L31 crank rods in early block (350 build) - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-27-2009, 09:36 PM
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L31 crank rods in early block (350 build)

Can this be done. If so, what are the advantages, and what caps do you use, early or the late main caps. I hear the late model rods are forged, and the cranks are nodular (lighter).

Any thoughts or opinions from anyone that has done this.

I have an L31 block, but many told me not to use because it was nuked (overheated). One piston was ruined and it slightly scared the bore. Not much, one shop said a light hone couild possibly clean it up and at min. it would take a .020 overbore. I have had some tell me to use it. I figure if I can't use the block, I can use the cranks and rods.


I know one is internally balanced and the other externally.

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Old 01-27-2009, 10:30 PM
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The late-model one-piece-rear-seal crank won't work in a block that's designed for a two-piece rear seal. The older block also won't accept the factory roller lifters, if you intend to use them. A machine shop can check your block for heat-related damage such as cracking and warping, and if it's fixable, they'll fix it for you.

SBC cranks have been made from nodular iron for the last 40 years or so, so it's nothing new.

Older SBC rods are forged steel, newer stuff is made from powdered metal, and is supposed to be just as good, if not better.
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Old 01-28-2009, 11:26 AM
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would there be any benefit in using a late rod in early block, or is the early rods just as good. I was told you can use arp on early rods, but not on late.
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Old 01-28-2009, 11:48 AM
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Quote:
Originally Posted by Carlos9
Can this be done. If so, what are the advantages, and what caps do you use, early or the late main caps. I hear the late model rods are forged, and the cranks are nodular (lighter).

Any thoughts or opinions from anyone that has done this.

I have an L31 block, but many told me not to use because it was nuked (overheated). One piston was ruined and it slightly scared the bore. Not much, one shop said a light hone couild possibly clean it up and at min. it would take a .020 overbore. I have had some tell me to use it. I figure if I can't use the block, I can use the cranks and rods.


I know one is internally balanced and the other externally.
A 2 piece block can be modified to accept a one piece crank see this URL
http://www.circletrack.com/enginetec...eal/index.html

But given the number of one piece seal blocks out there, why go thru the trouble when you can pick up an L31 from the wrecking yard for a couple hundred bucks.

Actually overheating the block while destructive to the engine can be beneficial to the block as a stress relief method. If there are residual stress the overheat will let them out. If the block warps but doesn't crack, it will be a very stable platform for re-machining. If the block didn't warp, then it wasn't carrying any residual casting stress, if it didn't form cracks in the overheating process then it's good to go with a clean up bore. Building, or rebuilding the engine gives you an opportunity to correct one of the factory's major design flaws which is the circular dish piston. Replacing these with a D dish piston with the appropriate volume allows you to take better advantage of the squish/quench function of the combustion chamber, you can then use a lower octane fuel at your current compression or take advantage of higher compression and high octane fuel to improve thermal efficiency and power output.

The L31 is roller cam block, certainly older two piece main seal blocks are not, if that is a concern. The older blocks can be modified to take a factory roller but its a somewhat complex and not always successful endeavor. Again why bother when there's so many Vortec blocks out there.

The one piece seal cranks lost a bit of counter weight on the rear, this is made up by the flywheel/flexplate for these engines. Going to a two piece seal engine (305 or 350) requires the use of the flywheel appropriate to the engine. This is really a no brainer as the bolt circle between one and two piece seal cranks is different so you can't cross fit these things. The damper unlike the 400 is neutral balanced.

If your running the Vortec injection, the L31 timing cover has to be fitted to what ever block your using as it contains the crank position sensor the computer needs to find the number one cylinder. You will also have to swap on the L31 damper for the same reason.

Old, pre 1986 rods are forged mild steel with machined caps. The new rods are made from a high temperature forging of powdered steel. This produces a near net part requiring little to no final machining. The cap is cracked from the total part so each is a unique mate to the shank portion. The powdered metal rods are advertised, and from all service appearances, as having the same strength as the pink rod.

Bogie
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