[AZ64GP]

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.

[b-man]

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.

[Stuart]

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.

[Scarebird]

What rear axle ratio does it have?

[AZ64GP]

Originally Posted by Scarebird

What rear axle ratio does it have?

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

[alleyCat]

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.

[Keith Seymore]

Originally Posted by Stuart

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.

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

[Poncho60]

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

[Ragtop Man]

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?

[stevep]

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.

[stevep]

Originally Posted by Poncho60

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

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

[stevep]

Originally Posted by Ragtop Man

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?

You can convert any THM 400 to a switch pitch.

[pfilean]

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.

[Willshire]

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.

[Willshire]

Originally Posted by pfilean

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.

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

[hurryinhoosier62]

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.

[Willshire]

Originally Posted by hurryinhoosier62

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.

[b-man]

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

[Dick Boneske]

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.

[AZ64GP]

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?