Allison 6 Speed GMC 401
 

2006 6 speed Allison from a GMC Kodiak RV bolted to 401M engine. Hows that for keeping it all GMC? Will be programmed as a standalone to operate as a 5 speed. Bendtsens is finishing the flexplate up this week. driveshaft is at the driveshaft shop. Should be driving this month. Junkyard transmission on a 50 year old motor in an almost 70 year old bus. bet that's never been done before.

Re: Allison 6 Speed GMC 401
 

If it's a 6 speed, how come you can only use 5 gears?

Re: Allison 6 Speed GMC 401
 

5th and 6th are both overdrive. 6th gear would only be usable for speeds we don't travel and it is more $ to make 6th gear work in standalone. 6th gear is more suited to electronic engines.

Re: Allison 6 Speed GMC 401
 

Years ago I picked up a 401 with an Allison MT40 6 speed attached. The engine was junk, but I had the 6 speed rebuilt and still have it in storage.

Thinking then I'd never find a good 478 (that took another 10 years) I scrapped the componennts to bolt the 6 speed to the GMC engine.

Re: Allison 6 Speed GMC 401
 

Nice!
Is a control module required for stand-alone operation?
If you don't mind, how much was the flex plate?
hh

Re: Allison 6 Speed GMC 401
 

The MT40 was a non overdrive trans. cast iron case I believe and was unbelievably sturdy in its day. Its interesting to see how this aluminum case 1000 trans with overdrive and weighing about 200 lbs less has eclipsed the 40. handles probably double the torque and HP input. Yes a factory TCU is required along with the standalone harness and a linkage mounted TPS. flexplate was $600 and includes bolts, torque converter spacer and a custom machined center hub pilot to keep the tc hub centered in the crank end bore. Bob is the man on these engine/trans conversion adapters and custom flexplates. has done a lot with the GMC V6 engine conversion to various automatics.

Re: Allison 6 Speed GMC 401
 

That'll be sweet! :thumbsup:

Re: Allison 6 Speed GMC 401
 

Thats interesting that the tps is the only input.
That not too bad on the price considering what you get.
I haven't been able to find a flywheel for my 478 yet so that might be an option.

Re: Allison 6 Speed GMC 401
 

Multiple cracks in the flywheel and a broken bellhousing pretty much left me no options for a transmission. being remotely shifted pretty much rules out any stickshift option. could probably spend $5k building new parts and repairing and freshening the clutch. and I would still have the obsolete 70 year old transmission setup that no replacement parts exist for. plus with the Allison will have much needed overdrive.

Re: Allison 6 Speed GMC 401
 

Sounds like a good choice to me. Im looking forward to seeing how it goes.

Re: Allison 6 Speed GMC 401
 

For something like 15 years, I have had an Allison AT from a 6500 or 7500 chassis. Would that likely be an MT40? I know I received a cable shifter assembly with it, that is about 25 feet too short for my rear engine configuration.
If I recall correctly, I was told it would have been a 5 speed, but I am not sure. Were they five speed, or six?

The plan back then, was to put it into my RM7500 'Motorhome', along with a 478M; to replace my 401M and the 5x2 Manual transmission. No progress yet, on that plan.
Rod J
Issaquah, WA

Re: Allison 6 Speed GMC 401
 

Flexplate assembly from Bendtsens. consists of a GM factory plate from an old MT40 application. Bob shaved the face and drilled it for a factory Allison adapter ring. Then he machined a center pilot assembly to center the torque converter hub into the center of the plate. All very nice and very precise. fits like a glove. Don't think anybody else on the planet had these parts and the exp on the GMC V6. Glad I found him. Driveshaft is being built this week. All new Spicer yokes, tubes and axle companion flange. Gonna be a nice piece.

Re: Allison 6 Speed GMC 401
 

That's a very nice Flxible (yes, I spelled it right) Clipper you have there! Many of those were built with Buick straight 8 engines and were converted to GMC V-6 power in the 60's. I have a friend with a Clipper that has a 478 in it. Good luck, hope you get it back on the road soon. It's a light bus and it should really fly.

Re: Allison 6 Speed GMC 401
 

Yes ours had a 320 Buick inline 8 originally. It was converted to the V6 in 1971. Gonna be interesting to see how well it pulls 5th. The 478 would be unbelievable in a Flx. Tom

Re: Allison 6 Speed GMC 401
 

youtube.com/watch?v=itSs8SYvpbMhttp://youtube.com/watch?v=itSs8SYvpbM Might have posted this before.

Re: Allison 6 Speed GMC 401
 

I have to admit there are some big gaps in my mechanical knowledge and automatics definitely fall into that category.
I have been puzzling over the bolt/ hole setup on the center pilot assembly. I assume that the 6 countersunk holes are for the crank thru bolts. Are the other 6 holes threaded, I can't tell for certain?
h

Re: Allison 6 Speed GMC 401
 

correct those are thru bolts to the crank. the other holes are threaded and not used for this app. Bob must have been repurposing something he had in inventory. thx.

Re: Allison 6 Speed GMC 401
 

Cant wait to see pics of your progress.

Re: Allison 6 Speed GMC 401
 

Did 4,000 miles in 23 days. the 401 needed 2 qts of oil and aside from a load of separated gas ran great. The Allison 1000 operated perfectly. 60 mph is 2100 rpm instead of 2800 rpm with the non synchro non overdrive 4 spd Spicer. Got an astounding high of 9.5 mpg on mostly flat highway only runs of 350 miles. Highway only was 8 mpg with the old trans. Not the jump I thought I would get with the rpm reduction but even the prior 8 was very good. Combined highway/city was low 7s. Used the biggest Griffin trans cooler they make and never ran more than 170 degrees. The 401 is still very able but slow in the big hills. pulled a crazy steep hill up to an overlook in 1st gear at 2400 rpm. The guy behind us couldn't believe we made it. The bus felt like it could have pulled it all day as long as youre not in a hurry. Tom

Re: Allison 6 Speed GMC 401
 

Nothing wrong with the hill climbing ability that a turbocharger won't fix! That's the only device that makes diesels powerful.

Re: Allison 6 Speed GMC 401
 

Next move on the motor will be the new GMC 8.0l engine Bill. will try to run it on propane but will do gasoline if I cant get components. Chevrolet was a sponsor this year and hopefully next year. the 8.0l now has a part # and a price attached in long block crate motor form. they introduced it last year but no units have been available til recently. will see what they can do going forward on price and if they can open up the parts bin. Tom

Re: Allison 6 Speed GMC 401
 

Hmmmm, sounds like it's a big block Chevy. Nothing wrong with those. I have a 454 (468 actually) that's been wearing a turbo since 1986.

I might be interested in the 401 if you decide to let it go. I'm also in the northeast.

Re: Allison 6 Speed GMC 401
 

Did you ever installall the 8.0?

Re: Allison 6 Speed GMC 401
 

Have not done anything Bill aside from the last body lighting harness and conversion to all LED lighting last year. Engine conversion is not possible until rear suspension and axle can be worked out. Rear axle is unserviceable as there are no replacement parts available. The very short driveshaft requires everything in the back be laid out as a unit to achieve acceptable driveshaft angles. More pressing is front suspension and axle. Needs complete replacement. All tall order stuff. Our last big trip was Myrtle Beach SC. Did 625 in a single day. The 401 with overdrive allowed speeds up to 65 mph @2200 rpm and delivered 8-9 mpg. Previously speed was pretty much limited to 55mph @ 2900 rpm. Motor has a lot of miles so keeping it under 3000 pm made sense. Leaning towards a Duramax. GMC is re entering the Medium Duty market in 2019 with the 4500, 5500 and 6500 Silverado. There will be an air brake option on the Class 6 6500 Series trucks with an engine driven air brake compressor. The 8.0l gas engine had air brake compressor mounting brackets available but Duramax did not until now. You kind of need that factory engineered mounting system on these serpentine belt drive systems for reliability. Not all that simple to devise your own mount system. In the old days of separate V belts stuff like that was fairly easy. In all bus needs tall order stuff to keep going. In the meantime its collecting dust in the warehouse. Tom

Re: Allison 6 Speed GMC 401
 

What is worn out in the front end/axle? Is there a problem with the rear axle or are you worried about the lack of parts?

As much as I like my Chevy engines, one does not belong where a GMC big block V6 resides lol. Besides, Chevy used them too, so it could be argued they are a Chevy engine!

Re: Allison 6 Speed GMC 401
 

You are right Bill on the Chevy/GMC point. ive always considered the Chevy a GMC product in the larger sense.
The bus rear axle is like the original 4 speed Spicer. Replacement internal parts exist nowhere. The Allison modernized that aspect of the bus. And most of the Allison swap can be retained with an engine update. My guess is that the axle would not handle the torque of a Duramax engine. The 401 had to relocated 6 inches aft and 1.25 inches off center to get a good driveshaft angle. So you need a replacement axle assembly in place before a new engine cradle can be fabricated.
Front suspension has original air brake chambers and they are unique to the Timken steer axle. Bus was garage kept in CA and sat East in a barn for 24 years. We store it inside. Air system up front makes almost no water so those rubber diaphrams are still working. But they cant last forever. Doesn't make sense to be out on the road when they time out so bus is parked. Brake drums for front and rear are not available and it should have 4 new ones. Drums could be hogged out of a piece of iron for crazy $. Modern air chambers could probably be adapted as well with machine work but all of it is expensive and you still have obsolete components. Front needs leaf springs replaced as well as spring eyes. Front is also a little primitive with a buggy spring ride up front. Theres no sway bar system either. A modern steer axle with disc brakes, conventional shocks and a sway bar is very doable. We need a component mfg to help us with dimensional data and maybe some engineering oversight. The guys at SAF Holland have offered some limited assistance with their air beam components but support would not include and actual axle assembly. The aim is to use new components throughout. As I said its all tall order stuff so we need buy in from one of the big axle builders. We have something unique with this vehicle and a unique purpose for it. Tom

Re: Allison 6 Speed GMC 401
 

Perhaps components (front and rear axles) along with springs etc. could be used from a more modern transit style pusher school bus. Most modern school buses (last 30 years or so) are diesel powered and thus the axle ratio's are more friendly towards lower speed engines. They often turn up cheap and are generally well cared for. Grabbing one just out of school bus service may be a very economical way to score some good usable later model parts.

As for the engine, the modern diesels (Duramax included) are complicated and very expensive to repair. The emission controls, electronics and electronic components can be an expensive nightmare.

Power wise, the turbocharger saved the modern diesel from extinction. A naturally aspirated diesel produces less HP and torque than a same displacement gasoline engine. My point here is that the turbocharger works wonders on a gasoline engine just as it did on the diesels. Mild boost (6 lbs or less)on the 401 would yield wonders in the power and feel of your bus without over stressing the engine. The 401 could be turbocharged with quality components for less than the cost of a set on injectors for the modern electronic diesels lol. With a lower numeric axle ratio modern axle, some boost might be necessary and still be less stressful than a diesel on the bus.

If you are worried about the age and mileage on the 401, over the years I have sourced rebuild parts for my GMC V6's pretty cheaply. I always felt that an already good running engine could always be freshened (rings/bearings/gaskets/seals) without going to the elaborate expense of boring/turning the crank, and having to find rare and expensive replacement oversize parts. I've had good success with just freshening suitable older engines over the years if that's all they needed.



Do you do all of the work on the bus or hire it out?

Re: Allison 6 Speed GMC 401
 

The bus builder made engine chassis mount kits available in the 60s when these V6 motors were starting to become available. An owner could swing an entire V6 with the GMC SAE 3 flywheel housing, air brake compressor, brackets and pulleys from a 6500, 7500 or 8500 Series truck. Its was a quick and cheap upgrade that was light years ahead of the Buick 8. While our motor is very good mechanically it is underpowered. It will climb any hill or grade but it is be slow. Ive thought of adding a turbo but just not inclined. Later Duramax is getting pretty complicated but the earlier pre 2012 is very dependable and components are readily available anywhere. Aftermarket support is huge as well. Torque and HP is very adequate to keep up with interstate traffic in hill country. With availability of engine driven air brake compressor bracketry the swap is much more attractive. Fuel mileage would also be better than a gas engine. Axles could come from a school bus but what we really need is dimensional data from an axle builder. You could climb under a lot of vehicles and maybe not find what you need. Ive pretty much done all the work on this. Ive had help with some aspects of rewiring. Subbed out some light mechanical issues when it came out of the barn and subbed out a clutch R&R. Tom

Re: Allison 6 Speed GMC 401
 

The pre Jan 1, 2007 D-Maxes and others are the simplest (for electronic engines anyway). That's when DPF became mandatory and urea came along around M/Y 2011. There might be delete kits for that stuff if you get a later engine.

Is there room width wise for a 90degree V engine in the bus?

Re: Allison 6 Speed GMC 401
 

Yes. the BB Chevy engines fit fine. Not all that different than in size to the V6. The 8.0L has made its way into new production school buses and those bus builders have designed a accessory drive system that incorporates an air brake compressor. It makes the BB Chevy an attractive option and crate engine prices are good. That said a Duramax eliminates 85 gallons of gasoline from the bus. 06.5-07 LBZ is the preferred Duramax swap engine. Aftermarket wiring is available new and reliability and parts availability is excellent. The 2012 and later LML/LMM Duramax has very little swap support from the aftermarket. You cant run them without the emissions. An entire body harness is required to accomplish that swap. Not likely a harness and a computer is ever available. But who knows?

Re: Allison 6 Speed GMC 401
 

My only diesels are a Cummins B series with a mechanical P7100 pump, an NH series mechanical 250HP engine, a few Oldsmobiles with the DB2 pumps and a 6.5 Turbo diesel with a DB4 pump. All parts are dirt cheap for those engines and they are as simple as a rock. Had a couple of early Power Strokes and they seemed reliable.

I do like BB Chevy's and the 496 would be a good choice. Simpler and cheaper to maintain than a Duramax too. Maybe a 478M will come along or even a 637 V8.

Re: Allison 6 Speed GMC 401
 

Cummins engines are very heavy and the 6 cylinder doesn't fit. The 4BT fits. HP and torque are not all that impressive though. You are right though on reliability. Bus has 70 year old and 50 year old components throughout. Its the reason it sits in the warehouse. Needs late model updates. Reliability with the Duramax /Allison is excellent and servicing it would be pretty conventional.

Re: Allison 6 Speed GMC 401
 

While I believe the 4BT's can be upgraded for more power, the torsional vibration of a big powerful 4 could become a problem with driveline/rear axle components. I recall some years ago, the 4BT conversions were popular in lighter vehicles but the torsional vibration created all kinds of breakage issues.

Years ago when I drove for Greyhound, we had some MCI 12's I believe that had the 4 cylinder series 50 Detroits. Sluggish of the line but held road speed well, even on grades. On the road, it felt much bigger than it was.

Re: Allison 6 Speed GMC 401
 

Had not heard that about the 4BT. I'm more inclined to stay with newer components. If we were using this only locally you could do minimal upgrading and use it for a long time. We have done 4,000 mile trips in 3 weeks. We need modern serviceable components. Everything needs repair and servicing--even late model. Parts availability is a big issue when you are 1000 miles from home.

Re: Allison 6 Speed GMC 401
 

If you eventually pull the 401, I'd still be interested in it and the original Spicer transmission and clutch parts if you still have all of it. In the event you change your mind and decide to turbocharge, I can be of assistance.

Re: Allison 6 Speed GMC 401
 

Will keep it in mind Bill. Transmission stuff is gone. Remote shift tower went to Australia. Transmission is running a water well drilling rig in IN. Driveshaft and clutch went to MA. Thx.

Re: Allison 6 Speed GMC 401
 

What is used in school busses now is an 8.8 litre version not manufactured by GM. Though the block and components are supplied by GM the engines themselves are assembled by "Powertrain Solutions International" (PSI) to their specs and sold to Navistar for school busses. GM is going back into the medium duty truck business but will only market medium duty trucks as Chevrolet's, these trucks will be manufactured by Navistar under contract with GM. They will use a GM cab and powertrain on a International chassis, the Duramax Diesel will be the powerplant not the 8.1 or 8.8 litre V8. Word is GM is coming out with a larger 7.0 gas V8 based on the 6.0 V8, most likely a taller deck block.

Re: Allison 6 Speed GMC 401
 

Powertrain Integration is using the 8.0l from GM. GM has made it available in crate form too. Part# 19328586. Power Solutions Intl has I'm assuming a modified version of the 8.0. Theirs is the 8.8l. Both have devised a plate system to mount accessories on the front of the engine. Makes hanging an air brake compressor simple. Cheaper gas has made gas attractive again esp with the EPA regs on diesel engines. Diesel engines are expensive and the latest emissions very expensive to service. Fleet owners don't need those surprises and downtime. Both of these outfits have LPG versions of this engine. Its kind of cool how whats old is new again.

Re: Allison 6 Speed GMC 401
 

Wasn't aware of those two companies. So I researched it and on the PSI website what do I see?.........a turbocharged big block Chevy. A good read too.

As you can probably tell, I'm a big fan of turbocharged gasoline engines, but their naturally aspirated 8.8L engine looks good on paper too. I hope it catches on as some seem unhappy with the V10 Fords.

At the last job I worked at, we had a fleet of diesel trucks. The medium duties spent a lot of idle and low power time and clogging DPF's was a huge, expensive problem. One new '16 Mack tractor I was driving had constant problems with the urea injection system and I spent a total of a two days on the road in various Mack shops while they tried to figure it out. A '14 Freightliner tractor with a Cummins was trouble free in the 75,000 miles I drove it though.

During my school bus days, regeneration of the DPF was a constant problem. I always knew when the regeneration started, so I'd drop it into 4th and run it on the governor. I never had to do a parked regen but most of the other drivers did, sometimes with students aboard.

You couldn't give me a new diesel today and it looks to me like the big gasoline engines may be on a comeback for medium duty work. Where I live, diesel fuel runs $.60 more a gallon than regular gas, so the fuel cost per mile advantage of a fuel sucking modern diesel evaporates.

Re: Allison 6 Speed GMC 401
 

I agree on the modern emissions standard for diesels being complex and expensive. I believe domestic drilling and resulting lower gasoline price has made it attractive to the Medium Duty market again. These 8.0 and 8.8 engines from these 2 companies debuted with LPG induction. The gasoline version is the 2nd version. A big thing with bus fleets is the early morning warm up period to get the interior up to temp. Gas and LPG have a lot cleaner warm up period. Have not seen the turboed 8.8. That's a pretty recent offering I think. Will take a look.