I've probably lost my mind

[convert2diesel]

This is most likely the strangest construct that I have ever envisioned. Have found a 1978, 26 ft. GMC Motorhome for sale with a blown Olds 403 engine. Been intrigued with these motorhomes since they were introduced some 40 years ago.

The better looking one and I have been planning on some long distance, 6 month per year traveling during our pending retirement and as such have been researching the appropriate RV for that purpose. We do like the GMC for its driving characteristics. Many describe it as a 26 foot hot rod with plumbing.

Having said that, it is after all a 40 year old motorhome with some inherent issues even when it was new. The beefed up Olds Toronado front subframe and drive system being top of the list.

My idea as to replace the entire front subframe with one from a GM 3/4 ton 4X4. Have done some initial measurements and it will work. That will allow me to install, hopefully a Duramax, 6.5 Turbo or maybe a Banks VM Moterie if the bank account allows. Coupled with either an Allison or a 6L80, it should make for a very tractable drive train.

My question is this. Would a good mechanical transfer case have a strong enough front drive to be used exclusively (the motorhome was front wheel drive) with the rear drive either covered over or a modified yoke installed to seal up the rear of the transfer case?

Will also be doing some major mods to the rest (stretch it 4 feet and install a living room slide) and a complete body off restoration, but the drive system is the biggy.

As always, any input, (other than I'm crazy...I already know that) would be appreciated.

Bill

[Rumpelstiltskin]

A gear driven Transfer case would take the abuse and would be strong enough, chain drive will have issues and oil problems.
I honestly would look into custom gear boxes, and redirect power units.
I wish I could offer more help, but I think you will be opening a can of worms trying to convert a transfer case.

[arveetek]

Very cool idea!

However, I'm afraid that the front diff from a 1 ton truck would not be strong enough for full time use lugging around a motorhome. The front diffs were never intended to be used for full power, only when split between the rear and front diffs for part time use. Think of it this way: in a 4WD 3500 truck application, the rear diff would usually have a 10.5" to 11.5" ring gear, while the front diff would only have a 9 to 9.5".

Having said that, I'm not sure what size the original Toronado "ring gear" is, but chances are, whichever engine you swap in will have more power output than the original motor had, so it would probably need a beefier drivetrain anyway....

What's so bad with the Toronado setup anyway? My dad is actually using that TH425 tranny/transaxle in a project he's currently building: a 1964 Corvair that will be converted to a mid-engine Chevy 350.

Casey

[convert2diesel]

A gear driven Transfer case would take the abuse and would be strong enough, chain drive will have issues and oil problems.
I honestly would look into custom gear boxes, and redirect power units.
I wish I could offer more help, but I think you will be opening a can of worms trying to convert a transfer case.

I agree. Atlas apparently makes a gear driven TC that is built to stand up to 700 ft/lbs or better. However, would have to see how they configure the output shaft bearings etc. Probably not intended for full time front drive only.

Very cool idea!

However, I'm afraid that the front diff from a 1 ton truck would not be strong enough for full time use lugging around a motorhome. The front diffs were never intended to be used for full power, only when split between the rear and front diffs for part time use. Think of it this way: in a 4WD 3500 truck application, the rear diff would usually have a 10.5" to 11.5" ring gear, while the front diff would only have a 9 to 9.5".

Having said that, I'm not sure what size the original Toronado "ring gear" is, but chances are, whichever engine you swap in will have more power output than the original motor had, so it would probably need a beefier drivetrain anyway....

What's so bad with the Toronado setup anyway? My dad is actually using that TH425 tranny/transaxle in a project he's currently building: a 1964 Corvair that will be converted to a mid-engine Chevy 350.

Casey

Not too concerned about the diff. My "B" and "D" body cars had 9.5 diffs and I have seen them built to significant specs. The 1/2 shafts on the 3500 are beefier and the output shafts also seem to be heavier.

My issue with the original setup is that the newest 425 available would still be over 35 years old, plus does not play well with the newer engines. Loved the older torgue monsters (the original Olds Toronado 455 was rated for over 500 ft/lbs.) but again they are also over 40 years old.

The reason I want to go with the swap over to the 3500 subframe is to allow me to go with newer powertrains and more substantial suspension bits. Just going with the newer unit front wheel bearings is reason enough. Better brakes, heavier torsion bars, etc. make for a better setup. The original GMC motorhome weighed in at 12,500 lbs. I assume I will be adding another 1,500 to 2,000 lbs with my modes. Well within the specs of a 3500.

I am going for reliability here and the ability to get it fixed when on the road. Finding a good used 425 in a junkyard while 2,000 miles from home is not my idea of a good time.

Thanks for the input. Finding the right transfer case is still my primary concern.

Bill

[trbankii]

Atlas is a standard in the off-road world. TWF is another: http://www.trailworthyfab.com/Hero-Transfer-Case/

I know they’ve built some transfer cases for custom applications such as 6x6 and such. They could likely build something for your requirements.

The other thing to consider is a high pinion front axle so that you’re operating on the drive side of the gears. A low pinion axle in the front is operating on the coast side of the gears - similar to a low pinion rear axle operating in reverse. The downside is that the high pinion axle will likely create some driveline clearance issues over a low pinion axle.

[convert2diesel]

Atlas is a standard in the off-road world. TWF is another: http://www.trailworthyfab.com/Hero-Transfer-Case/

I know they’ve built some transfer cases for custom applications such as 6x6 and such. They could likely build something for your requirements.

The other thing to consider is a high pinion front axle so that you’re operating on the drive side of the gears. A low pinion axle in the front is operating on the coast side of the gears - similar to a low pinion rear axle operating in reverse. The downside is that the high pinion axle will likely create some driveline clearance issues over a low pinion axle.

Good point about the high/low pinion. Never thought about it but you're right, it would be a consideration.

TWF is interesting. Wonder if they make a single speed TC. Either that or a 2 speed with OD? Would open up my trans selection to allow for a low speed/high torque engine. Actually considered an LS gas option but they seem to want to see reasonably high revs to achieve the peak torque numbers. As the engine essentially sits beside you in the GMC, noise is certainly a consideration.

Thanks. Obviously needs some more thinking before pulling the trigger on the project.

Bill

[Yukon6.2]

Hi Bill
I am sure that i read about someone using the Olds trans axle on a Dmax in the same motor home as you are looking for.
Might be worth some time searching the interwebs.

[trbankii]

TWF is interesting. Wonder if they make a single speed TC. Either that or a 2 speed with OD?

I had asked Todd about OD options and there was some reason that wasn’t possible. But I seem to recall a project with a single speed transfer case for a project - possibly military.

[convert2diesel]

Hi Bill
I am sure that i read about someone using the Olds trans axle on a Dmax in the same motor home as you are looking for.
Might be worth some time searching the interwebs.

When I first started thinking about this, that was my first option. The 425 trans is essentially a turbo 400 with the torque convertor and the front pump split off the main body and a chain drive transferring power to the forward oriented main trans body and thus to the front diff (bolted onto the tailstock). Built like the proverbial brick outhouse but lacks things like a lockup convertor and maintains the original gearing of a 400. The first Toronados (1968) had the circuit built into them that used the dual range torque convertor (giving you effectively a 6 speed trans) and the newer 425 can be built to accept it but finding that convertor would be an issue.

A simple adapter to convert the BOP bellhousing to the Chevy bolt pattern is required plus modification is required for a different motor mount arrangement (single front mount with two mounts on the back of the trans) and a different flywheel to accept the 400 convertor. Don't know if the trans would hold up to a Duramax over the long haul plus as I mentioned before, you would be starting out with a 40 year old transmission with limited parts availability.

The age of the components that would have to be overhauled, plus the uniqueness of the various powertrain and suspension parts is what got me thinking of the 3500 subframe and modern parts to update what was actually a brilliant design for a motorhome.

Bill

[convert2diesel]

I had asked Todd about OD options and there was some reason that wasn’t possible. But I seem to recall a project with a single speed transfer case for a project - possibly military.

Obviously some more digging is required. May have to go with a 2 speed and just lock it into high range or tighten up the ratio split to give it some advantage from a standing stop. Going to have to get under the crewcab with a tape measure and see how much room I have for another diff in the front now that you have added that issue to the list

Thank again

Bill

[trbankii]

Clearly, there are lots of vehicles out there with low pinion front diffs - but for the most part they are either part-time or at least take a lesser portion of the load for full-time use.

[joed]

For whatever this is worth...

A few years ago I was thinking about swapping a 6.5TD into one of the 1st gen toronados (1966-68 or so), but didn't like the 3 spd non-lockup transaxle.

I found that they did offer a 4-spd O.D. version from 1982-1985, with a lockup TC (I assume). Not sure if it could be built strong enough for a 6.5/motorhome use....Seems like there are some still out there, according to car-part.com.

Joe.

[convert2diesel]

For whatever this is worth...

A few years ago I was thinking about swapping a 6.5TD into one of the 1st gen toronados (1966-68 or so), but didn't like the 3 spd non-lockup transaxle.

I found that they did offer a 4-spd O.D. version from 1982-1985, with a lockup TC (I assume). Not sure if it could be built strong enough for a 6.5/motorhome use....Seems like there are some still out there, according to car-part.com.

Joe.

Before embarking on the subframe swap idea, I talked to a guy in California who is the guru of these transmissions about the 4 sp., lockup version. He was less than complimentary about them, saying they failed even with the V6 powerplants.

If memory serves, I believe he said it was based on the old thm200r4. Gear ratios in lower gears were great but would need a butt load of cash to strengthen them up to take the torque and even then, their lifespan would be suspect. Again old tech with limited parts availability.

Thanks.

Bill