Transmission Swaps & Modifications

Transmission Options for the Nailhead

Created By: Russ Martin at

There are several transmission choices for the Nailhead engines, I will start with the easiest to use. The 64-65-66 401-425 came with an excellent automatic trans, the ST400. They have a round shaped bell housing unlike other GM transmissions. This trans is super strong and the 65-66 version had a two speed (switch pitch)torque convertor. The stall is changed in the convertor with an electric solenoid so you can change the stall speed (RPM will raise gearing the trans down) with a flip of a switch! I like using the smaller 64-67 ST300 convertor in lighter cars for even a higher stall RPM. I have a picture of a wiring diagram posted to wire for best performance. You don't have to wire to your brake light switch (as in my diagram) but helps when using a larger camshaft for a better idle. THE SWITCH PITCH AND STANDARD CONVERTORS AND FRONT PUMPS ARE DIFFERENT so don't mix them up.
The 64 is better than the Dynaflow but not as a good as the 65-66 ST400 (available in 67 also but has the BOP pattern and will not bolt to the Nailhead). The 64 trans case and valve body are one year only so no shift kits are available and the convertor is the standard GM type. This trans can be identified by the single wire connecter (switch pitch has two) at the rear and a weld on shift lever (the 65 and later bolt on). The 64 also is lacking in controlling down shifts, it shift 3 times but only has a low and drive. If you want to hold it in each gear you start off in L, wind it up and shift to D, that puts it in second gear, now pull it back into L and that holds it in second, wind it up again and shift to D and you are in third. Here is the part I don't like, say you are driving in the mountains, the turns are tighter and slower, you drop in down to L holding it in second gear, then you have to slow down to about 25 mph to make a tighter turn and bam! it goes into low, then you faster, put it into D to put it back in second and back to low to hold it there.WHEW! I drove a 64 Riviera everyday for 13 years so I know. (A customer gave me a solution for this problem, add a toggle switch inside the car that powers the kick down switch and it will hold the trans in gear!) OK, now you want to know if the 64-66 trans will bolt to earlier Nailheads right? you can bolt this trans on to all Nailheads from 57-63 with my special crank adapter bushing and the correct flex plate, the 59-63 can use the 401-425 flexplate but because ALL Buicks are externally balanced and different flexplate balanced to the 364 spec's must be used. The 57-63 starters (yes 4 different ones used) have a 1/2" longer snout for the Dynaflow so the starter from the 64-66 (or my mini gear reduction starter) must be used and will bolt onto the 61-66 blocks. The 57-60 364-401's will need one of special starters with a different mounting bolt pattern. NEW OPTION, we now have starter spacers so you can keep your original 57-63 starter!! Best trans option is the 4l60-E overdrive, bolt on bell housing, clean looks and function, uses 57-63 Dynaflow flywheel cover or repro GS covers we sell. For all 364-401-425 engines. Clears floor of the car, no need to modify the floor for clearance like the other adapters that mount the trans 4" further back from stock.. NO crank spacers and ALL mounting bolts used.. This setup looks like the Buick factory made it and less money! We also have adapters to mount other Chevy automatics behind the Nailhead . BEWARE of companies selling UNBALANCED flex plates and flywheels for less money, you will have to pay as much as 150.00 to balance the flywheel so it will cost more! The 53-56 264/322 has very few options, the bell housing and flywheels look the same as the 57-66 but in fact they are about 1" larger. We sell an adapter for the Chevy 350-400-700R4-2004R and it is possible to adapt a Ford C-4 with a flat-o-matic adapter bolted to a flathead Ford trans adapter. A custom convertor will have to be built but it is a very neat and tight package when it is done and looks like like it was made for it.
Manual Transmissions
Buick's have more low end torque than most engines. The fastest Nailhead's I have driven were the 65-66 3 speed Gran Sports. There were a special Ford Top Loader that GM used the GS, GTO and 442. It was called the Dearborn 3 Speed. The gears had perfect spacing for the torque output. The T10 close ratio 4 speed was the WORST transmission to use and that was what was put in the Wildcat's and Gran Sports. With a 2.20 low gear it needed 3.50 (Gran Sports came with a 3.36) to launch the car smoothly and then second was useless so we would skip it and go to third. Here are a couple of wide radio transmissions that are strong and makes a more enjoyable driving car. late 70's to early 80's Firebird or Camaro Super T-10. You could use a 2.73 rear end ratio! plenty of power and a great cruising gear.. Mid to late 70's Ford 3 speed with overdrive, there were basically a 4 speed top loader, when shifted into 4th it went into the 3rd gear position and a overdrive gear. The front 3-4 lever points down making them easy to spot. Their nick name is the IMPOSTER .. Small block guys hate them and that makes them affordable. If you want to know what rear end gear you can use with a manual trans just multiply the rear end gear with the trans low gear.. You want around 9 to 1 so say you have a 3.0 low, that means you can run a 3.0 rear gear. if you had a 3.4 low and a 2.73 rear end you would get a 9.28.