" but not quite. If memory serves I think both 331 and 354 can go to a 3.90" or 3.94" bore and still leave .130" or more cylinder wall all around.

I had never gotten my hands on a 392 to check one, until now.

Recently I had the chance to check a clean 392 block. The block was already bored .060" over and was a 392 passenger car block in very good condition. I had heard the 392 may have core shift problems on the message boards. That turned out to be correct. While most of the cylinder walls were plenty thick with remaining material of .200" and over, and a few down in the .150" to .180" range, there were 3 holes with very thin THRUST walls. That's the walls directly in line with engine rotation that take the side loading. One was down to .117" on a MAJOR thrust. Runnable for a street car but by no means a good idea for boosted/NOS application, or ultra high compression i.e. 13.5 CR. The other 2 thin cylinders were around .122" and .125" On other makes of engines, one usually sees thin walls like that on the non-thrust sidewalls. I have run a Pontiac 455 at .060" with only .085" sidewall on a few holes, and it ran in the 12's and made about 500 HP no problem. But running thin walls on the thrust is a risky deal.

After seeing this, IMHO a 392 should only be bored out if absolutely necessary, and if so, the minimum required to clean up the cylinders, remove the ridge if any, and hone the block with a torque plate if you have one. 392's should DEFINITELY be sonic checked first before boring them out at all, it's money well spent. The 392 is not a block you would want to hog out looking for more CID using a much bigger bore size. The cores obviously did shift in this one, it appeared the cores sagged downwards making the upper cylinder walls facing the lifter valley on both sides, thinner on a few holes. Going to .080" over would be a moderate risk without first checking the walls with a sonic tester. If it splits then the block is junk.

Sleeving is not a fully valid repair method, it has drawbacks - every block I ever sleeved, would then not take the cam, because the cam journal bore was then out of alignment. I had to hone the cam journals on 3 blocks, to get them straight again, so the cam would go in the blocks, after having them sleeved. It was a difficult tricky process and the cam bearings had to be put in/out and checked each time with the cam, if it didn't go in, then remove the cam bearings again and keep honing the cam bore. Hone it too much and it won't hold the cam bearing. And you need a few sets of cam bearings because you go through a set in the process.

DavidBraley
Posts: 163
Joined: Sat Jan 30, 2010 7:40 pm
Location: Fort Collins, Colorado

Re: 392 Hemi maximum safe overbore- cylinder wall thickness

Postby DavidBraley » Mon May 21, 2012 1:23 am

I don't understand why you need to remove the cam bearings to check if the cam will fit. That makes no sense to me. A good dial bore gauge will tell you the size, and how out-of-round a hole is within 0.0001" in just a few seconds.

Also, if your machinist is machining a block for sleeves that have so much interference fit that it distorts your block that much, you need a new machinist.

I think it was here I read how drag racers in the late 50's and early 60's would sleeve all eight holes in their cast iron blocks. Something about having control over the final wall thickness.

If it's rare, I wouldn't hesitate for a second if I needed to sleeve all eight holes.
Horsepower determines how fast you hit the wall. Torque determines the size of the hole you make. Holzwarth's Law

TrWaters
Posts: 390
Joined: Fri Dec 14, 2001 6:00 pm
Location: Vermont
Contact:

Re: 392 Hemi maximum safe overbore- cylinder wall thickness

Postby TrWaters » Wed May 23, 2012 8:09 pm

:lol: Whatever you do, dont tell him he needs a new machinist. I did on the HAMB, and it didnt go over very well. :roll:
Early hemi to late sb Mopar trans adapters. Precision billet parts for early hemis.

DavidBraley
Posts: 163
Joined: Sat Jan 30, 2010 7:40 pm
Location: Fort Collins, Colorado

Re: 392 Hemi maximum safe overbore- cylinder wall thickness

Postby DavidBraley » Wed May 23, 2012 9:49 pm

Oh, sorry. I don't mean to upset anyone. Sometimes my responses don't come across very well.
Horsepower determines how fast you hit the wall. Torque determines the size of the hole you make. Holzwarth's Law


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