The Stovebolt Site Forums The Shop Area Engine Shop SM420 conversion

I am pretty sure about going with a change of rear end for my truck. I found a 3:42 94 Blazer 4X4 rearend for $130. Someone posted in the past that they changed their SM420 from a closed driveline to open drive line tranny. I don't read much about this on this site. Is it possible to keep the original 420 and convert the back side to accept an open driveline?

What parts am I looking for to do this?

Also, with the rearend I am looking at, would any/all of these changes be worth it?

I thought the SM-420 didn't start until around 1948. If you are refering to your '46, I believe it would have the earlier non synchronized 4 speed. I have the non synchro 4 speed in my '40 GMC and have thought about converting to a synchronized sm420. Jim Carter sells a modified later style bellhousing that fits the sm420 and allows the pedals of the earlier trucks to bolt to the bellhousing. I believe this is what you would need to change from the earlier non-synchro 4 speed.

I too would like to hear from someone who has done this conversion.

Steve

Yep...that's what I have is the non synro 4 speed. New one on me..I thougth it was an SM 420..I never knew they changed in 48 or whenever.

What's the width (flange to flange) of the rear end your getting? I'm gonna convert mine too. But I think I'm going to use a pumpkin from a '55 and keep the same rear end housing (going to weld in spring perches).

John,

As I understand it, the rear width of an S-10 Blazer Auto 4X4 Axle housing is 59 inches wide. Our 46's are 58?? I don't know flange to flange width, but I think it is close to 55. If you search E-bay, some place in Flordia is always selling an S-10 rear...

I looked at a trailblazer rear, but they wanted $550...I know it is not that much..but ouch...$550 is still pretty steep when I can go with a s-10 for 130 and they are running the same ratio.

I am trying to come up with the best of the gear ratios with my T-5 and the rear end without getting rid of the foot starter and all of the other stuff hooked to the bell housing. I want this to turn out right without going too crazy.

Sounds like a decent match for width. I wouldn't mind checking into a S-10 rear but I have these wheels that I want to use. That's why I'm kinda commited to a 6 lug rear end.

I hear ya. I just powder coated mine last January and got new hubcaps and tires also.

Both the non-syncro 4 speed used from as early as 1931 to 1946 and the SM420 can be converted to an open driveline. The rear bearing retainer on 41-46 has to be changed to a retainer used on 39-46 COE, school bus, or other large truck with an open drive line. Parts book indicates even 1 ton trucks in later years had the bearing retainer for open drive lines.

Thanks Rusty46, That's the info I am looking for.

Is that Blazer rear end you picked up 5 or 6 lug? Eventually, I may change over also. If it s a 6 bolt, it would probably work on my '40 also.
Steve

I have not picked it up yet. That is still my problem. I think it is 5...I want 6.

If you do get an SM 420, I thought it might be worth mentioning that Gear Vendors does make an overdrive for this transmission. Every gear can be overdriven, which would be useful on this transmission because you could overdrive the super-granny gear as well as 4th.

I'm thinking about that for my truck.

I was at the wrecking yard with my trusty tape measure the other day checking out the rearends. I noticed that the '78 Jeep Cherokee might be worth checking out. The outer width of the rearend (mounting face of the wheel) was approx. 60" which might be close enough. And it had a 6 lugs. The spacing between lugs was 2 3/4"....just like chevys. The middle portion of the housing where the pumpking goes in looked like it was closer the the driver side. Maybe a 2 wheel drive unit would have a centered pumpkin.

I used to run across a lot of limited slip units in the old Wagoneers and J-10's. ;=)
Speed