354 hemi. Overbore

Discussion of the 331-354-392 HEMIs.

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J-1450
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Location: Norway

354 hemi. Overbore

Post by J-1450 »

Hi.

I recently got an almost free 354 passenger block. It has got some really bad pittting in the cylinders...

I was wondering, will the 354 block take a 392 (4") .030 overbore? Or is 4" max ?

Anyone tried this before?

Thanks :wink:

Jens
ebbsspeed
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Re: 354 hemi. Overbore

Post by ebbsspeed »

J-1450 wrote:Hi.

I recently got an almost free 354 passenger block. It has got some really bad pittting in the cylinders...

I was wondering, will the 354 block take a 392 (4") .030 overbore? Or is 4" max ?

Anyone tried this before?

Thanks :wink:

Jens
Well, you're talking about a .0925 overbore here, which is going to put you into "thin wall" territory. The only way to know for sure is to sonic check the block for wall thickness.
Bailiesdad
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Post by Bailiesdad »

a 4 inch bore 354 is a couple of thousands over 60 and should be fine. .0625 to take it to 4.00. more than that it is leaning over the line.

I have taken a thick cylinder block to 4.030 but it was a full fill block.
ebbsspeed
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Post by ebbsspeed »

Bailiesdad wrote:a 4 inch bore 354 is a couple of thousands over 60 and should be fine. .0625 to take it to 4.00. more than that it is leaning over the line.

I have taken a thick cylinder block to 4.030 but it was a full fill block.
I got a little confused when he said a 392 .030 overbore, which is .0925 on a 354, pushing the limit. If it's just 4", and not 4.030, that's been done lots of times, but to make sure you're OK have it sonic checked, it's cheap insurance.....
wayfarer
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354 overbore

Post by wayfarer »

Stay to the 4", but remember, if you plan to use std bore 392 pistons you will loose about 1. of compression due to a smaller dome volume.
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Powerflite
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Re: 354 overbore

Post by Powerflite »

wayfarer wrote:Stay to the 4", but remember, if you plan to use std bore 392 pistons you will loose about 1. of compression due to a smaller dome volume.
I have heard that before, but I don't understand it. Doesn't the 392 have a larger combustion chamber? Thus, going with the smaller head should give higher compression - no? Does the 392 piston pin sit higher on the piston? What is the reason for the difference?
George
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Post by George »

Shorter stroke, less swept volume.
wayfarer
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Post by wayfarer »

You are compressing less 'stuff' into essentially the same size chamber.
There is some variation in combustion chamber volume, ranging from 105 cc up to 112 cc (as physically measured in our shop), but with a whole bunch of chambers at 108 cc in what appears to be stock configuration. This is for all 55-58 heads. Valve face design can account for several cc difference either way, as well as the quality of the last valve job.
Regarding the piston itself, if you mount a stock 354 piston and a stock 392 piston on a stock wrist pin you will immediatly see the difference in overall height of the dome.
And please do not believe what some of the current suppliers say about compression of their slugs. Make sure that returns are accepted if you check and then find compression does not meet advertisement.
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oldngood
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Post by oldngood »

Bailiesdad wrote:a 4 inch bore 354 is a couple of thousands over 60 and should be fine. .0625 to take it to 4.00. more than that it is leaning over the line.

I have taken a thick cylinder block to 4.030 but it was a full fill block.

did you sonic test any early hemis, if so what kind of numbers are there for bore thickness ?
wayfarer
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Post by wayfarer »

As a rule, we now sonic check all Hemi blocks before deciding on how much to cut. The 392 regularly shows alot of core shift and there have been several occasions where we were forced to move the bore center away from the thin side in order to preserve some thickness just to make a clean bore. We have measured 'spots' as thin as 0.125"... :o
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Early Hemi Parts and Pieces,

INVENTORY CLEARANCE IN PROGRESS. See website for details.
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