The Stovebolt Site Forums The Shop Area Engine Shop freeway flyi'n

i know this has probably been addressed to death but i wanted to make sure i get a few of my questions out of the way.

if you want a little back story read this.
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=514138#Post514138

so i read a few of the articles here on transmission swapping and i guess the T-5 is the way to go. only problem is that most of the info i got was for smaller trucks. i have a 1 1/5 ton dump bed army rig. so i just wanted to get a few concrete answers in regards to a transmission swap.

I'm running a 235, which one I'm not sure. i am pretty sure that my uncle updated it to a rather new engine when he made the swap from the original engine. its paired up to the 4 speed crashbox, which really needs to go.

i wont have any issues with fabbing up new mounts, that's no problem. i have access to a full fab shop and pretty much all the material i can get my hands on. its the drive line and pedal connections i need a little help with. i would like to keep as many of the original pedals and what not without a whole bunch of work. i mean if im going to go all the way i could just hook up a hydraulic clutch slave system and a dual piston master cylinder and . . . yeah i would rather not get into all that.

so gentle men, what would you recommend? and has this been done before or am i stepping onto new ground here?

Several options are available to you. I would try to find a SM420 transmission, which is the tranny used in trucks from 1947 to early 70's. This tranny is synchronized and will be much quieter. You will have to shorten the first driveshaft to make this work.

Is the rear end a two speed? There are several different ratios available for two speed rears--just make sure you don't get one with a low speed (high numerically) ratio. I had a 42 with a two speed Timken rear end that would go 50-60 comfortably with a full pressure 235.

If swapping to a two speed diffy is not an option, you might be able to find a "Brownie Box" which is an auxiliary transmission that would have underdrive, direct drive, and overdrive. There would be some fabrication involved including driveshafts and aux tranny mounts.

You may also consider a NV4500 or some other new transmsion with overdrive. I am not sure what type of bell housing these take so I can offer the least amount of info on this because I have not personally done this swap.

If you get an SM420 like Racecarl suggested, you may want to consider a Gear Vendors overdrive. I put one on my '51 3/4 ton and I love it. My top speed is now comfortably 70 MPH instead of 50 as it was with the 4.76 rear axle ratio. The Gear Vendors unit gives you overdrive in each of the top 3 gears (so you have 7 forward speeds if you really like shifting) and the overdrive is .78. I rarely use it in any gear but 4th. My truck had a throttle cable with a knob on the dashboard which I replaced with a push-pull switch and the original knob to activate the overdrive.

thanks for the replys.

well one thing i forgot to mention is this beast is a 4x4 and to be honest i have never looked at the connection from the output shaft to the transfer case. it donned on me today that this may be my biggest challenge. because of possible differing lengths of transmissions and if there is a drive shaft or not going to the transfer case, it may just be a direct connection at the transfer case. which i really really need and want to keep because its also where the PTO power comes from for the dump bed and the 12,000lb winch up front. and because I'm 1000 miles away i cant get any pictures of it or take a look myself. i could possibly get my dad to do it but hes busy most of the time.

also we need to keep in mind that both my front and rear differentials are 6.67:1 and my stock trans gears are.

1st: 7.06:1
2nd: 3.48:1
3rd: 1.71:1
4th: 1:1

also what is this Gear Vendors over drive? sounds like the ticket for me.

*Edit*

i just did a little bit of reading for the SM420 and i think it might be the best choice already, but i would need a taller 4th gear or possibly an over drive because its gears are basically the same as what i have now.

Here is a link to Gear Vendors.
http://www.gearvendors.com/index.html

if you're just going to use it to drive you could more than likely get away with a t5 trans; they just can't take much abuse. I just put one in my 4 x 4 behind a 283 and am going to see how it works. if it is to light for what I want , then I will put in an upgraded tremec unit. any o/d unit should go between the trans and transfer case as you always want both axles to be at the same ratio. there will always be some no brainer to get in it and have it in 4 w/d and have 1 axle in o/d and the other not and then you will find out which piece of your d/line that won't take the strain of opposing ratio's.
it does looks like a neat truck to fix up though.
ron

i think the SM420 is going to work especially if i up the tire size.

does anyone know if there are different gear ratios are available for the SM420? if i could get a .83 4th or any over drive kind of gear that would be perfect.

The SM420 is not an OD, it is 1 to 1 in 4th gear just like your original trans. T-5's are light duty and probably would be unsuitable for a larger vehicle. It is also unlikely that you can fit a Gear Vendors OD unit in the space between the trans & transfer case. You might look for a 5 speed from a late model full size truck. You are in for modifications to several components no matter what you select.
Fred

So is the truck here in Wasthington, or is it still in California? I'm just picturing it blowing a T-5 apart coming up out of California into Oregon or on another of the steep climbs along I-5...

That truck empty represents more load than the T-5 was designed to handle. I can't imagine it lasting very long.

As for the Gearvendors approach, first its a lot of money, and second they are long, and I suspect you'd be moving the transfer case a long way back to make it work.

I'm not clear on how you intend to use this truck, so I may be way off base, but if it were mine I'd look at getting both the transmission and the transfer case from a modern (careful--my definition of modern includes things built before you were born) four wheel drive, and adapting both.

Since you have a crashbox, you don't have the later bellhousing even though you may have a later motor. The bellhousing that became standard in '48 will take pretty much any GM manual transmission that would make sense in that truck. The 5-speed manual with OD and a transfer case would be a good choice.

The reason I suggest using both transmission and transfer case is two fold. First, I've broken some old transfer cases, and been completely disabled as a result. Second, if you get them together that is one less piece that has to be adapted.

What are your expectations? This truck isn't going to fly down the freeway no matter what transmission you put in it... It's heavy, not areodynamic, and underpowered...

well the Gear Vendors OD is out, that's more money than i was planning on spending on the entire truck project.

the trucks still in California and needs to come up to Washington. and all i need it to do is cruise at around 50mph instead of 35mph. which i think i can attain if i up the tire size and if i could possibly find a different 4th gear to put into the SM420. i know the SM420 is 1:1 which is what i want to change, i need a taller 4th than that. does anyone make different gears for these or am i stuck with stock gears? if so i may have to turn this trip into an adventure of the unbeaten path kind. which may take more time than i have.

i talked to a few people and they all agreed that they T5 would die a horrible death with the weight of this truck. and because it would take massive modifications to use.

the SM420 for the most part seems like its going to bolt up without a whole lot of issues, particularly because i can run the PTO i have now on it and keep my dump bed and my winch. what make the transfer case is i don't know, but i need to find out because its possible that there's another straight swap like the SM420 for the transfer case. if i cant find a direct swap for the transfer case then i cant change it, i have way way to much crap attached to that thing, my E brake is a drive line brake mounted to the transfer case, the PTO off of the transmission, the mounts, the drive shafts. all this would have to come out and change if i had to modify anything. which makes this into an entirely new headache.

thanks for all the help though.

If you are going to use the box and other things, yes it is to much for a t5. My statement was if you were to use it as just a driver with a light bed on it I think that t5 would work. the power is not overrated for a t5 and with a 6;whatever rear axle it would be usable, but not for working it .
what is on it for tires now?
ron

i believe it has 35" tires on it right now, im not positive but i vaguely remember measuring at one point. with the 6.67's in the axles i should be able to turn pretty much anything i want. im thinking of upping to a 40" tire.

what size are they?
ron

Is there a Clark 5 speed that has o/d for larger trucks that could be used.

I know it will make for a long drive, but I think I'd advise putting your time and money into brakes and reliability issues and dealing with transmission stuff after you are home.

In the early 80's I drove a '51 GMC mounted on earlier 1-ton military four wheel drive stuff several hundred miles. It was an adventure I'll never forget.

If you want to move faster than it goes now use a trailer.

if you look around you may find one. I just sold a NP 5spd o/d that I had in my 4 x 4 with a 453 detroit
ron

brakes, wiring, fluids, all that sort of stuff with be changed and re done. wheel bearings greased and so on. the engine is going to be rebuilt and possibly hopped up a little bit.

the transmission was what i was most worried about. i may want to do more serious work in the future but i think the SM420 paired up to bigger tires will probably get the job done. all this five speed and transfer case talk is good to know but all that would be plans for later in the works. the week build would just be to make a reliable driver out of it.

a little bird told me however that the CCKW, which is the 2 1/2 ton GMC version of my truck had a 5 speed in it. im going to look into that.