Tranny swap in '71 GMC B-series school bus

[Gary Gable]

Hi All,

This is my first post, though I've lurked and read quite a bit on this site relative to my bus. Lots of excellent information and helpful replies offered by members. I hope to be as helpful in the future.

I have a set of questions. My bus, a 19000 GVW B-Series school bus about 26' long, came stock with a 350 mated to an SM465 with a parking brake on the output of the transmission. Although the motor was replaced with a Goodwrench within the last 50k, the tranny is original and the synchros are shot plus it leaks pretty bad. Additionally, the bus winds out at 50 mph, which is pretty much unacceptable if I ever care to drive on the freeway.

To remedy this I bought an NV4500 5 speed in good condition, and since it was out of a Chevy 3500HD it too had a parking brake on the output.

I want to pre-run as many of the swap issues as I can before I start. I know in advance I'll need to shorten the drive shaft, but as I've done this on previous occasions with my Jeep and there is a local shop that's excellent I'm not worried about that. The NV4500 is some 9.5" longer. I know I'll need to fab a cross brace. No problem, I'm a metal fabricator by trade. The output shafts, pinion diameter, and bell housing hole diameter are identical, so no problem there I think. Although the NV4500 had a hydraulic T/O bearing and my stock bus has mechanical, since I'm retaining the stock bell and replacing the flywheel, clutch, pressure plate and T/O bearing with stock parts I think it should mate right up. The NV4500 has no speedo attachment so I'll need to change my speedo to full electric vs cable driven.

Here are my questions:
1) The NV4500 drum diameter is 9.5", and the SM465 drum diameter is 11.5". As I need this drum to hold the bus on a hill, does anyone think (as I suspect) that a parking brake drum that is essentially 20% smaller will pose a problem? The bus GVW is 19000, and a 3500HD series pickup comes in at a max of around 13000 I think when all loaded up.
2) Do these transmission mounted parking brakes tend to have similar mounting interfaces? My wet dream fantasy is that the drum assembly on the SM465 will bolt right up to the NV4500. Has anybody had any experience with this?
3) No matter what, the parking brake cable will be too short for whatever parking brake assembly I use. Has anyone here had a custom cable made up, or have knowledge of other longer cables being commonly available for other applications that may work here?

Thank you for any and all help and advice, in advance. Feel free to weigh in with partial answers. I'm happy to sift through. I've found some fairly decent schematics but nothing that is approaching the detail I need to answer my questions.

Sincerely,

Gary

[rkn463]

Originally Posted by Gary Gable

Hi All,

This is my first post, though I've lurked and read quite a bit on this site relative to my bus. Lots of excellent information and helpful replies offered by members. I hope to be as helpful in the future.

...

Here are my questions:
1) The NV4500 drum diameter is 9.5", and the SM465 drum diameter is 11.5". As I need this drum to hold the bus on a hill, does anyone think (as I suspect) that a parking brake drum that is essentially 20% smaller will pose a problem? The bus GVW is 19000, and a 3500HD series pickup comes in at a max of around 13000 I think when all loaded up.
2) Do these transmission mounted parking brakes tend to have similar mounting interfaces? My wet dream fantasy is that the drum assembly on the SM465 will bolt right up to the NV4500. Has anybody had any experience with this?
3) No matter what, the parking brake cable will be too short for whatever parking brake assembly I use. Has anyone here had a custom cable made up, or have knowledge of other longer cables being commonly available for other applications that may work here?

Thank you for any and all help and advice, in advance. Feel free to weigh in with partial answers. I'm happy to sift through. I've found some fairly decent schematics but nothing that is approaching the detail I need to answer my questions.

Sincerely,
Gary

Hi Gary,

First off, welcome to the forum! It sounds like a good swap, what will you be using the bus for? Got any pictures to share?

Now on to your questions... the reality is that I don't know any of the answers for sure but will share my thoughts.

1) I'm a big fan of bigger is better, however from what I've seen in the manuals that I looked at the drum size seemed to be tied to the trans type rather than GVW. So I think I'd give it a try as is.

2) I have no idea about the mounting. On one hand GM is the king of interchangeability on the other hand it is different trans manufacturers. Would it be possible to pull the drums and measure the bolt circle for the two mounts?

3) look up Morse cables, there is so much available I think you'll be able to find something. One other option, if the park brake handle is under the dash now you might consider moving it next to the seat which would give you extra cable length.

Looking forward to seeing more.

-Richard

[Gary Gable]

Thank you for the reply Richard, and thanks for the welcome too!

With regard to the drum brake, it's pretty hard to figure out the way it's bolting up to the back of the transmissions until the drum is pulled off, and since I'm driving the bus once a week or so I don't want to pull it apart until I actually go to do the swap. I'm pretty sure they've both got a 4 bolt flange, but beyond that it's anyone's guess. If it's not something that can be adapted with some relatively simple machine work I'll just go with the existing assembly on the NV4500.

Yes, I thought about relocating the lever too. I think some have a floor mount lever right next to the shifter which keeps the whole cable really short. I'd like to keep it as stock as possible, not because I'm going to restore it to original, but just because the lever is in a good position now and it's very positive. I once had an '04 GMC (Isuzu) cabover W3500 that had a transmission drum that was super spongy and barely held. The automatic had parking bands too, which was good because the brake alone was completely inadequate.

Thanks for the tip on the cable, no matter what that will be a big help.

I have a bus license and I drive back up routes when one of my guys is sick or absent at the school. I'm either going to convert this into an activity bus to run kids or convert it into a camper. It's a 26' bus. The body is made by Superior, and was first purchased by the Airforce, then sold inter-agency to the forestry service, and finally surplused again and bought by the local sheriff to lug their officers down to the tarmac twice a year to do police cruiser pursuit training. I picked it up at auction. I didn't need it, I just like old stuff like this so it's a bit of an indulgence. See the pic. I've named the bus "lucky number 8".

Thanks again!

Gary

[rkn463]

Cool bus, that would make an awesome camper!
-Richard