3.31” Crankshafts

-
StreetSleeper

StreetSleeper

Well-Known Member
Joined
Jul 21, 2015
Messages
263
Reaction score
300
Location
32202
Are poly 318 crankshafts: are they interchangeable with LA- does that include any difficulties ?

Not all 273 cranks are forged correct?

Would a 1960’s forged 273 crank need much metal (Mallory) attached to the crankshaft if hypereutectic (lightweight) pistons and I beam aftermarket rods were used in a 318?

Would a 1960’s forged 318 crank need much metal (mallory) attached to the crankshaft if hypereutectic (lightweight) pistons and I beam aftermarket rods were used in a 340?

Am shopping, and I want to know all the options. Thanks
 
Last edited:
You can use the poly cranks. They are good cranks.

I’ve used 273 cranks with aluminum rods so I didn’t need heavy metal.

If you get the rotating assembly light enough you don’t need Mallory metal.

I’ve run poly cranks with steel rods and didn’t need Mallory metal.
 
For ages, the HD 318-3 crank was the "go-to" for any serious performance build.
BUT, be aware of the fact that poly cranks (manufactured up to '66, '67 for our northern neighbors) have a crank flange with a smaller pocket for the convertor nose, and if you are running a '68 or later automatic transmission you'll need to have that pocket enlarged for the convertor to fit.
A lighter reciprocating assembly will not require addition of any Mallory metal, but would most likely require some metal to be removed from the counterweights to get it balanced.
 
Some forged pistons, like SRP, are lighter than hypereutectic pistons, at least in 360 applications.

A LA 360 with aftermarket I-beams & SRP pistons will take little, if any, mallory metal. But I would give up a little weight and go with Scat I-beam instead of Eagle I-beam because they are stronger, if you plan to run it hard. I haven't tried it with a 340 but the shorter 3.31" stroke would work in your favor if using a 273 or 318 crank & aftermarket I-beams.

The earliest 273 cranks have a small converter register that the common, later converters won't fit into.

Earlier, lighter 273 rods are weaker & heavier than aftermarket I beams.



Did early, forged 318 cranks come with the 273 style rods & went to the bigger 340/360 style with cast cranks???
 
66fs said:
No, it seems to have been a running change, I have seen small rods on 68 and later 273's and 318's with cast cranks.
Click to expand...
Thanks. I wasn't sure since (if I remember correctly), forged steel is heavier than cast iron so the crank counterweight would be a factor. May have changed piston weight?
 
What an interesting discussion. I have to wonder if Mallory metal gets used to fine tune. Lets say we took a little too much. I'm no machinist by any means. I can barely measure pushrods.
 
Newbomb Turk said:
You can use the poly cranks. They are good cranks.

I’ve used 273 cranks with aluminum rods so I didn’t need heavy metal.

If you get the rotating assembly light enough you don’t need Mallory metal.

I’ve run poly cranks with steel rods and didn’t need Mallory metal.
Click to expand...
How long did the aluminum rods last in what engine and application sir?
 
StreetSleeper said:
How long did the aluminum rods last in what engine and application sir?
Click to expand...

At 250 runs I changed the rods. Never pushed them past that. That was at 8500.

Bill Miller says you can run his aluminum rods on the street for some big number of miles. 50k comes to mind.

I think about doing it all the time but I can buy steel rods way cheaper.

If I was drag racing it would have aluminum rods. It’s not an option for me.
 
Professor Fate said:
For ages, the HD 318-3 crank was the "go-to" for any serious performance build.
BUT, be aware of the fact that poly cranks (manufactured up to '66, '67 for our northern neighbors) have a crank flange with a smaller pocket for the convertor nose, and if you are running a '68 or later automatic transmission you'll need to have that pocket enlarged for the convertor to fit.
A lighter reciprocating assembly will not require addition of any Mallory metal, but would most likely require some metal to be removed from the counterweights to get it balanced.
Click to expand...
No 318's whether poly or LA had a small register for the converter. That was a '67 and earlier forged crank slant 6 and 273 thing only.
 
StreetSleeper said:
Are poly 318 crankshafts: are they interchangeable with LA- does that include any difficulties ?

Not all 273 cranks are forged correct?

Would a 1960’s forged 273 crank need much metal (Mallory) attached to the crankshaft if hypereutectic (lightweight) pistons and I beam aftermarket rods were used in a 318?

Would a 1960’s forged 318 crank need much metal (mallory) attached to the crankshaft if hypereutectic (lightweight) pistons and I beam aftermarket rods were used in a 340?

Am shopping, and I want to know all the options. Thanks
Click to expand...
'62 and later poly 318 cranks will work in an LA 273/318/340 engine. Just rebalance. Don't use the '61 and older as they have extended 8 bolt flanges.
273 cranks were forged from '64-7. '68-9 273 cranks were forged (and the same crank as a 318 those years)
Adding Mallory Metal will depend on which reciprocating parts are being used. Sometimes it's just a matter of some minor grinding/drilling....other times it's adding various amounts of MM.
 
So later 273s did come with cast cranks. Interesting. I've never seen one. I will say this though, about the lighter 273 and early 318 rods. Evidently, Mopar Performance didn't think they were much weaker because they offered a performance version of them with ARP rod bolts and those rods are pretty sought after today. I've said it before. I've never seen a broken factory rod, early light one or later. Let me clarify that. I've never seen a rod "just let go". I've seen some that were broken, but something else always caused it. Broken rod bolt, broken crankshaft, or "something" that had fallen into the crankcase, but never just a rod itself "just break" with no outside influence. Never. Not one.
 
Poly 318s used the puny connecting rods found in the 273. The poly crank I have is heavier than my 340 steel crank, and the 1-2 rod throw is not drilled through like it is on the 340. At a guess, I'd say that material was removed from the throw when it was offset by the heavier 340 rod and piston.

I have Bill Miller rods for one of my 340s, but it's not together yet. Per Bill hisself, they're good for 100K+ street miles. I know the the guys at (long-defunct) Warrior Racing were been using them in street engines in the 1980s, and had at least one daily-driven street/strip engine with 'em that was well over 100K miles. With BME rods and SRP pistons replacing the stock 340 parts, my reciprocating parts weigh 9.2lb less than OE. There's going to be a few pounds of crankshaft left on some machine-shop floor after balancing.
 
RustyRatRod said:
So later 273s did come with cast cranks. Interesting. I've never seen one. I will say this though, about the lighter 273 and early 318 rods. Evidently, Mopar Performance didn't think they were much weaker because they offered a performance version of them with ARP rod bolts and those rods are pretty sought after today. I've said it before. I've never seen a broken factory rod, early light one or later. Let me clarify that. I've never seen a rod "just let go". I've seen some that were broken, but something else always caused it. Broken rod bolt, broken crankshaft, or "something" that had fallen into the crankcase, but never just a rod itself "just break" with no outside influence. Never. Not one.
Click to expand...
I've seen OEM 273 rods run successfully in race applications but the ones Mopar sold through Direct Connection, including their standard size 340/360 version, were forged from 4340 material, like Scat I-beam and many H-beams rods today. Eagle SIR I-beam are made from 5140 material and I'm pretty sure OEM rods were another step down, but obviously still strong enough to take a lot of abuse.

I had a couple of sets but have been selling off my stuff. They had "Mopar" cast right into them!
Direct Connection/Mopar 273 style aftermarket rods.
 
I don't mean to sidetrack (but I'm gonna do it anyway!), but are old mallory slugs reusable?
I've got a badly cracked alcohol crank, with 14 slugs of mallory in it, of various sizes. is it worth my time to knock em out?
 
Locomotion said:
I've seen OEM 273 rods run successfully in race applications but the ones Mopar sold through Direct Connection, including their standard size 340/360 version, were forged from 4340 material, like Scat I-beam and many H-beams rods today. Eagle SIR I-beam are made from 5140 material and I'm pretty sure OEM rods were another step down, but obviously still strong enough to take a lot of abuse.

I had a couple of sets but have been selling off my stuff. They had "Mopar" cast right into them!
Direct Connection/Mopar 273 style aftermarket rods.
Click to expand...
Yeah that makes sense. It does seem I remember that, too. Even still, I like the lighter rods. Always wanted to do a 340 using them. I do have one set left over, too. lol
 
Molnar Technologies 6.123" 4340 H-beam connecting rods only weigh 623 grams and with lite pistons you'll be removing metal from the crank instead of worrying about adding metal.

Tom
 
You must log in or register to reply here.
-

Back
Top