348 962 casting overbore limit

Hi all, need to sonic check a 962 348 truck block, hoping it will go 4.312 or standard 409 bore. Does anyone have experience with this? What is minimum cylinder wall thickness? Thank you.
 

Don Jacks

Well Seasoned Member
Supporting Member 3
Most of those blocks won't go .125 safely.Your minimum is about .130 and even that could be shaky with high compression/cylinder pressure.
 

1964SuperStocker

Well Known Member
Supporting Member 3
Pretty sure that isn't going to happen. Otherwise that would be the first thing every guy with a 348 would do. You will get plenty of answers but ultimately they are all going to say, no. Don't bore when you can stroke easy past 427ci.
 
Thanks for fast responses I will post results from the sonic test. I figured too good to be true but a reliable source says he has done it but did not back up his claim with sonic test numbers. Much appreciated!
 

Don Jacks

Well Seasoned Member
Supporting Member 3
The torque that you can achieve via stroke will far exceed what can be achieve via a masssive overbore.For example,an overbore of .060 and a 4 inch stroke will result in 440 cubic inches from a 348 block.
 
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1964SuperStocker

Well Known Member
Supporting Member 3
Only way I can think of to get the end result you want would be to stuff 409 sized sleeves in a 348 block. Expensive and just not worth it.
 
I have a 409 crankshaft and found a set of 340 HP 409 standard pistons if the 962 casting 348 truck block sonic checks to go to 4.312 then I would have a motor to make my project car mobile while I slowly move forward on the long term motor, a 409 truck block .060 over 4" crank H-beam rods Walla heads, McQuillen intake, and so on. I have been out of the game for 20 years and exited to get back in!
 

Don Jacks

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Supporting Member 3
Welcome back! For ban idea of what can be achieved from the parts that you listed,look at Plumcrazys thread about the STREET engine that he's finishing installing in his truck.
 

1964SuperStocker

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Supporting Member 3
What number do you have in mind to be "safe" once you have sonic checked each one of your cylinders? We are all curious to know what safety margin you following because these 60 year old blocks aren't made of the same materials or engineered like the blocks are today. I can stuff a piston down a ceramic cylinder and watch it explode at 100psi. The thinner your walls are the more likely you will have a structural failure of your motor destroying everything inside of it. If it were a sbc 350 I'd say have fun and see what happens because they are dime a dozen and a rebuild kit is less than $200. Old school parts mean respect them as such and adjust your safety margins accordingly. We would all be excited to know we could do more with the 348 blocks but out of the hundreds maybe thousands of blocks that this body of experienced individuals has built over the years I don't see anyone I trust saying this is a good idea. What is your safe number for cylinder wall thickness? :rub
 

tenxal

Well Known Member
All sound advice I know I am dreaming, safe wall thickness to me would be .125-.150 if it flies this would be a 9 1/4-9 3/4 to 1 compression motor with a mild hydraulic cam. Sonic check takes place next week.

I've broken .125-.130 cylinder walls with pistons 250 gms. lighter than you'll be slingin'. ;)
 

MRHP

 
Supporting Member 1
And remember, a sonic test does not account for pinholes, divots,pitting, or voids. It is just a sample of thickness at a given point. An inch away it could be much thinner. I would think the minimum overbore with no issues and buying pistons would be cheaper than a maximum overbore, a bunch of sleeves and pistons you already have.
 

427John

Well Known Member
Another possibility if you do sonic check in the range your talking about is to partial fill the block to shorten the amount of unsupported wall as much as possible to increase the rigidity.While max. wall thickness is always best,if you must go that far partial fill will increase your chance of success.
 

Greg Reimer

Well Known Member
348+.045"+3.760 stroke(396 crank) =409 cu. inches,although the bore and stroke is different than the OEM 409 engine. If doing this and ordering pistons specify that you are using the 6.135" big block rod.Good luck,however,finding a 348 block that will clean up at .045".
 

Carmine

Well Known Member
Supporting Member 10
What I did with my '61 348 engine to make it 434 cubes. Bored .030 over. 454 crank. 396 bb rods. Forged pistons. 9.1:1 cr. Isky hydraulic cam. 1-2 levels above stock; nothing crazy. Edelbrock heads, intake and 750 carb. Engine runs real well. Plenty of horsepower and torque. Have it in my '56 straight axle gasser wagon. Alot of fun doing hole shots. I think this motor is more powerful then the 409 in my '62, Carmine.
 
Hi all still waiting on sonic check. Side note: How much can 409 be decked befor encountering problem? Will be running Best composite head gaskets, standard bore 4.312 truck block with 340 HP pistons, figure 9:1 unless deck the block. Could you deck .080 when using composite gaskets where steel shims were OE? Thanks! Nate
 

Don Jacks

Well Seasoned Member
Supporting Member 3
Nate,I wouldn't deck that block .040,much less .080.It'll be better and cheaper to use the Cometic gaskets that are available in .027 and .040. When you reduce the blocks height,you then must modify the intake manifold[more money].Mill the block enough to get it square,mock up the assembly,and determine what the deck height would be.What you need is a total quench distance including gasket thickness of NO MORE than .050,no less than .034.
 
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