The Stovebolt Site Forums The Shop Area Engine Shop 5 Speed / Overdrive / . . . again?

This may have been discussed, like 5 million times here, but I wasn't really paying attention for the first 4 million or so times. (because I wasn't this far along yet.) I did a 'search' but have a question if someone can help.
On the following thread:
http://www.6066gmctrucks.org/spicer-5831-b.htm
there are several choices.
I have a SM-420 out of approx. a '55.
Can someone 'decipher' this 'code' and tell me if I can get a 5 speed with O/D??
or even the correct 'brownie box'
Oh yeah, it's for a 1.5 Ton.
:confused: Timmer

Yes, you could put another 4 speed behind the one you have but why? The linkage alone would be a challenge. You might find a 5 speed overdrive but likely the ones made for your truck will be direct in 5th. If your looking for more speed think about going to a 2 speed rear, assuming you don't already have one. Most common rears were 6:50/7:17 ratio. Twin trannies, AKA brownies, were not used in such a light duty truck.

Timmer, Yes there are 5 speed overdrives out there for these trucks. I'll get back with model numbers and makes etc. Redcoat

Clark 280-OD .8 overdrive - usually found in either 2 or 3 ton trucks and up.

OK
Check out this link:
http://www.classicchevy5speed.com/tremec_transmissions_tko_details.htm

Would this 'bolt up' direct if I have a SM420 now.

Timmer, I found three overdrives that ought to bolt right up. New Process 540G014, an .82 ratio
Clark 264VO, an .80 ratio and Spicer 315C which is a .79 ratio. I got these off a web site that don't remember. I'm looking for one of these transmissions for my 6100. If you find one I 'd like to hear all about it. Redcoat 54 6100

Timmer, thanks for the link to Tremec but man-o-man, you are talking about spending a BUNCH of money. These are what a lot of rodders use and they are nice trannys but much better suited to a car than your old truck. Yes it would likely bolt up but your talking $2K at least before this job is over. A decent used OD like the Clark one of the others suggested is a much better choice IMO. Good luck with your quest for speed.

I can tell you that the New Process 540 series of transmissions are not a bolt up replacement for the SM-420. I have one, the input shaft is longer and bigger around. the bolt holes are spaced different. I could be done, but you would need to pull every thing form the flywheel to the trans to make it happen.

Check out this page for some pics of the NP 540 trans; GMC Transmission Data , scroll down to "5-Speed Transmissions". Also be sure to check out the "Figuring RPM to MPH" info on the same page. then look at this page for pics of the bellhousing and clutch; GMC 401 Magnum V6 Engine , the bellhousing is for a GMC V6 but you would be looking at the same differences on the back side were the trans mounts.

Also these old OD transmissions have a Strange Shift Pattern strange pattern.

I'll be putting that Spicer 5831-B Auxiliary Transmission in my one ton, it will give me an overdrive & Underdrive. The trick to installing one of these is to be sure to get all the parts for it from the donor truck.

That TKO500 for $1,660 doesn't sound to bad. It sure beats scrounging the swap meets for JUNK then finding out that it has a crappy over-drive. There's something to be said for getting something nice that WORKS the first time around.

"The most valuable thing that you can spend is time."

Thanks guys for the info.
I may just sit tight for a while, but continue to research the 'correct' OD.
I just finished putting a Dana 70 with 4.56 in this truck this week. I was not sure that was going to work, but it did.
You can follow that post in "Big Bolts" under 'Finally, 65 mph @ 2500 rpm in my 1.5 Ton'
if you want to see the pics and the process.
Timmer