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The 915 heads with the original 400 pistons won't get you anywhere near 10-1 compression.

You probably wouldn't need to rebalance the assembly with a forged 383 crank because the piston weight isn't that much different. While the 400 piston is larger in diameter, the pin height is lower.

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The 915 heads with the original 400 pistons won't get you anywhere near 10-1 compression.

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Balance factors: 383 steel crank vs. 400 cast crank.
They both use the same rod.
They both use a piston that's the same or very close weight.
383 cast cranks used exactly the same factory imbalance as the 383 or 400 cast cranks. And they were somewhat interchangeable on the assembly line, between cast and forged 383s.

My opinion is if you remove the 400 cast crank and swap in a 383 forged crank with the 383 harmonic damper and flywheel/torque converter, you will have a typical Mopar factory balance job on the result.
The Mopar balance jobs aren't known for their precision, but we didn't hear about any problems back in the day. Now that everyone is balancing to a gram or less, ANY Mopar factory balance job can be improved. But don't expect a noticeable difference.
So swap away!

And, if you're hoping for 10:1 compression without changing pistons, dream on...you'll never get there.

R.

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Please excuse my lack of knowledge but what does the OP need to get 10-1 in his engine? Closed chambered heads, mill them, deck the block a bit, thin head gaskets, yes? How much more would he need?

Would turning down the rod journals to 010 undersized but doing it like 440 rod journals are offset ground to BBC size do anything to help or is that wasted effort?

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We'll just have to agree to disagree on that. I've balanced a lot of mopar engines and what your saying is not what i've seen in actual practice.

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We'll just have to agree to disagree on that. I've balanced a lot of mopar engines and what your saying is not what i've seen in actual practice.

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Some actual numbers would bolster your argument. As you know, machine shop balance and factory assembly line balance are way different.

The forged crank for the 1972 400 has the same part number (2268114) as the 383 from 1962-1971 which tells us there is no difference in the cranks.

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We'll just have to agree to disagree on that. I've balanced a lot of mopar engines and what your saying is not what i've seen in actual practice.

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Some actual numbers would bolster your argument. As you know, machine shop balance and factory assembly line balance are way different.

The forged crank for the 1972 400 has the same part number (2268114) as the 383 from 1962-1971 which tells us there is no difference in the cranks.

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John, 71 was the last year for the 383 and as you know, 72 was the first year for the 400. It stands to reason they wanted to deplete the old 383 cranks. I think we all know they'll physically interchange dimensionally. That does not mean that the component weights were the same. At the factory all they had to do was drill the counterweights differently. The pistons are not close enough in weight to just swap cranks from a 383 to a 400 and not rebalance it.
At some point you'd have to decide what is close enough, right? So in your opinion, what would be close enough? 30, 40, 60, 100 grams? typically anything more than 1 Oz.-inch will cause problems. (28 grams)

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The factory tried to maintain 14 grams or less.. but
I know that varied somewhat

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We'll just have to agree to disagree on that. I've balanced a lot of mopar engines and what your saying is not what i've seen in actual practice.

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Some actual numbers would bolster your argument. As you know, machine shop balance and factory assembly line balance are way different.

The forged crank for the 1972 400 has the same part number (2268114) as the 383 from 1962-1971 which tells us there is no difference in the cranks.

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John, 71 was the last year for the 383 and as you know, 72 was the first year for the 400. It stands to reason they wanted to deplete the old 383 cranks. I think we all know they'll physically interchange dimensionally. That does not mean that the component weights were the same. At the factory all they had to do was drill the counterweights differently. The pistons are not close enough in weight to just swap cranks from a 383 to a 400 and not rebalance it.
At some point you'd have to decide what is close enough, right? So in your opinion, what would be close enough? 30, 40, 60, 100 grams? typically anything more than 1 Oz.-inch will cause problems. (28 grams)

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The big issue is whether or not the crank balance is internal (no added weight to flywheel/converter) due to being forged instead of cast. Weight balance is way differemt due to offset balancers and weight pads (flywheel/converter) on the cast crank assemblies. CANNOT mix parts between the two, otherwise you will eventually have a bad vibration and a very short engine life! ( BOOM, BOOM, POW!! ) Either type of rotating assembly, I would DEFINITELY spend the extra cash re-balancing it!! Free horsepower, way smoother operation and extended engine life.

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Thanks everyone for all the good info.i wasnt sure what the compression was on a stock 400 but was hopeing that useing the 915s and some mill work that i could get real closr to 10-1 comp.i knew that the comp was low but did not realize that it was that low......yes its just going to b a half ass street motor for my 17 year old son.i had some parts laying around so i thought instead of having the 400 crank polished,i would polish a 383 crank and b somewhat better. Lol

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If you use the 915's, zero deck/flat top piston combo, choose a cam that will pump up cylinder pressure to "create" a ratio closer to 10 to 1 cr (effective). Definitely need to play with the ignition timing for MORE SNAP!!

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John, 71 was the last year for the 383 and as you know, 72 was the first year for the 400. It stands to reason they wanted to deplete the old 383 cranks. I think we all know they'll physically interchange dimensionally. That does not mean that the component weights were the same. At the factory all they had to do was drill the counterweights differently.

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The pistons are not close enough in weight to just swap cranks from a 383 to a 400 and not rebalance it.

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John, 71 was the last year for the 383 and as you know, 72 was the first year for the 400. It stands to reason they wanted to deplete the old 383 cranks. I think we all know they'll physically interchange dimensionally. That does not mean that the component weights were the same. At the factory all they had to do was drill the counterweights differently.

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I understand that but the crankshaft was individually balanced to a set bobweight, if the 400 bobweight was that much different than the 383 they would have given the crank a different part number and a different balance but they didn't...so a parts room 2268114 crank would have been supplied as a direct replacement for either the 383 or the 400.

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The pistons are not close enough in weight to just swap cranks from a 383 to a 400 and not rebalance it.

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Again, depends on how much imbalance is considered "too much"....evidently the factory thought it wasn't enough to warrant a different part number.

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John, 71 was the last year for the 383 and as you know, 72 was the first year for the 400. It stands to reason they wanted to deplete the old 383 cranks. I think we all know they'll physically interchange dimensionally. That does not mean that the component weights were the same. At the factory all they had to do was drill the counterweights differently.

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I understand that but the crankshaft was individually balanced to a set bobweight, if the 400 bobweight was that much different than the 383 they would have given the crank a different part number and a different balance but they didn't...so a parts room 2268114 crank would have been supplied as a direct replacement for either the 383 or the 400.

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The pistons are not close enough in weight to just swap cranks from a 383 to a 400 and not rebalance it.

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Again, depends on how much imbalance is considered "too much"....evidently the factory thought it wasn't enough to warrant a different part number.

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A crankshaft bought over the counter did not have balance holes drilled in it. Aside from that, how many forged crank 400's were ever built? I've never seen one.

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Can you bore a 400 block large enough to take a .030" over 440 piston?

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You could spend yourself poor at the machinist to reinvent the whole thing, or you could spend a few hundred on pistons. I've driven myself bats with the math on this puzzle... just buy the damn pistons!

That said... i have a No-buck zero-deck 400 ready for the rebuild in my shop. Found a standard bore re-cond. 400 at the junkyard, free in trade. Had some .030" over 440 pistons (stock low comp, perfect comp ht. for lowdeck) kicking around. Enough rods to build a V700, and a couple steel 383 cranks. Bore the 400 to just 4.35", flycut the pistons, balance it all and pow! Zero-deck/steel crank/balanced 400 to go with some nice 516 heads and .040" quench. Total cost? $125 + machining/balancing. Kinda hokey... but i bet it screams.

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All that and not one use of your sillyass ov ... ..

Did you add valve notches to those pistons? If you didn't don't get crazy with your cam choice.

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A crankshaft bought over the counter did not have balance holes drilled in it.

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You could spend yourself poor at the machinist to reinvent the whole thing, or you could spend a few hundred on pistons. I've driven myself bats with the math on this puzzle... just buy the damn pistons!

That said... i have a No-buck zero-deck 400 ready for the rebuild in my shop. Found a standard bore re-cond. 400 at the junkyard, free in trade. Had some .030" over 440 pistons (stock low comp, perfect comp ht. for lowdeck) kicking around. Enough rods to build a V700, and a couple steel 383 cranks. Bore the 400 to just 4.35", flycut the pistons, balance it all and pow! Zero-deck/steel crank/balanced 400 to go with some nice 516 heads and .040" quench. Total cost? $125 + machining/balancing. Kinda hokey... but i bet it screams.

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All that and not one use of your sillyass ov ... ..

Did you add valve notches to those pistons? If you didn't don't get crazy with your cam choice.

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You could spend yourself poor at the machinist to reinvent the whole thing, or you could spend a few hundred on pistons. I've driven myself bats with the math on this puzzle... just buy the damn pistons!

That said... i have a No-buck zero-deck 400 ready for the rebuild in my shop. Found a standard bore re-cond. 400 at the junkyard, free in trade. Had some .030" over 440 pistons (stock low comp, perfect comp ht. for lowdeck) kicking around. Enough rods to build a V700, and a couple steel 383 cranks. Bore the 400 to just 4.35", flycut the pistons, balance it all and pow! Zero-deck/steel crank/balanced 400 to go with some nice 516 heads and .040" quench. Total cost? $125 + machining/balancing. Kinda hokey... but i bet it screams.

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All that and not one use of your sillyass ov ... ..

Did you add valve notches to those pistons? If you didn't don't get crazy with your cam choice.

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DAMMIT!!!!!

I'll try harder next time. I WILL, i promise!

Flycut pistons (for valve clearance). Yes, part ov the budget (see?? see???). And that actually was the hangup that back-burnered the whole deal for now... flycutting the pistons. I'm too lazy to get that neat Isky tool and do it myself, and my machinist was too far away for the back and forth needed to get it going. No way i'd put together a neat combo like this and limit myself on cam size.

I wish there was a standard template for flycutting a piston for valve notches. Something that didn't require giving him the entire pile or assembling it to figure it out. Or maybe i'm overthinking things?

The whole point with this combo was NOT to succumb to the 'might-as-wells'...

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When you said flycut I thought that was to trim the top of it to get it to zero , my bad.

You could have bought some small carbides and soldered them to valves and cut them that way , keeping with your bucks down theme

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A crankshaft bought over the counter did not have balance holes drilled in it.

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I disagree. A parts room crank would have been exactly the same part that was supplied to the engine assembler at the Trenton engine plant.....spin balanced at another part of the plant using a specific bobweight.

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okay, if you say so it must be true. I've only bought a few cranks over the counter at a Plymouth dealership and none of them had balance holes in them.

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So in the end what your saying is that a forged internal 383 crank would have been balanced exactly the same as a cast crank external balance 400 crank

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okay, if you say so it must be true. I've only bought a few cranks over the counter at a Plymouth dealership and none of them had balance holes in them.

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So I guess back in the day "Joe the dealer mechanic" at the small town dealership went to the parts room to get a replacement crank for 383 or 400 and the parts room guy would say "here's your crank, Joe, but you'll have to find somebody to balance it....nearest place is 100 miles away".

Okay, I'm convinced...not.

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So in the end what your saying is that a forged internal 383 crank would have been balanced exactly the same as a cast crank external balance 400 crank

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Never said anything of the sort...what I said is that the FORGED crank for a 383 is the same as the FORGED crank for the 400 as evidenced by the same part number. If the bobweight that the crank was balanced to was significantly different for each they would be different cranks and would have gotten a different part number....they didn't.

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My 1970 FSM shows the 383 as 3.375

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In the end it doesn't really matter. One would be foolish to replace one crank with the other and not balance it. The factory balance was not that close to begin with

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So anybody that assembles a motor with a stock replacement crank is "foolish"? Okay.

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and the 383 and 400 DO have a different bobweight, believe it or not.

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Okay... heres a hokey question...

How much piston/bore clearance is just too much?

Could you put a standard-bore 440 piston in a standard bore 400? (4.32" vs 4.3422") Assume compression height was viable (which it is, in fact). If so, what would make it last as long as possible?

Only reason i'd even consider that was because this infamous old street racer (a Chevy guy but had done a lot ov time with nitroused junkyard 440's), said the craziest thing he'd done with a Dodge was running with .020" clearance in a 440 (.040" over pistons in a .060" over bore). Obviously done outta desperation, but he said it worked, and "probably didn't burn any more oil than a normal 440..." (bit ov a stab here). That was a pretty fast car apparently. His stuff always was.

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