quick MT to ATM swap question

[B17CRX]

55 2nd series originally a 4spd MT truck, when swapping in an ATM is it correct that I will need to remove the cross member that the original trans mounted to? (grey in pic) or does the ATM trans clear that and I just need to add my new cross member behind it?


-Joel

[pugs]

you will need to remove it

[mr48chev]

You will need to reinforce the frame though as the weight of the cab will roll the frame rails outward at the top if that covrossmember is removed. That's why a lot of these trucks look like they are sagging in the middle (they actually are) with the nose out of line with the cab.

I would either build a removable round tube crossemenber that could go under the trans or figure out how to trim the stock crossmember down. If you put an independent front end on it that may help stiffen it a bit. I junked the first frame I had under the 48 because of that sagging after I cut the crossmember out to put in an automatic.

[B17CRX]

Originally Posted by mr48chev

You will need to reinforce the frame though as the weight of the cab will roll the frame rails outward at the top if that covrossmember is removed. That's why a lot of these trucks look like they are sagging in the middle (they actually are) with the nose out of line with the cab.

I would either build a removable round tube crossemenber that could go under the trans or figure out how to trim the stock crossmember down. If you put an independent front end on it that may help stiffen it a bit. I junked the first frame I had under the 48 because of that sagging after I cut the crossmember out to put in an automatic.

What did the trucks that came with a factory automatic have for crossmembers? What you are saying is that the new transmission mount crossmember that I will have to add for the automatic won't be enough bracing right?

-Joel

[55Series2]

My truck was originally a Hydramatic. It had a removable cross member in the same location. I installed an independent front end and a new tailshaft mount cross member. Engine and trans mounted at three points. Original was engine mounted at 4 corners with trans hung off the back of the engine even with the Hydramatic.

[mr48chev]

The crossmember has been hacked out since the first engine/trans swap was done in these trucks but eventually it will cause problems with cab alignment.

Take a look at the photo you posted of your chassis and imagine the weight of the cab plus the weight of it's contents and driver and passenger pressing down on the end of the outriggers that the cab mounts to. Then imagine this weight working on that lever to twist the frame rail for several thousand miles. Now remember that otherwise really nice truck you spotted at an event in the past couple of years that looked great but somehow the cab and front clip didn't exactly line up right and the cab didn't line up square with the bed? If you were to look under it you would see that the bottom of the cab is riding on the frame rails because the frame has twisted out at the top.
I'd trial fit the engine and trans and see if I could maybe section the crossmember for trans clearance if it even needed modification. As big as the original bellhousings for a stick setup are I think a modern auto trans will clear the crossmember without modifications. The rub might be the ability to easily remove or service the trans but you could modify it for trans pan clearance.