[56GMC]

Well the response from this thread certainly favors replacing the Roto with a different auto or converting to a manual. Living with the 1-2 upshift is not a desirable plan even if I take care of all the leaks. Great to hear about the operational characteristics & background on the Roto. Would love to convert to a manual but I may have another option. My ‘56 GMC has a ‘73 vintage 455 coupled to a TH400 with 3:73 gears in the back. Thought about replacing that trans with a 4-speed OD auto as the cruising RPM is just too high & put the 400 into the Catalina after getting it resealed. Does anybody have a suggestion for the OD transmission source on the truck?

[Poncho60]

Wouldn't it be simpler to change the rear gears in the GMC?

[Bill Hanlon]

Originally Posted by Poncho60

Wouldn't it be simpler to change the rear gears in the GMC?

Depends. Some '56 half tons had an GM H033 rear axle (big round rear cover) with a 3.90: ratio like the Chevy trucks. Others had an optional Spicer 45 axle which was available with 3.92:1 and 3.07:1 gear ratios. I had a 3.07. 60 mph = 2000 rpm with 28" tires.

[carcrazy]

If it was my car it would be getting a turbo 400 unless you wanted to change it to a manual. I have had turbo 400's in the last two 62 GP's I've owned. I've never liked the way a "slim jim" performed.

[Poncho60]

3.73's in the rear per the original post.

[mgarblik]

Originally Posted by carcrazy

If it was my car it would be getting a turbo 400 unless you wanted to change it to a manual. I have had turbo 400's in the last two 62 GP's I've owned. I've never liked the way a "slim jim" performed.

The beauty of a Turbo 400 swap is very little needs to be done to make it plenty strong stock. For up to 450 HP and 500 Ft. Lbs. torque, one in good condition with a $50.00 Transgo shift kit is ready to rock. So simple to work on, parts very inexpensive. Only drawback is no overdrive with a stiff gear in the rear. A little heavy but may not be heavier than a Slim Jim. Not sure about that.

[redhawk44]

Where would I find that Blueprint?

Originally Posted by Keith Seymore

I threw a TH400 into my Grand Prix, mostly because I already had it.

It was from a '67 Riveria; since the bell housing was the same as the block an "adapter plate" per se was not needed but I needed someplace to hang the starter. Bendsten makes a "starter plate*, which I used and which allowed re-use of the production starter.

New one piece drive shaft and fabricated the console shift linkage using a coupling nut, threaded rod and a tie rod end from a John Deere garden tractor.

K




*blueprint for the starter plate can also be found in one of Pete McCarty's books, if you have the means to make one yourself.

[Stuart]

Like he said, it's in one of Pete McCarthy's books.

Specifically, the "Pontiac Racers and High Performance Handbook."

[Sirrotica]

Originally Posted by 56GMC

Well the response from this thread certainly favors replacing the Roto with a different auto or converting to a manual. Living with the 1-2 upshift is not a desirable plan even if I take care of all the leaks. Great to hear about the operational characteristics & background on the Roto. Would love to convert to a manual but I may have another option. My ‘56 GMC has a ‘73 vintage 455 coupled to a TH400 with 3:73 gears in the back. Thought about replacing that trans with a 4-speed OD auto as the cruising RPM is just too high & put the 400 into the Catalina after getting it resealed. Does anybody have a suggestion for the OD transmission source on the truck?

FWIW, if you have a truck and want an OD automatic, I would suggest the truck transmission that GM used for most of their higher HP light duty trucks, The 4L80E transmission. It has the same guts as a T400 with an OD gear added by the Hydramatic division. It is however a chevy bell pattern that can be easily adapted to a BOP pattern with a $50 plate.

It also requires a control module as it's electronically controlled. Lots of information around about swapping these tough transmissions based on the old reliable T400. These transmissions were being installed from 1991 through 2013 so there are plenty of used transmissions, and cores out there to pull a donor from. A rear axle from 3.50-4.10 would work out fine.

I have one in my turbo diesel dually with 270,000 miles on it so they are plenty durable.

I also have seen plenty of rotos replaced with T400s for the Pontiac I would also recommend one of those with the 2.69 axle.

[389]

Don't let them talk you out of the Roto Hydramatic, they are a fun transmission once there understood and you learn how to drive them. Most people just leave them in D and fry them in city traffic because they don't know any better... You need to hold them in gear and shift manually to keep the pump rpm up. Much like the 200r4 needs to be left in 3 and only put in over drive when on the expressway above 70 mph.

I would drive one of these big crates with a clutch before going through all that aggravation and money to swap one in. I've driven quite a few of them and my view is there much more fun with an automatic transmission, they are defiantly faster.. I once had a 64 Bonneville with a turbo 400, the trans had a shift kit so I could hold gears. It was shifter by a Hurst Autostick I shifter, this was one of the best set ups I ever had. I had a 2.75 first gear set installed, this worked well with a 2.56 rear end. I could pull out real hard and top end with most of the modern super cars, usually 140-150 (I did extensive suspension work, 8 degrees positive caster). I have the same shifter in my 69 el Camino 427/ Th400 and like it even more.

Frankly, I would learn how to rebuild the Slim Jim and have one ready to go. They are a fun transmission. You have to hold them in L to get them to preform. I had one in a 64 Grand Prix behind a 421, it was a super fun car!! The guy down the alley from me rebuilt those transmissions for a living so I had access.. He used to do a couple things to them but I don't know what. He said he got the ideas from a Hurst booklet in the sixties. Anyhow that GP really flew. You just put it in L and left it there, when it would wind out of first, it would kind of wind into second thru the stator (torque multiplication) those transmissions had, it would catch rubber and go like hell! It really is a cool transmission..
I used to love shifting into second and lugging around the city with the cool sound of that 421 and when you nailed it in second and it hit that super low first, all hell would brake loose..

So if your mechanically inclined, I'd buy another, build it up and have a spare. Its not like you depend on this car for transportation..

As for a stick, I'd really look into that, With a stock suspension, these cars are allready a handful in modern traffic. The last thing you need is a clutch at a red light on a steep hill with some nit wit right on your bumper..

I'll bet somebody on HAMB knows where to get that Hurst booklet for the Roto. https://www.jalopyjournal.com/forum/

[8LUG]

Well put 389, between 1963 & 1964 the slim jim had 7 different transmissions depending on horsepower ratings and type of vehicle. good rebuilders retired and shops just changed out the trans that was rebuilt.You get a rebuilt 267 hp unit and install it in a 350 hp grand prix. when it goes the trans is no good or you got the wrong trans for the hp rating as original and a rebuilder that said thats good enough

[Keith Seymore]

Originally Posted by 389

Don't let them talk you out of the Roto Hydramatic, they are a fun transmission once there understood and you learn how to drive them. Most people just leave them in D and fry them in city traffic because they don't know any better... You need to hold them in gear and shift manually to keep the pump rpm up. Much like the 200r4 needs to be left in 3 and only put in over drive when on the expressway above 70 mph.

I would drive one of these big crates with a clutch before going through all that aggravation and money to swap one in. I've driven quite a few of them and my view is there much more fun with an automatic transmission, they are defiantly faster.. I once had a 64 Bonneville with a turbo 400, the trans had a shift kit so I could hold gears. It was shifter by a Hurst Autostick I shifter, this was one of the best set ups I ever had. I had a 2.75 first gear set installed, this worked well with a 2.56 rear end. I could pull out real hard and top end with most of the modern super cars, usually 140-150 (I did extensive suspension work, 8 degrees positive caster). I have the same shifter in my 69 el Camino 427/ Th400 and like it even more.

Frankly, I would learn how to rebuild the Slim Jim and have one ready to go. They are a fun transmission. You have to hold them in L to get them to preform. I had one in a 64 Grand Prix behind a 421, it was a super fun car!! The guy down the alley from me rebuilt those transmissions for a living so I had access.. He used to do a couple things to them but I don't know what. He said he got the ideas from a Hurst booklet in the sixties. Anyhow that GP really flew. You just put it in L and left it there, when it would wind out of first, it would kind of wind into second thru the stator (torque multiplication) those transmissions had, it would catch rubber and go like hell! It really is a cool transmission..
I used to love shifting into second and lugging around the city with the cool sound of that 421 and when you nailed it in second and it hit that super low first, all hell would brake loose..

So if your mechanically inclined, I'd buy another, build it up and have a spare. Its not like you depend on this car for transportation..

As for a stick, I'd really look into that, With a stock suspension, these cars are allready a handful in modern traffic. The last thing you need is a clutch at a red light on a steep hill with some nit wit right on your bumper..

I'll bet somebody on HAMB knows where to get that Hurst booklet for the Roto. https://www.jalopyjournal.com/forum/

I have one available if you want it.

K

[Keith Seymore]

Originally Posted by redhawk44

Where would I find that Blueprint?

Pete's book, page 49.

K

[Keith Seymore]

Originally Posted by 389

Don't let them talk you out of the Roto Hydramatic, they are a fun transmission once there understood and you learn how to drive them.

I was being a bit facetious, since I have one that's junk and can't give it away.

K

[geeteeohguy]

Love the look of your car. If it were mine, I would do what another poster said: leave the 2.69 rear gear and install a '79-'82 BW ST-10 transmission with the steep 3.42 first gear. It'll haul the freight off the line AND get killer gas mileage at high speeds. Following that, a TH400 or TH350 would work just fine. I installed a 2.56 gear in the back of my '67 GTO with a TH400, and although off the line suffered, it runs 2450 RPM at 80 mph and gets over 20 mpg doing so. 90 mph cruising is 3,000 rpm. Hard not to like on a car that's driven a lot on $4.00 per gallon gas.

[tom s]

That is whats in my 421 62 GP,LOVE the combo.Tom