The Stovebolt Site Forums The Shop Area Engine Shop SM420 trans question

OK fellas, I have a `54 3100 truck, 3spd on the column, that is converted to open driveline. I was also given this sm420 to eliminate all the worn out shift box madness. So I`m looking at this trans, and it has a yoke coming out of the back of it. So it must have been in a two piece driveshaft truck. My question is: Is the spline on the output shaft going to be the same? Do I need to change out that rear housing, or look for another sm420 from a short wheelbase truck? As always, I appreciate your help. JD

I think all SM420's had the yoke attached, regardless of driveline type (tube or open) and number of driveshafts. There was no provision for a slip joint on the output shaft. Instead, vehicles with a single driveshaft had the slip joint integral to the shaft behind the front u-joint, similar to the enclosed type.

Yes, I have multiple version and none of them have a slip joint. You need a telescoping or 2 piece drive shaft with this transmission.

You can use a later Saginaw 4 speed with a floor shifter or convert your 3 speed to floor shift if you don't want to mess with the driveshaft, or get a new telescoping driveshaft made or find one that fits.

JD,

I'd look for another SM420 out of a 1/2 ton- it will hook right up to the torque tube. My '52 1/2 ton has one, which was obviously a replacement for the original 3-speed. You could probably trade your current 420 for the 1/2 ton style.

Tyler

Tyler, thats what I was thinking, however my 1/2 ton has already been converted from torque tube to an open driveline. Some how I was thinking the `55,`56,`57,`58,and`59 short 1/2 tons, with a 4spd. all used a one piece, slip yoke type driveshaft. Either way, I guess I will need to use either "their" driveshaft, or "their trans. Is that right? Thanks again, JD

JD, is there any particular reason you want to use the 4-speed, aside from eliminating the column shifter?

If the main goal is to switch to a 4-speed purely to eliminate the column shifter, and you are happy with the gearing as-is, I'd say forget it and just convert the 3-speed to floor shift as 4ontheFloor suggested. The shifter companies (such as Hurst) make universal 3-speed shifters just for that purpose, and it would be cheaper and require much less work than fooling with a tranny/driveshaft swap. I don't see an advantage to using the SM420, as you'll probably end up driving it like a 3-speed anyway, skipping the granny low 1st gear. In fact, with the SM420, even 2nd gear is lower than the 3-speed 1st gear.

I dunno, I use the super-granny gear sometimes. Especially if I'm stuck in rush hour traffic and my foot gets tired of stepping on the clutch constantly. Leave it in first and just chug along.

Also, that steep first gear is pretty good to use in really rough terrain.

there are several reasons I prefer the 420 over the stock 3 speed in early model chevies. First the 420 is synchronized in 2nd 3rd and 4th, the 3spd is only in 2nd and 3rd.the 420 is a top loader transmission-no linkage to screw up. And reverse is really low-great for backing large heavy trailers in tight places-something I do quite often with my 66 C30. I have had trucks with both and if you haul or tow anything with your truck the the 420 is the better of the 2 transmissions....Mike

I've got a 235 with a 420 on a 1/2 ton chasis with a 57 half ton rear end. I had to build the drive shaft from parts. The top drive shaft yoke is on a spline shaft. I was not aware a 420 was offerd on a 1/2 ton truck. Mine came out of a 1-1/2 ton truck. Yes, I usually start off in 2nd on pavement. I switched for the same reasons mentioned above. I thought the 4 speed floor shift offerd in 47 was a lot smaller transmission than then 420 but I may be wrong.