The Stovebolt Site Forums The Parking Lot Tons 'o Fun 4BT swap

Grigg,
After reading through your P30 swap tech article I came to a little confusion on 4BT/bellhousing compatibility.

You claim that with a GM outfitted 4BT, installing the SM420 or 465 is nearly bolt in. I recently bought a rear end from a guy who had a P30 and was removing the 4BT from it. It had an SM465 coupled via an adapter to the 4BT. For a "bolt in" installation, would that 4BT happen to have the same bolt pattern as my 1954 1-ton 235 block? My bellhousing is the kind that wraps around the flywheel and attaches to the narrow 235 block, and the tranny mounts are on the bellhousing. I guess I'm confused at what bolt pattern the GM-outfitted 4BT has.

Any clarification would be greatly appreciated.

Thanks,
Skybum

You need the 4BT with everything except the actual transmission, so the adapter, flywheel, and bellhousing from the GM chassis 4BT.
Then a GM manual transmission bolts to it. If using an SM420 the bolt pattern is the same but would need to check the index diameter on the input shaft bearing retainer. The SM465 is 5.125" I think, and I'm thinking the SM420 is 4.937" or something like that? this can be dealt with one way or the other.

In reality an OD transmission is a good idea in most cases.
So before getting wrapped up in using the original transmission consider it's usefulness in the newly converted vehicle.

Grigg

Okay, thanks. Right now I have a good running 283 out of the wife's Camaro that I could drop in there but I don't really want to deal with all of the clearance issues with the steering box that I keep hearing about. I'm not even sure that block hugger headers would work in there.

If a 4BT was a viable option then I'd seriously consider that. I was hoping it would be easier than that though. That darn 235 bellhousing with it's tranny mounts causes a whole other realm of problems. I was trying to get away from having to "float" the engine and tranny in there while locating new engine mounts and new tranny tailshaft mounts. I think if I'm going to have to do that either way, then I would probably just go with the "free" 283 that I already have and figure out interference constraints.

Thanks Grigg, you're one heck of a knowledgeable source of information.

While your 235 bellhousing is limited in what you can bolt to it, the bellhousing mounts on a '54 will accept a later V-8 bellhousing.

That both simplifies at V-8 swap and opens the door to things like like a 250 or 292 six.

Like a bellhousing from a 60's or 70's truck?

I'll have to look into that as well. Hopefully there would be enough room with V8 heads to clear the firewall doghouse, and steering box.

The mounting pads are the same from '54 through at least '72. After that I'm not sure.

The clutch linkage varies and some of the bellhousings in the 60s are designed to use an hydraulic clutch. You probably don't want one of those. On the other hand a hydraulic throwout bearing can solve most clutch linkage problems and might be your solution. I'd just convert to an automatic if it were me.

After '67 the center hole on the transmission side is larger for the the SM465 and the SM420 would need an adapter ring of some sort (see Grigg's post above).

I'd look for a '55 to '59 bellhousing if I were making this swap. Others probably have more experience and might have other ideas.

Head clearance isn't as big a concern as distributor. The big cap HEI might hit the firewall. Of course a 283 did not come with one of those so that may not be a concern.

And you will still have steering box issues to deal with. I like the power steering conversion that puts the box ahead of the axle, but those kits are designed for 1/2-ton trucks and I've concluded would require some modification to work on a 1-ton.

Going back to your original question, that 4BT swap could be easier than the V-8 swap expect you can buy engine mounts for the V-8 and may have to fab your own for the Cummins.

My sense is you are looking for a swap that requires very little fabrication or re-engineering. You might be better off looking for a Chevy 261 which is a near bolt-in swap or perhaps a GMC 270 or 302, which is not quite as simple as the 261 but requires a lot less modification than a V-8 swap or the 4BT or a later six like the 250/292 motors.