3-speed to 4-speed automatic transmission conversion in a TF truck

Background:

I own a 1956 Chevy Stepside pickup (Task Force or TF series) with a 283 CID V-8 engine and TH350 3-speed transmission. The rearend is original with either 3.90 or 4.10 ratio. At 65 MPH the engine was turning 3200 RPM (a little too fast for my liking), so I had the choice of either replacing the rearend with a later model and lower gear ratio or changing the transmission to a 4-speed overdrive with lock-up torque converter.

With a basically stock suspension (6-lug front and rear drums) I didn’t want to mix-and-match the wheels/bolt patterns and wasn’t all that knowledgeable in doing rearend swaps, but I had experience with engine and trans work so I opted for a transmission conversion.

Research:

I consulted several mailing lists for classic trucks and streetrods and got a lot of free advice and comments. My choices appeared to be either a 700R4 or 2004R 4-speed transmission (both available with or without lock-up torque converters), manufactured by GM.

The 700R4 is a robust trans (good for hotrodding and towing), but I was cautioned to be careful about buying what was called the ‘early’ years of this model (not so good until GM had the bugs worked out) or at least making sure whatever I bought had been rebuilt by a reputable shop/specialist with the recommended upgrades. This model is longer than the TH350, and requires some driveshaft modification (shortening and/or a new u-joint yoke depending on the number of splines on the trans output shaft). Depending on the application a new transmission crossmember for the frame may be needed.

The 2004R is within 1 inch of the same length of the TH350 and has the same number of splines on the output shaft, so generally no driveshaft modifications are needed. Because the trans rear mounting location is different than the TH350, you will need to move your existing crossmember back between 4-6 inches on the frame. Some folks say the 2004R is not as strong as the 700R4, but I received several messages stating the 2004R was used in the Buick Grand National cars in the mid 80’s (supercharged V-6 engine with fuel injection, a real screamer!), and again finding a rebuilt model from a reputable shop/specialist would assure few problems. Since I don’t race or hotrod my truck, the 2004R was sounding better all the time.

I emailed and called two companies that regularly advertise 4-speed automatics, BowTie Overdrives in CA and Phoenix Transmissions in TX. Both were very helpful with emails and follow-up phone calls. I finally selected Phoenix Transmissions for the following reasons: · Approximately $300 less for the same model · 2-year 24,000 mile warranty versus 1-year 12,000 mile · Less shipping cost (TX to IL versus CA to IL by truck carrier) · Easier installation (TV/shift cable hookup to engine carb and single 12 volt wire HOT with ignition ON) versus cables, wires, shift sensor and hookup a pressure gage to check shift points during test drive.

Installation: The transmission arrived in a wooden crate with the trans and torque converter wrapped in heavy plastic. Installation instructions were simple yet thorough and had a ‘hotline’ phone number for Q&A. I prepped my truck by measuring the output shaft centerline from the frame rails, removing the driveshaft, old TH350 trans, torque converter and rear crossmember. I removed the old fluid lines from the trans to the radiator/cooler and blew-out the radiator cooler with compressed air. I bought a new lines kit from BowTie Overdrives which fit perfectly and uses braided stainless hoses for the radiator connections. Trans dipstick was purchased from Lokar and was a firewall-mount (versus tranny mount), which I could locate in an out-of-the-way position for easy access.

Installation went well, and after bolting-up the front of the trans and torque converter I measured and redrilled the frame for the rear crossmember installation. Bought a new rear trans mount (rubber isolator) and added/removed shims until the new trans output shaft centerline matched the original TH350 (double-checked angle from trans to rearend pinion angle after installing the driveshaft). Had just put in new front and rear universal joints on the driveshaft the year before, but when in doubt – replace (you don’t want to do it later).

Hooking-up the TV/shift cable from the trans to the engine carb went well (used the same cable bracket on the engine intake as the old trans). Made sure cable tension and full-open carb position provided the necessary travel for good shifts at the right engine RPM’s. Ran the single wire (12 volt HOT with ignition ON) for the torque converter lock-up and made sure I had a fuse in the electrical circuit. I installed a new Genie “Swan-Stick” (nostalgic) floor shift and made sure the Park/Neutral safety switch was adjusted correctly.

Filled the trans with new fluid, started and checked for leaks (there were none) and went for a test drive. Made several adjustments to the TV/shift cable to get the shift points at the right engine speed, but within an hour I was finished “fiddling’ with the adjustments.

Results

Engine RPM @ 65 MPH is now 2000 (versus 3200 with the TH350), 2200 @ 70 MPH and 2500 @ 75 MPH. With my new engine intake, carb and distributor installed this spring I’m getting 15-16 MPG driving 60-70 MPH. Only minor inconvenience I’ve noticed is the shifts are solid (I’m used to my Buick Park Avenue with barely noticeable shifts). I called the vendor and they explained they put in shift kits on all their rebuilds and solid shifts are to be expected.

I’ve put approximately 1800 miles on my truck since the trans conversion and am very happy with the results. I hope what I’ve written here may be helpful to others considering transmission changes.