The Stovebolt Site Forums The Shop Area Engine Shop Motor Mounts for 261 in '53 3100

I am planning to put a 261 in my '53 3100, using the original bell housing and SM420 transmission that were on the 216. I am converting to open driveline.

Should I use the front engine mount and the original rear engine/transmission mounts?

Without the torque tube, does the transmission need additional support?

Lynn,
I have a 62 261 in my 51 truck. I used the 216 bellhousing and clutch setup, sm420 transmission, original 6 volt stomp starter, I modified the original 261 front plate but modified it to be a 216 plate so it bolted right up (so I assume using the 216 plate is what to do) and I also used the stock rear cross member. I do not have any additional support for the transmission. It is just hanging from the four 1/2”Bolts off the back of the bell housing. I have 770 miles on my truck since rebuild with no issues.

I didn’t have to figure out driveshaft stuff because my truck is a 1 ton and it had a bolt in shaft already.

The 4 speed needs no additional rear mount.
Mount the engine as original at front and bellhousing.

There was a thread on this many months back. It was determined that 1/2 ton torque tube trucks with 3 and 4 speed transmissions had the rear cross member installed from the factory.
Now you want to go open driveshaft, ok not a problem, you have the open yoke now, just need the rear bearing retainer for the sm420 to accept the open yoke and rear seal. There is not a reason in the world to remove your cross member, unless you want to purposely weaken your installation. My thoughts are that sm weighs over 160 pounds and now you want to remove the rear cross member and cantilever that weight off of 4 bolts . Like Grigg says the engineers of the time said it didnt need one but I will add those trucks that didn't have one also had a propeller shaft support bearing installed mid ship, that you do not have. There lies the difference IMHO.

Stock,
True. The one ton has the driveshaft steady bearing. 1/2 tons do not. I forgot to mention that. Thank you.

From the earlier thread linked by sstock and even the discussion here, I am still wondering about the presence/absence of a rear transmission cross member in a '53 3100 with SM420.

I did find the crossmember in the Factory Assembly Manual (section 7, sheet 3.05) for Automatic Transmission.

My truck had a 216 with the SM420 and torque tube. Front and rear engine mounts. There was no crossmember at the rear of the transmission.

All my agriculture mechanic experience tells me there should be something supporting the rear of the transmission.

Also, the question of a one piece open driveshaft VS a two piece with a carrier bearing. The driveline shop I do business with tells me they can build either, whichever I want.

"I did find the crossmember in the Factory Assembly Manual (section 7, sheet 3.05) for Automatic Transmission."

The reproduction "1947-1954/55" Factory Assembly Manual is actually a 1954/55st FAM.

There was no rear-transmission mount (or rear-transmission frame cross-member) in 1954. The rear-engine cross-member (and motor mounts) changed in 1954, allowing for the elimination for a rear transmission crossmember (unless you had a Hydra-Matic transmission).

It is quite common to find an old 47-53 truck with a missing rear transmission crossmember/mount (it was removed and never replaced). GM installed that cross-member and rear transmission mount because GM thought it was necessary.

I agree that the rear bolted-on crossmember needs to be there, but I t does not need to be attached to any part of a transmission which does not have a torque tube.
It is my opinion that the original wimpy mount bracket which was attached to the torque ball collar was there simply to stabilize the torque ball from undue vibration, extending the life of the cork gasket inside. This mount just was not sturdy enough to support any transmission weight. In fact, when the tiny rubber pad rots away, the two metal parts of the mount remain separated.
Carl

When braking, there is a lot of down force on the tail end of the transmission, when the drive train incorporates a torque tube.
When braking, there is a almost zero down force on the tail end of the transmission, when the drive train incorporates a Hotchkiss or open drive.

So I would say that the 47-53 truck rear-transmission frame cross-member is not needed when the drive train incorporates a Hotchkiss or open drive.
That said extra support does not hurt anything.
The rear-transmission frame cross-member might aid in preventing the frame from twisting.

Because of the design of the 47-53 rear engine mount cross member.
The engine & tranny weight want to pull the frame rails inward.
Hence the need for a rear-transmission frame cross-member.

GM redesigned the Rear Engine mount cross member in 1954 hence no need for a rear-transmission frame cross-member for trucks equipped with manual transmissions..

This weekend I am going to search the barn for the rear transmission crossmember. If it was ever removed from the truck in the past, by someone else in the family, it should be in there.

Here is a picture of what the cross member looks like. and HERE