The Stovebolt Site Forums The Shop Area Driveline Rear transmission support sm420

Looking at the forums on driveline, I need information on rear transmission supports. I’m putting a 1964 SM420 into my 1951 1/2 ton truck that use to have a 3 speed in it. I think I read that the bolt pattern on the bellhousing is the same but I couldn’t see agreement on whether or not as SM420 needed a rear transmission support. There is a cross member under the truck there but it is farther back than the length of the SM420. The flat plate with 6 oval holes was used to support the car? 3 speed but it doesn’t have the l-bracket with the two holes, just the flat plate. Please advise what is the proper support/ nonsupport. As always, this site is a wealth of information. Thanks.

Excellent question, I am watching as well. I am about to swap in an SM420 into my '53 in place of my 3-speed.

I just put my SM420 back in the truck, and unfortunately I didn't take pictures of the rear support while still out of the truck. I just took pictures as best I could. At least it's all clean and painted

Let me know if you still have questions. As you can see, the transmission is short of the rear support crossmember. There's a plate that bolts to the crossmember with rubber strips between it and the bracket that bolts to the back of the ransmission.

Wow- thanks for the quick reply. I forgot to mention I am going with an open driveshaft. The 1964 SM420 is also an open driveshaft so the rear bell of the torque tube shown in your pictures is not the one I have.

my my trucks with SM420 the transmission has no support, only front engine mount and one either side of bell housing.

The transmission with provisions for a rear mount likely came from an application with two side engine mounts, and the back of transmission was the third, no front engine mount.
Or with a torque tube rear a rear transmission mount may be necessary?

I've never seen an SM-420 with a rear transmission mount...all the big trucks had parking brake drums on the rear.

Mike B

Mike, what about in a 1/2 ton with a torque tube?

Torque tube or open, no rear mount for the SM420. When I converted to open, I removed the useless 3 speed cross member that was still there. It simply unbolted; 4 bolts each side. Good to have it gone since the power brake booster moves the m/c back and there would have been interference.

I just did a SM420 install after removing the 3 speed on my 53. It has the torque tube enclosed driveline. I kept the cross member and the rear transmission mount. The assembly manual does not say whether a cross member and mount was used or not on the 3100 torque tube. Are you guys saying there was no transmission crossmember or transmission mount used in the optional 4 speed on 3100 trucks with torque tubes?

The cross member I removed can be seen in the assembly manual sect 7, sheet 3.05. Looks like there were 5 bolts each side. Manual implies that it was used for the automatic, but I'm guessing the the 3 speed might have used it too. Anyway, the truck came to me with the 4 speed and a torque tube. The only use for the cross member was it provided a place to rest the torque tube on when the engine/trans was removed. I bought a 1967 SM420 and replaced the old SM420 and torque tube with the "new" trans and open drive shaft.

That is the same crossmember I have that was used on the three speed, but it also had the rear mount attached to it. See this: https://www.classicparts.com/1947-54-Rear-Transmission-Mount/productinfo/79-204/

It retains the rear two bolts of the torque tube to the mount and the mount is attached to the cross member so it does have a function other than to rest the torque tube when the trans is removed. I am inclined to think the sm420 used the cross member and rear mount as well to support the torque tube otherwise all the load of the driveshaft will be all on the four transmission mounting bolts themselves.

1947-53 trucks used a rear of transmission mount on a frame cross member.
These were sometimes removed when servicing transmissions from below.
This mount was there for a reason (necessary support).

The rear of engine crossmember changed in 1954.
1954/55st trucks with manual transmissions did not use a rear of manual transmission crossmember (and, therefore, no year of transmission mount.

However, a 1954/55st truck with a Hydra-Matic transmission had removable cross members at the front and rear of the transmission.

I have one of those cross members in my truck with a SM420. Whenever I've dropped the gearbox, I've always removed that cross member first otherwise it would be in my way. The TT would be temporarily supported via different means such as a bottle jack, nylon strap from frame rail to frame rail.

Thank you Tim for your clarification on this matter, it makes total sense.
Steve

Tim, it does not make sense to me. What size truck are you speaking of?
My 48-52 1 tons and 2 ton trucks did NOT have a cross member or mount at the rear of transmission.

Grigg

Sorry, and thanks, Grigg

47-53 31XX had the rear transmission support

1954/51st 31XX Hydra-Matic had a different style rear transmission support
No other size trucks in 47-55st had a rear transmission support.

http://chevy.oldcarmanualproject.com/parts/1929_54/258.htm

If you go back up to the 3rd post, you'll see that I attached pictures of the rear mount in my '49 3100 w/ SM420 and TT. I'm pretty sure this is all 100% original, though I've learned you can never be sure after 70 years.

For clarification: in both of Gord's posts, it looks like he/they is/are referring to a 1954 truck &/or the 1954 drawing in the FAM (for the Hydra-Matic transmission)?

[quote=Grigg]Tim, it does not make sense to me. What size truck are you speaking of? My 48-52 1 tons and 2 ton trucks did NOT have a cross member or mount at the rear of transmission. Grigg[/quote] 1/2 and 3/4 ton trucks with torque tubes, not the open driveshafts, you guys have the carrier bearings. Well the 3/4 ton trucks have carrier bearing but also have the front part of a torque tube setup, the enclosed part.

[quote=Greg Brown]If you go back up to the 3rd post, you'll see that I attached pictures of the rear mount in my '49 3100 w/ SM420 and TT. I'm pretty sure this is all 100% original, though I've learned you can never be sure after 70 years.[/quote] Thanks Greg for your post, mine is setup the same way as yours. Keep in mind that 1/2 ton trucks with sm420 are not depicted in the assembly manual.

As long as you have support near the front of the engine and either at the bellhousing OR under the trans you will be fine. Once the engine/bell/tranny are bolted together it's almost like having one piece from front to rear, pretty stiff combo.

I respectfully disagree with you 78Buckshot. I think the torque tube (enclosed driveshaft) that is solid from the rear end to the trans, needs the extra support of the crossmember and rear trans mount because of the up down motion of the rear end. Apparently the engineers at Chev. GMC thinks the same way.

Yup, I agree with you on that one, I might have misread as I thought he was going with an open driveshaft.

Transmission Cross member .......This has peak my interest..........I have owned 5 AD trucks all 1/2 ton short wheel base over the years ( all of unknown history). Not one has had a rear transmission cross member. Suspect all were lost over the years ?

A little research does indicate there should be 5 cross member on all trucks. See attached
My 1950 GMC 100 only has 4 as it sits.
Engine trans support diagrams do not really show the rear of the trans but only give the torque tube function. See attached

I am not going to look for a transmission support member as it appears to function fine as is.

Do you drive on a lot of bumpy roads?

In your count of crossmembers do you count the “rear engine support” as one or not? I’m not sure GM did.

Best I can tell they were counting four rivet in place members and one bolt in transmission cross member ????
There is a call out on the bottom of the frame chart that says:
“Structural cross members: those are attached to resist torsional frame stresses”
Guessing that means items like bed cross members are not in the 5 count.

Mine is missing the transmission cross member. Reposting my picture of the frame showing 4 members.

No bumpy roads.........but adding the cross member with trans mount can be added for $100. Considering adding the trans mount as a small winter project.
I see several new cross members have notes in the description they work with the CPP disk brake upgrade.

To beat the dead the horse just a little more.........

Reading the 1950 GMC shop manual. Under the 3 speed torque tube transmission removal it says .....” removed rear transmission support member ( only on 101 model).”
Same book on 4 speed transmission, torque tube and open drive shaft, long and short wheel base, no transmission rear support cross member is used. There is a drive shaft center bearing support member for the long drive shaft.

It is omitted for some reason in the manual but nevertheless it does exist. Here is a picture of a 52 survivor truck that just underwent a cab off. Seem to remember GMC used a longer bed on their 1/2 tons and had a carrier being didnt they?

^^^^^^^^^^^^^Good info and data point ^^^^^^^

GMC had a "long bed" 1/2 ton model (Chevrolet did not).
GMC had an "extension joint" to extend the enclosed driveshaft.

Tim, I seem to remember that picture maybe 6 months back on here. Did it still had the transmission crossmember and mount plate?

I do not know about the cross-member mount plate.

this illustration shows some sort of plate

Per the following link, you can see in the 5th pic, the cross member support plate.

sm420 support

Looks like it has four drilled holes for the cross member Tim

Bigger trucks (over 3/4 ton) had a supported carrier bearing. Needed for the longer bed and longer drive shaft. The torque tube needed some support and on the short bed trucks that was the cross member behind the trans mission. With out it some whip forces would only have the rubber in the rear axle to spring mount for control. The bigger trucks had the carrier bearing and a 2 piece drive shaft. and a cross member to suport it. You do have to be able to remove the support and cross member if you want to service the torque tube. The E brake set up (drum on the drive shaft) was also supported. The SM420 transmission will have either the torque tube bell end or the flat end with a seal depending on the driveshaft used. Those will interchange by the way.Older torque tube to a newer transmission? no problem. Just swap the bell end for the flat one and you are good to go. Older transmission to newer drive shaft? Again, no problem. Just swap the plate for the bell and you are good too. The output yoke matches the bell(torque tube) or it matches the the open drive shaft. I keep them together for each application. Does the length of the drive shaft matter? Probably. But the drive shaft in the Emissions cheating "heavy Half ton is a long one with no carrier bearing. Late 70s maybe. Keep the mount and the cross member. The Engineer had this discussion with the Accountant already. The Engineer won.

I like the way you put things Steve.
Steve

FYI, here,s my survivor 48-3609 heavy 3/4. I'm doing a clutch job on it. Prior to those new mounts the bellhousing was setting on the frame.... making getting the clutch and p-plate impossible to get out. I had to jack up the back of the engine about a inch to remove the parts. With the trans out it was then easy to put new mounts in. This truck is the 125" w-base flatbed, with the short carrier bearing drive shaft.

Wrenchguy..........what does the rear transmission cross member look like if it has one ?

Here is a look at one, the torque tube drive shaft is resting on it. Dont have a pic of it removed.
Steve