Stovebolt.com Forums The Parking Lot Big Bolts SM420 - Swap or Gear Vendor Overdrive

I would like to ask for input on the options that are available when replacing an SM420 and maintaining the strength and durability if not exceeding it while increasing highway speeds at a suitable RPM.

At this point Differential Gearing is not an option that I am ready to explore.

The first thing on my list is purchasing a Gear Vendor Overdrive unit. Although I would like to explore the options of a newer 5 or maybe even 6 speed Manual Transmission?

Right now I have a Small Block Chevrolet (SBC) mated to the original SM420 in a 1955 2 ton truck. I am working towards building a truck that will do long highway runs with a goose neck hauler (Even if it happens to be at a slower pace than the heavy haulers that are coming out now 60 years later.

I did find a bit of information regarding someone installing an Eaton 5 speed transmission behind a SBC which came out of a 1984 chevrolet C something or other.

Anyone familiar with any model of old truck I should be searching out/researching for a good manual transmission to snag as an alternative to the Gear Vendor Overdrive unit?

Thank you !

What truck and engine?

I haven’t used a Gear Vendors unit but some friends have and won’t again. In their experience a lot of money for not a terribly robust OD if using a truck as a truck. They thought failure was brought on by holding back with the engine on down hills.

NV-4500 should give OD. Be careful, normal 5 speed trans' that have OD added where 4th gear was located, NP-540, Clark, IHC, etc., do so by removing that gear, thus leaving a large gap in ratios just under direct. A two speed rear helps sometimes.

Eaton makes several OD trans, however many use SAE mounting systems. There are, however, bellhousing adapters that go from SAE engines to conventional flat trans' mounting systems. I have several if you would like a px.

Ed

Don't use gear vendors. It's way overpriced for what it is, and it won't hold up if you are actually going to be hauling like you say.

The simplest old school swap will probably be a clark vo transmission which will be a 5 speed overdrive. They are kind of rare, but they turn up every now and then. I'd like to hear from someone who has experience with the older ones because I'm curious if they will bolt to an sm420 bell.

One obvious choice is an nv4500. You can find these in '90's to 2007 chevy pickups, and they will bolt right up to a small block, but you will need to remove your bellhousing crossmember. If you want to keep your bellhousing crossmember, you will need an adapter plate to mate the transmission to the original bell housing. I did my swap this way nv4500 with sm420 bellhousing

Another option is the spicer 3053a which is an overdrive 5 speed. I've seen those transmissions adapted to chevy motors. Again, bellhousing crossmember removal

I just posted a transmission in the parts for sale forum. It's an eaton fuller fs4005b with all the factory parts to bolt it to a big block. It would be a good choice except for the lack of an overdrive and bellhousing crossmember removal

If you want an overdrive fuller transmission (apart from some of the newer units they've been putting out), you need to step up to a 6 speed, but that makes the weight go up as well. Their smaller 5 speeds weigh 230lbs which is in the nv4500 range as far as weight goes. The 6 speed overdrives weigh around 350lbs which is in nv5600 territory, but I would choose that over an nv5600 any day.

Another option would be to try and adapt a g56 or zf6 to your motor. Those are both 6 speed overdrive transmissions that are reasonably light.

If you are still reading, haha, I would suggest swapping your sbc for a 12 valve cummins (you should have room in there), and mating it to an fso6406 or fso8406. If I couldn't find a decent fso trans, my second choice would be a g56. Both of those transmissions will bolt to the cummins with factory parts, and you would end up with a simple, reliable, and strong drivetrain.

Advance Adapters sells the Ranger Torque splitter. It gows in front of the transmission, between it and the bellhousing. Your speedo and park brake remain intact. I just put one in my 58 2 ton. I am running a 327 that was in the truck when I got it. I dropped from 3600rpm @ 60 mph down to just under 3000rpm.

That said, mine is an older cast iron one (no idea how old, or what shape the insides are in, I just put it in along with the trans that was attached when I bough it!) Mine added some noise and vibration but, I believe a lt of the noise is the transmission. One of these days, I will pull it back out an pull it apart to replace bearings in both trans and OD. We hauled our 35' vintage travel trailer about 7 hours away in July with it and had no problems. We are heading a couple hours north of here with it later this week.

https://www.advanceadapters.com/pro...overdrive-27-with-a-4686-retainer-index/

The Gear Vendors unit is basically the same Laycock OD that was installed in 1950's and 60's European and British cars like the Jaguar, Austin-Healey, Volvo, etc. It's not terribly rugged, and it's very expensive to repair. It uses a cone clutch to engage the OD and it has its own small hydraulic pump built into the unit to provide pressure to apply the clutch. It didn't work well 50 years ago, and I haven't seen much evidence that any upgrades have been made to the 1950's technology. Run, do not walk away from that one!
Jerry

Thank you all for the information ! Wow !

I will do a bit of research on the NV4500

@Edpruss

Can you explain this a bit better for me I'm not familiar with these terms

"There are, however, bellhousing adapters that go from SAE engines to conventional flat trans' mounting systems"

I actually have a Corporate 2 speed rear in place right now that I will most likely lock in high gear.
I also have a spare set - Eaton 2 speed rear with the proper 6 bolt pattern that I will swap in when I am ready to purchase updated rims and radials.

@Whateverpruss

Do you happen to know what vehicles used the overdrive fuller 6 speed transmissions that you had mentioned above ?

Who makes the g56 or zf6 transmissions? What vehicles might have run these?

I definitely wont be going to a diesel engine anytime soon.

@69Cuda

I am looking into the "Chevy Ranger Torque Splitter Overdrive" that you have sent me as well. Looks like it could be a cost effective method of doing things until there is no way around a swap.

It would be nice to have an updated gearbox moving forward.

There is a a lot to think about here

Thank you all again

Has anyone come across the addition of some sort of E-brake assembly when running a NV4500 ??

I am really liking what I have read so far regarding the NV4500.

Readily available. Relatively inexpensive to purchase Re-manufactured.

All sorts of online support on swaps and tech !

Has anyone come across the addition of some sort of E-brake assembly when running a NV4500 ??

An NV4500 from a 90’s GM 3500hd or a step van (both with 10 lug wheels and therefore disc brakes rear) come with parking brake on the transmission.

Grigg is spot on with the ebrake applications. I've only seen one for sale that has the ebrake on it, but they are out there.

Those fuller 6 speeds came in all kinds of trucks. A lot of guys will buy a used '90's fl70 (freight liner) just for the drivetrain (cummins 12 valve and fuller 6 speed). Otherwise, hit the medium duty junkyards.

ZF makes the zf s6-650 and s6-750 (zf6 for short). These were used in ford trucks, but they were also used behind the duramax and the chevy 8.1 (496 big block) until 2007. The chevy versions are more rare, but they can be found. A chevy zf6 would bolt right up to your motor and only weighs around 230lbs.

The g56 is a Mercedes transmission that Dodge has been putting behind the cummins in their trucks since 2005. It weighs about the same as the zf6 and is constructed similarly as well.

Easiest swaps will be nv4500 and Chevy zf6 since they will both bolt up to your small block without adapters.

The main advantage of a 6 speed is that you don't have the bad 3-4 split that you see in all the 4 speeds as well as the nv4500. It's not a real bad split unless you are really hauling a lot of weight.

Ok Great !

I have also taken the time to research the advance adapters ranger overdrive and so far with regards to durability (from what I've read) the old school 4 speeds are still bit more reliable (noisy but more reliable) than a NV4500. The cost of a new Advance Adapters O.D. unit is cheaper than a re maunfactured NV4500. Even when factoring in then price of a rebuild kit for my SM420. Installation is straight forward and to the point.

This 2 speed rear will eliminate that 3-4 split issue that was mentioned along with the O.D. unit. !

I will call Advance Adapters tomorrow and get a bit more information on their aux trans.

Vrooommm

I am very interested in what you find out and decide to run.
69 cuda - website says overdrive you installed is 7 1/2 inches.
Is that total length? Did you move back your trans shifter that amount? What other details about install? Shorter driveline? Fabricate new clutch linkage?
Thanks! Mike

You all might consider that only the first couple of years of Dodge NV4500, I think, it's Dodge, had a compound low that I believe you'll want for crawling around in RV parks or other tight spaces when loaded. The rock crawler guys like the SM420 for that reason. SM465s have a good gear spread too but the front bearing retainer is too big for the sm420 bell housing.

The combination of an SM420 or 465 and a Spicer auxiliary either a 3-speed or a progressive 4-speed with give you good gear splits (~300 RPM) and a 17% overdrive. Grigg has such a setup as do I. Ernie, who advertises in the ATHS magazine has a wrecking yard in Riverside CA and when last I talked to him had at least one of these.

The common Spicer model is a 6231A or a 6241A or a 5831A. You want the "A" because the others have a deep under that was used with a five speed that didn't have a compound low. These were used in all sorts of heavy trucks like tandem-axle dump trucks where 2-speed axles either weren't available or were prohibitively expensive. Gas powered or 4-71 GMC diesel tandem-axle truck tractors also used them. The next larger 8xxx series is total overkill and was used behind like a 250, 335 or 350 Hp Cummins, (855 cu. In.) or a Hall Scott 1090 cu in.

Once properly set up, this combo will be reliable as a stone axe but the overdrive on the brownie will have a whine. I will take some practice to get on the the split-shift but in time it becomes second nature.

All of the nv4500s had a granny gear first. All of the Dodge nv4500's had a 5.61 1st gear. Chevy nv4500's had a 6.34 1st gear for '93 and '94, and then they went to the same gearing as the dodge transmissions.

Rock crawlers like the sm420 because of the small size, light weight, and the 7.05 first gear which is the deepest you'll find in a light duty transmission. The sm465 will give you a 6.55 1st gear. All of these granny gears are nice to start a heavy load off the line or to idle around parking lots; however, the main gripe that serious haulers have with these transmissions is the 3-4 split.

The sm420 and sm465 both have 1.7 3rd gears, and the nv4500 has a 1.67 3rd gear. It's a big jump when pulling a lot of weight to go from 1.7:1 or 1.67:1(3rd) to 1:1(4th). That's why the 6 speeds are sought out by haulers, and why you see the medium duty transmissions having a 1.2:1-1.4:1 gear before the direct 1:1. If you've hauled any weight with one of these 4 speeds or the nv4500 with a gas motor, you know about that 3-4 gear split.

Brownie boxes are great if you can find one at a reasonable price, but more likely than not, it will be cheaper to use a ranger overdrive or an nv4500.

NP-540 series have a nice 4th that lets one go 55 or 60 before shifting, no OD though. They do have rare OD ratios, however the gear splits are not very useful.

If one wishes to replace a 420, with something with a large retainer, it is easy enough to bore out the bellhousing.

Ed