Does anyone know if the rods are different between the forged crank and the cast crank engines?
The engine I have is a 72 or a 73 340. The numbers are not immediately clear but the prior owner said that it was a 73. It is supposed to run great but I'm considering a crank swap. I'd expect the journals to be the same size but it is the issue of balance that I'm not familiar with.

Side question: If I keep the engine as-is, what is the amount of torque converter balance weight?
Thanks, Greg

Rods are the same for all 340's, stock pistons weigh the same; the difference is the compression height between the cast/forged crank 340's. B&M makes a flex plate with the correct scallop for a cast crank motor, or most converters can be ordered with the correct weight.

Crank swap isn't really necessary unless you are pushing it really hard. The cast 340 cranks are pretty stout.

You would need a forged crank balancer as well and if you get one from a 68 or 69 the timing marks will be set for the timing tab on the right side of the motor. These cars had the cast iron water pump with a left side lower rad hose.

Great points made. I may just keep the cast crank in there.

Stock piston weight the same?,,,,if you change the compression height by almost a tenth of an inch from 1.84 to around 1.74...they are going to weight the same?

nhra shows the 71 piston at 672 grams..and the 72-73 piston at 658....so hell..dont worry about the balance ....lol

Those are NHRA weights. Stock weights for both are 720 grams plus pin.

There are two different rod forgings, the 496 and the 645 and they are the same, interchangeable, except for pins, pressed or floating.

Chrysler went to great lengths to make things interchangeable. Their balancing tolerances were pretty wide, as well.

R.

I put a forged crank in my 73 340 with stock replacement cast pistons. I had the assembly balanced. I asked them how far it was off and they said not too bad but it needed it.

You really didn't expect them to tell you the balance wasn't needed and you wasted your money, did you? Gene

He told me that if it wasn`t necessary he wouldn`t do it. I believe he was being honest with me. Does everyone in this world need to be a crook?

[/quote]Stock piston weight the same?,,,,if you change the compression height by almost a tenth of an inch from 1.84 to around 1.74...they are going to weight the same?[/quote]

Piston is physically the same size, pin location is drilled 0.10" higher.

This book has some good info: HP1405 Hot Rod Small Block Mopars

Mopar factory balance jobs were the worst of the Big Three and if you took any brand new '69 engine to a modern shop they'd tell you it needed to be balanced.

However, Mopar engines did not have a reputation of running rough or in any other way showing that they NEEDED a better balance.

Just goes to show how fussy we are.

R.