273 crankshaft, 5.2 Magnum

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Maistrelli said:
What's the average cost of -.020 rod & main journals? Anyone have crank ground lately? What others things won't be right?
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Balance would be my first concern. It would take some digging and comparison between the 2 cranks but there could be some dimension differences. A simple thing to do is look up the bearings. If they are the same or the same size you are 1 step closer. Let's put it this way, I have never heard of anyone putting a 273 crank in a Magnum. I wonder the reason? Don't take me wrong but if the expense of turning a crank is an issue you are in the wrong hobby. Engine building can be expensive and you want it right.
 
toolmanmike said:
Balance would be my first concern. It would take some digging and comparison between the 2 cranks but there could be some dimension differences. A simple thing to do is look up the bearings. If they are the same or the same size you are 1 step closer. Let's put it this way, I have never heard of anyone putting a 273 crank in a Magnum. I wonder the reason? Don't take me wrong but if the expense of turning a crank is an issue you are in the wrong hobby. Engine building can be expensive and you want it right.
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Reground crank kits are available at advance auto.com and Rockauto as well, for 5.2 magnums.
 
nm9stheham said:
Balancing is going to cost 2-3 times grinding a crank. If you are not racing, then changing pistons to oversize does not necessitate a re-balance. So haw hard are you going to be running this engine?

Just be aware: Since both the 273 and 5.2 are 'internal balance' engines, there is no need to balance the crank with the external parts. Each of those external parts are individually balanced.

$125-$150 as a guess on the regrind.
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Oh I disagree. The 273, 318, 340, and Magnum cranks will all interchange, they are not all within balance of each other. Each crank has different counterweights for their respective rod and piston weight. Otherwise all the cranks would be identical and have the same casting and part numbers.
 
Not sure which part you are disagreeing one ????

No.... the 318 and 273 were set up early on with the same bobweight. The 273's used a heavier thick wall pin to make up for the lighter pistons and reach the same reciprocating weight portion. (Where they MAY deviate is when the 318's switched to the heavier rod in the mid 70's... still digging on that question....)

But this is why I have presented the info that I have.... I have been digging up and measuring weight info for quite some time on all of this. Actually computing bobweight when the data is known is trivial.... the formulas are easy to find.
  • 273 2 BBL ACTUAL WEIGHED AND CONFIRMED: Bobweight = 2168 grams (4 BBL pistons weight more but the pins are lighter so the piston + pin weight is the same, so bobweight is the same.)
  • 5.2 Magnum from reseached part weights: Bobweight 2161 grams
If you have better numbers or actual part weights, I'd really and truly LOVE to have that info. But if you don't have better info, then I am not sure on what basis you are disagreeing.....????

The 340's were set up for a different bobweight. Not any issue there.

One other aspect of balancing that most folks don't realize: It is rare to actually do a 100% rebalance on all 4 journals. Usually what happens is that weight is changed on the 2 large end counterweights. This does nothing to directly change the weighting for the 2 central crankpins (for cylinder 3&4 and 5&6). What is being counted on for those 2 is that, IF the rods and pistons are reasonably well matched, then the 2 pairs of pistons counterbalance each other for the most part. It is more complex than that, but the bottom line, is that 'balancing' only really goes part way to an optimum balance. Since the small counterweights by those 2 center crankpins usually go untouched, and the other small counterweights by the end crankpins often don't get touched, then it really is not all that well optimized. Field re-balancing not as scientific as you might think.

So, that is why for lo-po stuff, you can swap out STD bore pistons with overbore ones, never touch the balance, and never feel it. The bobweight changes but not all that much as to be felt in lo-po use.
 
In the next couple days I will search for specs about balance. I feel balance is important and I wouldn't mix-match components without rebalancing. Period.
 
I also think that the idea of a 273 crank in a Magnum is not good regardless whether it will "work" or not. Other than the fact that the 273 crankshaft is a forged steel crank it has absolutely no other benefit to go in a Magnum engine.
 
One thought I did have on this matter.. .while 30' up on a ladder in 28 degree temps this AM LOL..... I don't know of a reason for the Mopar engineers TO keep the same bobweight into the Magnum... but on the other hand, there might be production reasons to do so. (Like not changing tooling setups....)

BTW, I just corrected my ring weight numbers for the 273 2 BBL...my original number was a tad heavy. Now bobweight is 2163 grams (measured) vs 2161 gr for the 5.2....

toolmanmike said:
I also think that the idea of a 273 crank in a Magnum is not good regardless whether it will "work" or not. Other than the fact that the 273 crankshaft is a forged steel crank it has absolutely no other benefit to go in a Magnum engine.
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Can't see why not..... if it is a crank that does the same job, has the same oiling, same dimensions, etc..... what is the detriment to doing so??? Would anyone really object to the news that we put a 273 cast crank in my son's 340? Balanced up easily (by design), has the strength to do the HP/torque we wanted.... My Mitsubishi Starion 2.6L turbo rally engine has VW flat top forged pistons in it... running 1.75 HP per CI.

toolmanmike said:
In the next couple days I will search for specs about balance. I feel balance is important and I wouldn't mix-match components without rebalancing. Period.
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It would be interesting to see what can be dug up on factory balance tolerances....
 
nm9stheham said:
One thought I did have on this matter.. .while 30' up on a ladder in 28 degree temps this AM LOL..... I don't know of a reason for the Mopar engineers TO keep the same bobweight into the Magnum... but on the other hand, there might be production reasons to do so. (Like not changing tooling setups....)

BTW, I just corrected my ring weight numbers for the 273 2 BBL...my original number was a tad heavy. Now bobweight is 2163 grams (measured) vs 2161 gr for the 5.2....


Can't see why not..... if it is a crank that does the same job, has the same oiling, same dimensions, etc..... what is the detriment to doing so??? Would anyone really object to the news that we put a 273 cast crank in my son's 340? Balanced up easily (by design), has the strength to do the HP/torque we wanted.... My Mitsubishi Starion 2.6L turbo rally engine has VW flat top forged pistons in it... running 1.75 HP per CI.


It would be interesting to see what can be dug up on factory balance tolerances....
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Ok I'm calling it. I have been searching for specs and can't find much at all. I have more important things to do than argue a point that I can't find the answer to. You win. If you or the op wants to put a 273 crank in a 318 Magnum knock yourself out. Post your results when you do. I would be curious as to what has to be done to make it work correctly. (balancer, flywheel, flex plate, converter, balance etc) I thought Magnum rods were lighter and pistons as well. The OP would be lucky if the original 273 bob weight was the same as the Magnum 318. Good luck.
 
Maistrelli said:
Can a 273 crank be used in a 5.2 Magnum engine? Magnum damper on 273 crank?
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toolmanmike said:
In the next couple days I will search for specs about balance. I feel balance is important and I wouldn't mix-match components without rebalancing. Period.
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@nm9stheham I must apologize and I'm glad I found the answer about the interchange from reliable sources. I didn't want to shoot from the hip about them interchangeable unless it were true.

" LA and Magnum production crankshafts are very similar and can be interchanged as long as the proper rear main oil seal and pilot bearing are used. Early 273,318, and 340 engines used a production-forged crankshaft." Jim Szilagyi (Dodge Motorsports Engineering Dept.) How to Build Big Inch Mopar Small Blocks."
 
Hey, please, no apologies at all TMM! This is all in the spirit of getting to the truth of the matter, of everyone's benefit. You have listed a pretty definitive statement that helps things quite a bit.
  • Of course, we know that the 340 is not a balance interchange, with its higher bobweight.
  • And, as things evolved and combinations changed, the right sets of parts for the 273/318/5.2 need to be used to meet the target bobweight for those. So, in that sense, you are 100% right: a random interchange of parts may produce problems. For example: You can't put the super heavy 2 BBL wrist pins with 318 pistons and the later (758 gr) rods ... that would be waaay off.
Thanks!
 
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