Looking to do some planning on my engine swap and want to start with the transmission. I have my original '48 SM420 in the truck behind the 216 and it needs new seals, but seems to be in great shape. I also have an SM420 out of a '56 truck or around there. I'm going to be doing a new gasket in whichever i go with, but should there be any difference between using one or the other? The original has the longer 'snout' on the back with that ball-joint thing. Does this mean my driveshaft would be too short if I went without it?
FYI, I will be dropping in a 261 and the rear has been changed to 4.10.
The '48 two piece driveshaft is half enclosed on the front. The '56 driveshaft is open on both halves. Not sure if it would be a bolt in, may require some length modification.
Photo #1 is indeed the torque tube. Photo #2 is for a conventional open drive shaft.
Martin '62 Chevy C-10 Stepside Shortbed (Restomod in progress) '47 Chevy 3100 5 Window (long term project) ‘65 Chevy Biscayne (Emily) ‘39 Dodge Business Coupe (Clarence) “I fought the law and the law won" now I are a retired one! Support those brave men/women who stand the "Thin Blue Line"! Hug a cop! USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)
Ok, I guess that answers a question that should have been obvious So is there anything to gain by putting the later model trans in place with the torque tube?
You won't be able to make the later model trans work with a torque tube. You need to match the driveline with the tranny and rear axle that you're using. If you still have the torque tube rear axle, you'll need to use the earlier tranny.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
Ok, I was told that the trans case was the same and I would only need to swap the rear bits. If this is not the case, I suppose I'll leave it at that.
What are the chances that the speedo gears in the newer trans would be for 4.10 gears and would make my current speedo more accurate? Pretty sure it came out of a later 3100...
The open shaft transmission is indeed out of a small truck with parking brakes in the rear axle drums. All of the big trucks have a parking brake attached at the rear of the transmission.
Mike B
Mike Boteler
1956 Chevy 3100 Resto Rod 1952 Willys M38 Army Jeep 1983 Jeep CJ7 Laredo 1984 Jeep CJ7 +++++ Silver Spring, MD
Thanks Mike, so being that this is out of a 3100, would the speedo gear in the other trans be more accurate since I've gone from 4.53 to 4.10? Can I just replace one or do they have to change as a set?
Goach2. The housing on the end of the trans can be swapped. The open trans will fit just fine, in place of the "closed" trans. You will need to take all the bits from the torque tube trans and put them on the open one if that is the way you choose to go. The speedometer drive is a matched pair. So that will have to be changed over as well. 4.10 is standard for the 1/2 ton. So that set should work just fine. The housing on the tail will match right up, so will the trunnion bearing support. Gaskets sets often come with both sets in one box. There are some differences inside the 48 vs the 56. Some loose bearings vs a caged set. But the gaskets are all the same. I even think the speedo drive will swap just fine. The 48 is a good one, so just re-gasket, swap the speedo drive and you should be fine. No major disassembly would be needed. Read the manual to set the "ball" seal shims and put it back in. Washing will be the worst part of the job.
I think it’s a little loose there are bearings under there that you can replace ,there are kits and some videos online showing rebuilding the sm420.check you newer trans the same way and see how much it moves for comparison .I would also take the top of the trans off to see what it looks like inside
Last edited by KEVINSKI; 11/06/202110:59 PM. Reason: More imfo
Put a kit into it. Pretty easy includes all new bearings and needle bearings too and gaskets for about $125 on flea bay. You will need a press and some bearing separators from harbor freight. I've done about a dozen of the sm420, pretty straightforward. Syncros are still available but if it is shifting well wouldnt bother.
1953 Chevrolet 3100 261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY Gallery Video of the 261 running
1964 GMC 1000 305 Big Block V6, sm420, the next cab off restoration
Spare (newer) trans has less wiggle, but clearly it was exposed to a bit of weather. Has different coloured synchros too. Not sure if there's any value in rebuilding one over the other...
I have one like that ,looks like you will have enough parts to make one good one there are some parts that are not available for the sm 420 .look up Denny Graham up top on the member list he has a good pictorial of a rebuild as well as a stand for the transmission.
When I got this truck, it had no rear motor mounts. I would say that would likely cause ths input bearing wear. Since everything else looks good and I've been driving it for 5 years with no complaint, can I change out this bearing only and regasket and move on? I don't have access to a press and the secondary shaft with the freeze plug type end worries me.
I have only taken one apart so far ,there are a few videos on YouTube.If it worked okay before then that may be all that needs to be done .did you open up the top of the original transmission to see how it is .My truck has the origional still in it and I plan on putting a newer trans from a 67 truck as it looks good inside and has the switch for the back up lights in case I want to add the lights later.
I have no complaints about how it drove, but 2nd to 3rd was always the most difficult and it looks like this is about where it would pivot, so perhaps changing this bearing would be enough
This first set of synchros has some wear to them. The ones in the other trans look really good, however it's not brass. I've read that the brass ones are actually better than the later ones. So is it worth replacing some that have wear with some that aren't brass but in good shape?
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
Got a part number off the box in the youtube rebuild video. 6209NR 45x85x19 ball bearing with snap ring. I can get this locally, so hopefully it's the right one. Are input and output bearings the same?
The one I found said 40mm ID, so different than your 6209NR at 45mm.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
I'd go with the one from the video. I watched that and saw that number as well. I'm an expert on the SM420 now.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
Tried to get at the bearing ,but no luck I don’t have the proper tool ,I tried a punch but doesn’t move also the diagram I posted above does not look like a sm 420 but is similar .so there may only be the ball type bearing.
Not a whole lot of feedback on this, so here is my plan (up for critique):
1 - Replace the input bearing with 6209NR 45x85x19, still not sure what the rear shaft bearing is, so will leave it 2 - Leave the worn-looking 4th gear and synchro, as I have a set in perfect condition from a later model, but they're not brass and no one could confirm if it was worth swapping 3 - Change out all gaskets 4 - Replace the 10-3/4" clutch and pressure plate, if I can locate one.
4 - Replace the 10-3/4" clutch and pressure plate, if I can locate one.
I just ordered a 9" clutch kit from NAPA and saw that they had the 10-3/4" clutch listed as well.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
So is there anything to gain by putting the later model trans in place with the torque tube?
