The Stovebolt Site Forums The Parking Lot Big Bolts SM 420 in a 1945 1.5 ton

I have one of these, (Came out of a low milage 53 truck) want to replace the 4 speed crashbox in my 45. Will I need to cut the driveshaft?

Hy Drydock, unfortunately it's not all that easy. The old sliding gear transmission has a smaller bolt pattern than the SM420, add to that the fact that the clutch/brake pedal shaft is bolted to the bellhousing. There is no factory bellhousing which will allow the swap. Apparently Jim Carter sells a bellhousing (custom) that will allow this swap. I do believe the SM420 is a bit longer than the sliding gear transmission, so a driveshaft adjustment will probably have to be made, good luck with the swap.

Recieved an engine, clutch/housing and transmission together, wondering if the whole unit would go in. Came out of a 53, (engine is a rebuilt 57) so the mounts are in the right place, but it sounds like no place to mount the clutch+brake assembly. Darn it!

Well, can always take it apart and sling in the engine. Might have to look up that Carter bell housing though.

Went to Jim Carter, they had a modified housing in stock, went ahead and got it. Not as much as I thought! Thanks!

Good find, now one can bolt up any GM bolt pattern to that bellhousing-465, 540, Clark, etc., driveline will not all be the same, however. I have welded clutch/brake pedals to original bellhousing, also.

Drydock,

As you are most likely aware, the SM420 is a little longer than the 45 transmission. I have a similar setup put together from a previous owner, and the difference in length was accommodated in the driveshaft splined slipjoint. If you plan to stumpjump, it may bottom out but otherwise should be okay.

Paul

Lovely thought! Unfortunatly, on a 1.5 ton the drive shaft is one piece back to a carrier bearing, then you get a slip joint!

Not a problem, got a good local machine shop that can shortent the shaft.

Hey drydock, i am considering a very similar conversion. I have a 1941 Chevy 1.5 ton long wheelbase. I came into a 1961 Chevy 261 with the SM420 out of a K10. I'm putting the engine into my truck, but now you have me thinking about the transmission. Is this transmission (the SM420) far superior to my stock transmission? Like everyone, I'm on a budget. But, I want to get this right the 1st time around. And, I should probably mention that I'm not much of a mechanic and I will be doing this myself with some professional input. Also, when you get an estimate on having the drive shaft shortened, could you shoot me a note? If it appears to complex, I will opt for using my oem bell housing and transmission. I'm just gonna drive it around town. I don't anticipate any awards or media coverage. : )

The SM-420 is a sycronized transmission and is far better than your non-sycronized trans. They both have 1:1 high gear so the travel speed will be the same.

As far as the drive shaft, that's the easiest part of the project...don't let that be a deciding factor. Grab the drive shaft out of the K10 so you have the correct trans U-joint flanged yoke. Then contact you local drive shaft shop and have them tell you how they want you to measure for the new shaft. Between the K10 shaft and the one in the truck now you should have all of the expensive parts needed.

Mike B

If you've got the 420, I'd use it. Driving around town is where the Syncroed Tranny will help you the most, and it has a better torque rating than the crash box in it now. As you said, easier to do it now than later.

Be a while before I get the shaft shortened, but I consider that the least of my worries/expenses for now.

Spent 20 years in Uncle Sams canoe club myself,(85-05) went alongside the Coral Sea a few times.