I have a 1964 Grand Prix with 421 HO Tripower (not original motor) and a Rotohydramatic auto transmission. The trans was rebuilt 10 yrs ago and has never worked well with this motor. It skips 2nd when cold and grabs higher gear at 1200 rpm with light throttle. Considering a swap for a TH400 - I'm told it bolts right in with no modifications to the trans mounts or driveshaft. Anyone have experience with this type of swap? Is it as straight-forward as I want it to be?

First off congrats on having such a great classic Pontiac, wow a 421 tri-power, what a torque monster and great motor! And btw welcome to the forum! I can't answer your question for sure because I wasn't familiar with the rotohydramatic trans until I researched it a bit. Apparently this trans was also known as the slim jim because it lacked a torque converter and used vanes for shifting, similar to the chevy turboglide, but with an additional gear. One sight I looked at said you would only feel one gear shift, although the big downside to this transmission was the gap between gear ratios especially when it tried to lock up. It sounded like to me that this transmission would be far from optimal in realizing the performance potential of your 421. I'm guessing the TH400 would bolt to the back of the block with no problems, but as far as the trans crossmember and driveline I have no idea. At any rate from what I've heard and read over the years, the TH400 is a great heavy duty auto transmission, although it does suck the hp, up to 45 hp at redline, but as a 3 pedal guy I'm prejudiced.

Ya that 421 has tons of potential. Now lets see if I can bring a 1964 expert in on this thread.
Luckily Mark who is building his own torque monster, may have some trans tips/help etc.
Stand By...while I try to make contact...

Please jump to the next post because I screwed up and will explain there.


Thanks Art for alerting me to rnelsons situation. I'm a big lover of the '59 -'64 full size Pontiac platform. The GP'* are even nicer, and have some body differences that make for great looks and sometimes problem procurements. The rear glass is fairly unique but I do think they are available new, just not sure.
Anywho. The trans you have is a really nice "old mans" trans when working properly. But by now it is old in parts and design. Finding someone that even knows how to spell it'* name let alone work on them is quickly fading too. The logical replacement is a TH400.
The bellhousing bolt pattern was changed to the '63 pattern in the '61 Pontiac year. The only problem with '61-'64 Pontiac block patterns versus the '65 and up is the starter location and bolt up location. '61-'64 ('64 full sized-will get to that in a minute) positioned the starter on the bell housing front plate face, with bolts that went in horizontally. Starting in '64, when the GTO/Le Mans/Tempest chassis/body was changed, the full sized Pontiacs kept the bell housing bolt up location and the GTO series had the newer location of the starter bolting to the rear block bell housing flange with vertical bolts. Some '64 Pontiacs had the newer block casting with the flange bolt drilled and tapped but the old trans with the bellhousing location. All mid sized (GTO) had the new blocks.
I do not think there were any '63'* with the new bolt location but I am not absolutely sure. Especially the 421 blocks may have the bolt holes and the wider flange cast in but I still do not think so until '64.
The TH400 will bolt directly to the '63 block, the patterns match. But the TH400 requires the block mounting location for the starter as the TH400 has no provision for a starter, or even material to add one. The easy solution is two companies that make a block/trans adaptor that sandwiches between the block and trans, spacing the trans back about '7/8ths of an inch but provides all the hardware, spacers (a crank spacers is provided) , I believe a special flex plate for the converter, and a special starter that works with their unique parts combination. The down side is that this "system' is pretty expensive. Upwards of $700 and maybe more since I have not looked into them for a while. One company is Bendtsen'* but I can't think of the others. I think a search of Pontiac transmission adaptors for '59-'64 (try 1963-1964 Pontiac transmission adaptors for a search) Pontiacs should turn up the other one. OK, Wilcap.com is the other one. Their products are very similar.
With that said. If you are a good fabricator, you could adapt a later vertical bolt pattern Pontiac starter to your block but it is not the easiest. You will have to have the flywheel or flex plate (later '65 and up Pontiac) bolted up and you'll have to mount the starter up tot he block flange to locate it. The block flange is where the bottom drivers side trans bolt and bell housing locating dowel is located. The early flange does not stick out far enough to bolt the second, outside vertical starter bolt to. The casting is not there. Plus, because the block was not intended to hold a starter there, the casting is a little thinner. If you build a strong enough adaptor for the starter I think the flange will be just strong enough.
Just a note. I'm using a '60 389 block for my twin turbo car. The '60 block had an entirely different bolt pattern, the bell housing face, the casting itself, is 1/4 inch farther forward than the late style bell housings too. I built an adaptor using a late model Pontiac to Chevy bellhousing bolt pattern adaptor (1/8th I think) and a rear motor mounting plate (another 1/8th thickness ) to make up the 1/4 inch difference, make the adaptation from early pattern to later pattern and serve as a motor mounting plate. I added a heavy thickness 1 inch flanged angle iron up against the block flange so I could mount my starter with vertical bolts. I located the starter in a proper spot, drilled the inside bolt thru the angle and into the flange, tapping the angle iron and block for the 3/8ths bolt. It'* pretty involved but that'* where the ready made adaptors are almost invaluable. They make it a bolt together situation. Although expensive.
With all of that said, the next thing becomes the trans tunnel clearance. The Rotohydramatic is a narrow trans. The early Pontiacs had two automatics. The Roto and another larger/wider automatic. Your GP only came with the roto (slim Jim) and the floor configuration had a narrower trans tunnel because of it. The floor was shared with the Catalina body series. The Bonnevilles (different wheelbase so they were a different stamping) used the bigger trans so the tunnels were stamped wider. I have never done the change myself but have read about it. You can get the necessary clearance by putting the trans in location, seeing the interference points and hammering out the metal to clear. Not a big thing and does not generally cause any problems. Some have actually cut out a Bonneville floor pan and welded it in to a GP/Catalina and fixed it that way. I personally would go the hammer and clearance route. I think that is the most cost effective. Remember that, if you use one of the available adaptors, they will move the trans back up to one inch so if you are mocking up the trans clearance by bolting the trans directly to your 421 (can be done, but again, won't have provisions for a starter) take into account the trans setback with the adaptors. Plus remember the fluid cooler lines up behind the bellhousing area need enough room to get the fittings and hoses attached. I think there are 90 degree fitting adaptors out there that would help with all of that.
Now to type of trans. The '65 and up full sized Pontiacs used a "long" tail shaft TH400. They were the same as a short tail shaft except for a longer output shaft and tail housing. You do not require the longer trans but you will need an extra long driveshaft. Even the long shaft trans is different in length than the roto so either way you will require a different driveshaft. The output trans yoke will be a 32 spline yoke, again different than the roto.
Of course the converter will have to be a TH400 converter too. I am not at all sure what will be required as far as a shifter is concerned. I don't know if you can adapt the stock '63 console system or not??
Sorry this got so long but it is not a bolt in situation.

OK, I just realized you HAVE a '64 (getting stupid in my old age, LOL). You may have the new block castings that has the provision for the newer vertical starter bolt up and all that I wrote may not be part of your situation. If you ca get under to look at the drivers side block flange, the flange that sticks out from the block at the bell housing, and where the bottom drivers side bell housing bolts to the block. Look along the very bottom of that flange to see if you have two vertical bolt holes already drilled and tapped for 3/8ths course bolts. If you do you are in a very good place. In the previous post I talked about adaptors and spacers etc etc. All of that will not be necessary if you have those bolt holes. Some cars got those newer blocks and some did not. It was a transition year for that starter location.
Please read through the previous post with what I just posted here in mind. Then it will make more sense.
I'm going to PM you my phone number if you decide you need more help. I'm more than willing to talk you through all of this. Sorry for the miscue.

Mark

Everything I talk about here will still entail the same starter adaptations as previously discussed in case you follow any of the next considerations.
I wanted to add that, if you are using this car in a near stock fashion, just enjoying it as it is power wise, I would recommend that you look into one of the overdrive automatics out there. The first would be to find a 2004R OD trans. Most of them came with dual bell housing patterns (both Chevy and the Pontiac patterns) cast into the bell and I would pass on a Chevy only pattern trans and keep looking for a dual pattern trans. The 2004R is a little bit weaker trans though. Your 421 tri-power is a pretty strong motor and it is in a heavy car so it can strain a 2004R. I would recommend finding one from a trans builder that has been enhanced a little to take additional power and you will be fine. If you are a cruiser with no intensions of a lot of power displays, a stock one with a shift kit may survive for a long time. Easy bolt in trans., again shorter than your roto so a drive shaft will be required, but, the 2004R is a little narrower than a th400 and may fit with little or no tunnel clearance problems, which would be a plus. Again watch your cooler inlet and outlet fittings. Shifter will probably be a problem requiring an after market install.
The 700R4 is next up in the strength department. That or one of the newer electronic controlled 4L60E (same trans just newer design) would work but you will need one of the thin trans adaptors from Pontiac to Chevy patters as the 700R4/4L60E'* were all Chevy patterns. This trans will take a little more abuse in stock form and there are a lot of them out there. Lots of builders because they were used heavily in Camaros and 'Vettes. The downside of any of the "E" version are they are all electronically controlled, requiring some kind of electronic control unit. They can cost up to and over 400-500 bucks.
The big daddy is the 4L80E/4L85E used in Chevy/GMC trucks. Chevy only bolt pattern so an adaptor is needed again. They are the big brother to the TH400, as strong and uses many of the same parts, just bigger to add the extra sun gear cluster. It'* also a heavy *****. But in your GOP, adapted in and starter location taken care of, with a shift kit and new driveshaft, it will run for ever at your power level. You will definitely be doing some kind of trans tunnel modifying for this bad boy though. No doubt about it. But the finished product will be so enjoyable, the OD and lock up torque converter will make that GP a high speed cruising dream, and the gas mileage will go up.
Although the benefits from all the OD transmissions will be the same.
I hope I have helped. If not I've probably help you to fall asleep--LOL.
Any questions don't hesitate to ask. I'll give you what I have or know. We all had to start somewhere.

Mark

Thanks for responding to my post. Like every question about old cars, there'* usually not a simple answer. My engine is a '64 421 HO with 4 bolt mains (50Q cast code on distributer pad.). The starter mounts to the trans bell housing. I'm told (but don't know for sure) that starters for earlier engines mounted differently. The trans doesn't work well with the engine. The best advice received so far is to tie a chain around the trans and use it as a boat anchor. I just want an idea of what I'm getting myself into before deciding to swap the trans out.

Rick