Required or . . . Crank/converter bushing

[Workingdog1]

Swapping a 318 to 340 that was a 4 speed. The 340 bushing/pilot came out ez enough. The 318 auto was being stubborn and decided to leave in for various reasons.

I had converted a manual to auto 45 years ago and do not recall dealing with the bushing in crank to accommodate the converter. Of course being more ignorant then ( maybe ), I probably bolted up and ran it.

After a frustrating amount of time looking for info and the actual part, I'd like to understand how the bushing relates to the assembly.

The only source found for a 1.81x1.55 inch bushing is in the picture. It took about a week to get and was the last piece of the puzzle needed for the swap. Engine biscuits needed were also a pain with Classic Industries being the savior.

 

[Professor Fate]

You didn't say what years your engine and transmission combination is, BUT-
The only time you need a convertor bushing is when you're mounting a '67 and earlier transmission/torque convertor to a '68 and up crankshaft.
The convertor noses changed diameter in approximately the '68 model year, becoming larger from then on. Therefore the crank flange had a larger diameter pocket. Using a early (small diameter nose) convertor on a later larger diameter crank with the larger pocket would supply no support and allow the convertor to move, causing all kinds of vibrations and harmonic issues, eventually cracking your flexplate and probably trashing the front pump.
The spacer (it's not really a bushing, since there is no difference in rotation between the convertor and crank, thus not requiring a plain bearing surface like on a true "bushing") makes up for the differences in diameter between the late crank pocket and the early convertor nose.
If you are running a late crank and late convertor, no spacer bushing is needed. If you are running an early crank and an early convertor, no spacer/bushing is needed. Only the early convertor and late crank gets a spacer. To run a late convertor on an early crank, machine work would be required to open up the pocket on the crank flange. Custom convertors can also be used to accomplish the swap in either combination, since factory convertors cannot be swapped between early and late transmissions- the input diameter and spline count changed at that time, too.

 

[Workingdog1]

You didn't say what years your engine and transmission combination is, BUT-
The only time you need a convertor bushing is when you're mounting a '67 and earlier transmission/torque convertor to a '68 and up crankshaft.
The convertor noses changed diameter in approximately the '68 model year, becoming larger from then on. Therefore the crank flange had a larger diameter pocket. Using a early (small diameter nose) convertor on a later larger diameter crank with the larger pocket would supply no support and allow the convertor to move, causing all kinds of vibrations and harmonic issues, eventually cracking your flexplate and probably trashing the front pump.
The spacer (it's not really a bushing, since there is no difference in rotation between the convertor and crank, thus not requiring a plain bearing surface like on a true "bushing") makes up for the differences in diameter between the late crank pocket and the early convertor nose.
If you are running a late crank and late convertor, no spacer bushing is needed. If you are running an early crank and an early convertor, no spacer/bushing is needed. Only the early convertor and late crank gets a spacer. To run a late convertor on an early crank, machine work would be required to open up the pocket on the crank flange. Custom convertors can also be used to accomplish the swap in either combination, since factory convertors cannot be swapped between early and late transmissions- the input diameter and spline count changed at that time, too.

Finally I get it and thanks for the details.
It's a '73 340 block supposedly forged crank and unknown year. Pushbutton 904 transmission guessing out of early Barracuda. Trans was rebuilt and existing converter redone to 3000 stall.
This makes more sense now as searched multiple posts trying to make sense of it all.

Thanks again !

 

[Workingdog1]

What's odd to me is/was availability or lack thereof.

 

[transman]

From 1968 and up, all 904 converters had a pilot (one that goes into the crank) size of 1.810 inches
From 1960 to 1967 the converter pilot size is 1.550 inch
Now, I may be wrong on this, but I think that the crank for a standard transmission was different from crank for an automatic transmission. I'm not 100% sure on this for a Chrysler (mine have all been automatics) but GM had two different cranks depending if the car had an automatic or a standard. The pilot size of a converter is larger than the size of the pilot of the sticks input shaft.