Rebuilt Slim Jim vs TH400 Swap
 

I'm restoring my '64 GP which is a very original car and I'm trying to keep it as close to stock appearance as I can, except for safety/reliability upgrades like front disc brakes/dual chamber master cylinder, replacing the wiring, etc. It's unique in that it's a factory A/C car with power windows and other options, but it's no 421 tri-power, 4-speed which will be worth high 5 figures either.

Anyway, since it is a "numbers matching car," I've been debating trying to keep the stock Roto 10 (Slim Jim) transmission and having it rebuilt. I know the Slim Jim in it will need to be completely rebuilt (the car sat for 20 years in a dirt lot and transmission wouldn't shift when I bought it), but I'm debating between doing that or swapping in a rebuilt Pontiac TH-400 from the late 60s.

I have read on this forum of all the trouble and expense of rebuilding the Slim Jim and how much better the car will run with the TH-400. For the sake of keeping the car original and what the car may be worth when it's restored, is it worth trying to get the Slim Jim rebuilt? From the folks on this forum who have driven the Slim Jim cars and ones with later model transmissions who did the swap, what would you do? Would swapping a TH-400 make the car less or more desirable than keeping the transmission "numbers matching" with the Slim Jim? I appreciate others experiences, knowledge and opinions. Thank you.

Dump​ the Slim Jim.

Nobody's paying big dollars for these cars in 100% correct restored condition. Most potential buyers would likely be thrilled that the troublesome Slim Jim was replaced with a strong and reliable TH400, the automatic transmissions were a sore point on these cars and buyers will be hesitant to own one just because of that horrible trans.

Dump the Slim Jim and don't look back.

If the slim jim is properly rebuilt by someone who knows what they're doing, it should be good for 100,000 miles - however, they always did shift 'funny' (they kind of slide from one gear to the next, and the 1-2 shift is a weird big jump in ratios) so they just never had the performance of a TH400. I'd agree that except for maybe with a 100 point restoration, swapping to the TH400 makes more sense and will make the car a lot more enjoyable and reliable.

I'm not positive but I believe it has the standard 3.08 open rear end.

Scarebird is hitting on the downside. My 61 Safari was converted to a T400 . It has a 2:87 factory rear gear ratio. Probably excellent with an original hydro but with the 400 its hard for it to get out of its own way from a stoplight. I am going to have to address this with a 3 series gear set sooner or later.

x 2.

My impression is that the Slim Jim is not a very enjoyable trans even when functioning to design intent (which mine didn't).

The TH400 swap was not bad. I got mine used out of a '76 Riv for $75. The only expense for me was a starter mounting plate (since the starter mounts on the trans bellhousing in '63) which if yours is a '64 you may not have to deal with.

K

So based on your 61 Safari the huge majority of full size Pontiacs were dogs off the line (w turbo 400s)...??? Interesting.

Kieth's comment sparks a question: is it possible to fit the Buick "Switch Pitch" convertor in a TH400 swap for a long geared, heavy car that might take extra oomph getting out of the gate?

I just rebuilt a Slim Jim for a 64 Olds and the guy is going to spend well over 3 grand installed. Do yourself and you car a favor and put a 400 in it.
We rebuild a lot of classic transmissions, but in my opinion converting to a 400 is a selling point.

Many 65-later Catalinas had 2.56 gears and they would spin the wheels off the line with Turbo 400s.

You can convert any THM 400 to a switch pitch.

If the Slim Jim is toast anyway then it makes a lot of sense to change to something better. Slim Jim had a 3.50 first (and the better HM was about the same) so it could get off the line with the 2.XX rear ends. The TH400 will not be as good and needs a 3.xx something rear end. You might also consider the 200 or 700 OD transmissions and still change to a 3.42 or 3.55. You will still get a good first and with the OD retain a good highway gear. With a 64 block you don't have the adaptor and starter problems. Get a BOP flanged 200 and it may be a lot easier. Either 400 200 or 700 will have some floor interference problems but those are solvable.

If you are going to put a TH400, why not put a 200-4R (with a 64 bonne driveshaft)? the extra gear is nice (just ask me lol) and you can put whatever gear in the rear. B-man is right, not huge money in a numbers matching GP. But, if you keep the trans and gears in a corner of your shed you could sell the "numbers matching possibility" for the purist. For me, i built what i wanted and the tripower more than makes up for the non numbers matching. I've been watching your build for a while and i really think you should put in what you want. I have a good shifting slim jim and fluid coupled transmissions are garbage. the 1-2 shift is as long as a beaver's tooth waaaaaaaaaaaaaaaaaaahboooooooooooaaaaaah! takes a second or so to drain the fluid to shift into second lol.

I had to add a metal plate to the crossmember for the relocated 200 mount and hit the floor in 2 spots to clearance the 200. not noticeable inside the car under the carpet. 3.90 rear. A little much but less rpm than 3rd in a slim jim with 3.08s

I'm not a fan of using the TH200R4 or the 700R4 in a "B" body. It's not a question of IF they'll break; it's a question of WHEN they will break. If a Buick Grand National can "scatter" a TH20R4, what can the torque of a strong 389/400/421/428/455 do to one? I'll stick with the TH400 I've planned to use. If I can figure out the sheet metal work involved, a 4L80E looks like a "doable" sweep.

I guess it depends who builds it. Lonnie Dyers has a great 200. Jake's has one too. I blew second gear out of mine because the cable wasn't right. my trans guy fixed me up and the trans works very well. He said all great parts on the inside and should last a very long time.

Turbo 400 plus 2.75 low gear set, win-win.

After you've already put disc brakes and dual master on your '64, what's the point of staying with a Slim-Jim? The transmission change is not as visible as the dual master cyl.

I've had two Pontiacs with Slim-Jim's---both were terrible when working properly. Your '64 with the starter mounting pads already in place is a no-brainer for a TH400 or 200r4. Many swear by the 200r4 for both reliability and overdrive. Even with a 3.08 or 3.23 rearend, highway driving at 70 mph puts your tach at 3,000 rpm, which is noisy and uses gas unnecessarily. The fuel usage and noise level is a good reason to pick the 200r4.

Many '60's Pontiacs with TH400 ('65 and later) were dogs off the line due to the low stall converter--about 2000 rpm. I think Pontiac did this for fuel economy. JMO. The switch-pitch converter was Buick's answer to fix that problem, but they're not that plentiful.

Thanks everyone for your input!!! I had thought about putting a 700R4 or 200R4 in my GP, but the cost of building an overdrive trans to handle the 389 is a bit too steep for me. I will probably only keep the car for a year or so once I'm done (time to sell it and build something else ;)). It will only be driven to local car shows and out for cruising once in a while. If I was going to keep it for long time or drive it more often, I think the overdrive trans would be worth it.

There's also a lot more written on this forum (I'm still trying to track down the article in Smoke Signals too) about how to do the TH-400 swap and it was what the next generation B-bodies after mine used, so I'm hoping it will be easier to install than doing an overdrive trans swap.

My engine had a mild rebuild...bored 30 over with a RV cam, but is other stock. Given my suspected 3.08 rear end and my mild 389, what would be a good stall speed for the torque converter in the TH-400?

My old man had a 66 cat 389 2bbl auto trans with 2.41 gears.......he beat many late 70's performance cars off the line. He always had it in top running condition though!

X 2, Bart.

I was just reading about the 2.75 low gear set. I had never heard of such a thing! That seems like a pretty neat idea...you get better off-the-line performance like I would with a higher (numerically) rear end gear, but without increasing the RPM on the highway at speed. They sure cost a pretty penny! Cheaper than a rear end gear swap or going to an OD transmission, but $750 is quite a bit when the trans rebuild may only cost $1,200.

I did the Slim Jim swap to 400 Turbo over 10 years ago on my 62 Catalina. DO IT MAN, JUST DO IT! You'll be glad you did. The only thing I would have done differently on my project was to also swap my original 389 for a later year or 400 or 455. The adaptor kit is expensive. I just didn't want to take out the original motor since it only had 71,000 miles. Hindsight is 20/20. I also put in 3:08 gears. I had a hard time locating 3:23 gears for the right price. 3:08s work just fine with the 400T. Good Luck!

You run a 2.75 low gear set (they make sets that are even lower!) with a 3.08 rear gear. In low gear, the actual ratio is 5.83:1, yet at highway speed it's still 3.08. You get the best of both world....One of the guys that used to post here had a '64 Bonnie with the 4 speed Hydramatic (3.27 low gear) with a 2.92 rear gear....

a 64 block should not need the adapter.Tom

Your 2.75 first w a 3.08 rear gives you 8.47 effective off the line. The hydro in the 64 Bonneville has trans low gear of 3.97. W a 3.08 rear you get an effective 12.23 off the line. My 60 has that same setup.

8.47? what figure did I leave out of the calculation? As for the Hydramatic, I remembered it as 3.27 low gear, but it's been ten years since the guy has posted here: I accept your info as correct.

Multiply. Don't add.

I'm going through the exact same thing. I have the 4-speed automatic version with 2.87 rear gear I believe. I've been trying to research as much as I can for the pros and cons of each transmission. I originally wanted a TH-350 however the first gear is not so good and getting the planetary conversion to get it closer to usable for this car is about $500-600 by itself. A 4L60-E would be awesome however the controller to run it is about $600 and as with any overdrive, the 2.87 rear gear I assume would render OD useless on the highway. I am leaning towards using a 700-R4 and just regearing the rear end.

Two points. I have a T350 in my 1964 GP with a 421 4bbl. Its been in there for over 10 years with no problems. I changed the rear gear from 3.23's to a 2.69 and have not had a problem with lack of pick up. Second if the Turbo 200-4R is built correctly you will have no problem with that either. How do I know? I have been running one in my 1967 supercharged 428 GTO for the last 16 years. It was built by Bow Tie Overdrives.

Iam in the same boat with my 64 Grand Prix....got that gawd awful slim Jim.....but does work...if you call it that. Trying to gather up all the parts for a 4 speed conversion....without the holy grail console...LoL

Thank you.

BTW- a great site for finding out information on transmissions is here:
http://novak-adapt.com/

I know it mainly pertains to Jeep conversions but they do have a good bit of information regarding engines and transmissions.

Not many shops have the required tools to rebuild a Roto. When you say you rebuilt a Roto for a 64 Olds does that mean you replaced the EIGHT thrust bearings and thrust washers, Check all specifications on all the FORTY springs, check, replace the sprag and roller clutches, Plus the usual neutral clutch plates, front clutch plates. gaskets/"O" rings?

BTW , Just look at the planetary gears in D type HydraMatic, Super HydraMatic, Roto HydraMatic compared to 2004R or 700R4 side by side & only then you will know the latter transmissions are not designed for the HP and weight of a 4,000+ lb. car.

Why does Roto feel like there is a big ratio between 2nd range and 3rd range? It feels this way because in 3rd range the coupling is empty and the trans is in full mechanical connection so... there is no fluid cushion from 2nd and 3rd ranges. If you are careful you can adjust this inconvenience out. Why is there a lazy 3rd to 4th range shift? The fluid coupling drains 3rd range and fills 4th range in 4 tenths of a second. There are ways to speed this up by playing with the valves, playing with the pressure regulator valve and a few other tricks.

BTW, look at the A/SA 1963 Nationals at Indy winner. A 63 426 wedge Plymouth wagon with a 727 Torqueflight against a 62 Catalina. You will notice both cars come out of the whole about even, but when the Catalina gets into 3rd range ( 2nd gear) and is in full mechanical connection it jumps and pulls away from the Plymouth and so on for the win.
https://www.youtube.com/watch?v=9jQsd1fZV8k

FYI, HydraMatic, Dual Coupling HydraMatic(, Cadillac calls 315 HydraMatic , Pontiac calls Super HydraMatic, Olds calls Jetaway) and Roto HydraMatic all these transmissions have split torque and in Roto HydraMatic's case it means when the trans is in 4th range ( 3rd Gear) only the coupling only handles 40% of the engine torque and the rest 60% is in mechanical connection. This means all three hydros in high gear are more efficient than T400 or any trans without a lock up torque converter.

Forgot to mention my wagon has a 350 Pontiac in it....not exactly brisk with a T400 and a 2:87

My 64 Cat has a beautiful shifting slim Jim it her. But because I was planning on mildly building the 389 I decided to do the 4 speed conversion. I've found a non cracked bell housing and original flywheel. Went through Fabcraft for everything needed and picked up a 64 Gto Muncie 4 speed. With all these parts I'm at just under 4 grand spent. And still needing more to complete the conversion. So I Guess what I'm saying is it depends what you want to spend your money on too do it right.

Back around 2001 I had the slim-jim in the 2 dr sedan rebuilt by a 78 yr old transmission guy who worked on those cars back in the 60's. That car was my daily driver for several yrs and I had no problems out of the rebuild. Sold the car in 2008 and it was then changed over to a Muncie.

Now have a th400 behind the 389 in my '63 with a 2.87 rear end. I haven't done any highway cruising yet but I don't regret having a th400.

Bet that don't even include a console.....Yup...how bad do you want it....that's why I haven't pulled the trigger on a 4Speed yet....Yous ain't getting that money back....cuz these cars ain't worth crap.

I'd guess that out of anyone who has responded to this thread, Steve is the one person who would really know how to rebuild one of these things. ;)

a 61-64 4 speed car virtually always demands more money than a auto car,even a T400 convert.Condition being equal.JMO,Tom

Looks like it. I debated rebuilding this myself but I'd have a headache if something went wrong. I doubt any shops near me can rebuild it either, usually with this technology it is an older guy or an enthusiast, not an advertised transmission shop that does it.

But your 61 SChief does not have the Roto trans if the trans is original. It should be the Super Hydro which is a he!! of a lot better unit (imo).

I often get the two mixed up, I do have the Super Hydro.

My father at 72 now tells me he can't believe how well the slim Jim works in my 64 Cat. He was a owner of three transmission shops and the Master rebuilder for many as I was growing up. He said they're a monster when working right but to get them there was fun. His shops is where I learned to work on my own stuff.

Super Hydro IS a better transmission than the Roto (Slim JIm). The problem is finding anyone who is familiar with them.

That's why I asked the question about all the parts and measuring, plus the 40 springs that need to have their tensions measured. The thrust bearings and washers, springs etc. are not in a master rebuilding kit, neither are the sprag and roller clutches.. There are 27 special tools for this trans so does he have them?
My question was does he have all these things?? Measuring devises, spring scale, parts access to thrust washers, and thrust bearings, springs, roller clutch, sprag clutch. Pump etc.
There was no response.

It's not as hard to rebuild as it is to find the parts for it or to adjust the throttle pressure. You have to be do some research and find all the updates. I found a Motors Automatic Transmission Manual 5th edition that had the updates. Then after some more reading I bought this book considered to be the Bible of secrets and tricks.

http://transgo.com/products.php?cat=...&parent_id=167