The Stovebolt Site Forums The Shop Area Engine Shop Overdrive for SM420 4-speed

Does anyone know if there is a company that offers an overdrive for the SM420? If so, does it hang off the back end of the tranny? How long is it? Thanks. Michael

try all these, in no particular order:

http://www.gearvendors.com/
http://www.usgear.com/
http://www.advanceadapters.com/acrobat/saturn.pdf
http://www.high-impact.net/transmission_and_gear/rangeroverdrive.htm

What truck is it in, and what are your goals?
there are many overdrive transmissions that can be swapped for the same and mostly less money that the above "add on" overdrives.

Grigg

Thanks for the links Grigg. I have a NAPCO 4x4 setup on my 55 3100 shortbed. The truck starts screaming at 45mph so it would be nice to have that extra gear for tooling down the roads with 55mph speed limits. The problem is that it has a divorced transfer case so there is limited space between the tranny and the T.C. I was hoping that an overdrive would fit on the side of the SM420 where the PTO cover is rather than the back of the tranny or transfer case. Though I may look into the gearvendors because they claim that you install on the back of the T.C. with no need to modify the driveshaft. My only other solution is to get an NV4500 but it is 7" longer than the SM420. I have room to move the engine forward, but it would be nice to keep everything where it is. I could have a custom built mini shaft made. What is the shortest shaft that can be made? I think after the NV4500 went in I would have about 6" left for a shaft. And it would need to have a slip shaft for movement.

Have you thought about the NV4500 with a transfer case attached, like from a Dodge PU?

There is no such thing as an overdrive to go on the side of a transmission or on the PTO opening. It will have to be either before or after the transmission, and in line with the center of the input/output.

If you could attach the OD unit to the back of the NAPCO transfer case that may be an option. But I doubt they have a kit to do that. Is it a regular transfer case as used in other applications, or only NAPCO?

Many of the newer 2wd transmissions are quite long with the 2wd tail housings, this makes them more difficult to fit between the engine and your existing transfer case.

You can get some pretty short shafts, but 6" may be pushing it. Yhe need for aslip joint is part of the problem, as you need enough spline engagement to take the torque, that takes a little bit of length to make it work. I agree that you must have a slip joint there.
You can call a driveshaft shop, like http://www.4xshaft.com/index.html and ask what they can do.

Grigg

You might investigate changing the differential gears instead of putting in an overdrive. While you'll loose some of the low end it has now, you might be able to pick up some speed.

A gear change for a NAPCO front axle may be difficult?
Are different gear ratios available?

Grigg

Why would the shaft between the transmission and transfer case need a slip joint? Both the transmission and the Spicer 23 should be mounted pretty solid to the frame. The slip joints should be installed in the driveshafts coming from the transfer case and going out to the drive axles because of the articulation of the springs and suspension.

I believe the NV4500 would be the eaziest overdrive swap. If your truck is truly scraming at 45 mph perhaps you have the 5.14 gears. You could get gears that are as high as 3.90 to replace them, but those are very rare. 4.11s are much more available. The front and rear axles use the same gearsets, which were basically the same from about 47- 72 3/4 ton trucks.

Changing the gears would affect the offroad character of your truck. For wheeling the lower gears are usually desireable. If you were to change the transmission out to a NV4500, you would lose some of that low gearing as the NV4500 has a higher 1st gear ratio than the sm420... 5.61:1 compared to 7.05:1. The Spicer 23 transfer case is also pretty whimpy with the low range being less than 2:1 ratio. For a few dollars more you could get a married transfercase for the NV4500 with a better reduction for creeping along.

You do need a slip joint because the engine is mounted in rubber, the frame flexes, and the relationship between transmission and transfer case will change.
It will be much easier to install if there is some slip.
If there is no slip there will be extra end thrust loads on the output bearing in the transmission and input bearing on the transfer case. this binding could also cause problems with the engine and transfer case mounts, and also transmit noise and vibration between the two, and to the frame if the transfer case is solidly mounted.

Grigg

Thanks Grigg,
I knew there was a reason, I just couldn't visualize it.

I actually have no idea what axle was used in the Napco conversion. Given the challenges of the other approaches, I'd want to find out before committing to an overdrive in this case.

I've spent some time thinking about overdrive on a four-wheel-drive and aside from an OD transmission I don't like any of the choices, and without moving the transfer case it may be very difficult to convert this truck.

But I'll admit I know very little about the Napco stuf...

The original mini shaft had a slip joint so I figure that would be best. I thought about changing the differential gears, but they are in good original shape and it is difficult to find replacements. Yes, the front and the rear are the same. I really don't know what kind of axle the NAPCO is based on but it is a military design with the steering tie bar behind the axle and it has a round pumpkin. They look really cool. I could go with a married transfer case, but the divorced is what makes these old trucks so cool and rare. Also, it came with a Braden PTO winch so it has to keep the Spicer. Drive shafts are everywhere! I think I will stick with the NV4500 conversion, I know I can move the engine forward a bit and make it work. But I will definitely look into the shorter shaft first cause I want to be able to return the truck to original at a later date. Hell I might save the money and just stick with the SM420, you have to start out in 2nd and 4th keeps me from speeding! By the way, the 93-94 year NV4500 has a lower first gear 6.34 to 1, 95 and up had the higher first you mentioned. These are harder to find but there are shops that sell the package with the lower first. Still not as low as the SM420 though. Thanks for all the advice.

There may be a way to delete the 2wd long tail housing from a NV4500 and replace with a special short one. someone may have a kit, or I am sure something could be made to work.

I would swap to NV4500 with married transfer, and if you want to later you can still bolt the old stuff back in.
I am sure you can still drive the winch off the PTO opening on the NV4500.

Moving the engine forward sounds like more work than it's worth, along with the other small problems it will create.

Grigg

Grigg,
I think your right about the shorter NV4500. I spoke to a place in Tallahassee, FL and they said they can shorten it down a bit and add the fixed yoke I will need. It will still be longer but I think it may work. I forgot that the NV4500 had a PTO, I would have to figure out a lever system, but that may work. I'm building a webshot site and will post some pics of my truck adventure so you can see what these cool trucks look like.

Your axles are the same as std 1/2 ton, should be 3.90 gears. There was also a 4.10 and a 3.38 gears available as an option, but I don't beleive that either was used with the 4x4. There is a 3.38 aftermarket gearset available. Both axles use the same gear sets.

Bill, are you sure a 55 would have the 1/2 ton axles? I thought the 55 and 56 1/2 ton Napco conversions used a 3/4 ton 8 lug axle up front.

Now again I was wondering about the comment of the truck screaming at 45mph. Is it possible the transfer case is in the low range? Seems to me a truck with 3.90 gears and about a 30 inch tall tire should have no problems doing 65 mph at about 3000 rpm. I have a 59 2wd 1/2 ton longbed with the 3.90 rear gears. When I got my truck it had a tired sm420 which I have since changed out to a NV4500 from a dodge 2wd. I used the dodge version because it was eazier to adapt to my chassis, and it can be modified for a mechanical speedo. You won't have the problem with the speedo because it will hook to your transfer case.