Transmission swap


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Powerglide to 700R4 conversion

Swapping an automatic transmission offers a number of options. Leaving the electronic (E) versions out, I could choose from TH350, TH400, 200R4, 700R4/4L60 and 4L80. Because I wanted an overdrive and no 4L80, the latter 200 and 700 were my only real options. The 200R4 is said to be a straight swap for the Powerglide (or TH350), while the 700R4 needs shortening the driveshaft. The dimensions (inches):

center of mount"
overall "
PG, TH350
700-R4, 4L60, 4L60E


Because I had ever overheated a TH200 (w/o overdrive) when towing and I am using a 4T60 in my daily driver, I chose the 700R4. Good advice said to seek a 1987 or later version, because earlier versions suffered from known bugs. This version also has a 30 spline input shaft and the same 27 spline output shaft as the Powerglide. Thus the yoke will fit the 700R4.

This conversion is often described as a "straight forward" swap, the well known adjustments concern shortening the driveshaft, changing the connection to the shift lever (and speedo lens) and building the lockup circuitry. Apparently this is more or less like the proverb "quality lies in the eye of the beholder". The 700R4 is really bigger than the Powerglide so you will need to create space by moving the sheet metal of the body some 3/4 inch inward at 4 places:

  • in the center on top for an area of 5 inches wide and 2 inches high (for the housing)
  • on the front of the passenger side just above the floor some 4 inches wide and 3 inches high (for the servo)
  • in the middle on the driver's side some 9 inches wide and some 4 inches high
  • on the front of the driver's side just above the floor, some 2 inches wide and 1 inch high (for the shift lever)

The end of the output shaft of the 700R4 protrudes nearly 3 inches more backward so the driveshaft needs shortening and of course rebalancing. After mounting the shaft I noticed that the angle the yoke can make is in 2 positions limited by the inside of the yoke Lowering the rear axle as much as possible shows what I mean by this. I still had 0.006 inch clearance, but had I known this I would have ground both ends of the inside up front.

The rear transmission mount can be reused, but the bolts to the tranny need to be swapped for M10 ISO. The position of the rear mount is at the same height, but lies some 3 inches backward. The crossmember can be moved backward so its rear holes line up with the rear (4th) holes in the frame rail. The front hole will not line up and has to be redrilled in the crossmember.

Moving the crossmember might have an effect the drive angles, so you will have to re-check that. Too large an angle or too much difference between front and rear will cause vibrations due to the variation in RPM of the driveshaft within each revolution.



You will need a protractor to measure angles E(ngine), P(ropeller) and A(axle) (for instance E at the u-joint that sits on the transmission yoke, P at the u-joint(s) on the propeller shaft and A at the u-joint that connects to the rear axle). Then calculate F=E-P and R=A+P. Your goal at curb position should be some 2 degrees for F and 4 for R. When the car is moving the A angle will decrease some 2 degrees due to traction. Thus when driving angles F and R are almost equal. Neither F nor R should be greater than 5 degrees.

The bolt circle of the new lockup converter is 10 3/4 " and fits the old 153 tooth flexplate (partnr GM 471529). The OD is 12 3/4 ''. The crank hub bolt circle is 3.58 ".

You will have to mount and adjust a Throttle Valve (TV) cable to have the throttle direct the shifts. One end mounts to the transmission, the other to the carburetor and depends on the type you have. I mounted a q-jet and made my own bracket. Adjustment of the TV is very well addressed by Sumner.

The 700R4 is not an electronic version but it does need a circuitry to control the lockup of the converter. The general descriptions I have read state : lockup will take place above some 40 mph with the engine temp above some 60 C. It releases the lock when the throttle position changes or when manifold vacuum drops to a certain level (no specification found yet) or the brakes pedal is depressed. Lockup will only occur if the internal pressures in the tranny are allowing it. So you will have to create a circuit that prohibits lockup under heavy load, during braking and during cold engine periods.

Many sources advertise their own lockup kits. I chose to create my own circuit by using the manifold vacuum (switch partnr 14014519) as an indication of the engine load and a brake switch that opens during braking. I have built in a 6 seconds delay after unlock to avoid shudder caused by small changes in engine load during upshifts. Works like a charm.

Update after 3 years of daily driving with the 700R4. I wanted a more accurate indication of engine load so I built an adjustable vacuum switch. This one allows locking the convertor between 22 and 55 cm (app. 10 - 21 inches) Hg. This helps part throttle downshift by releasing the lockup in time.)

The shift indicator will need extra positions for L2 and OD. I had ordered a new TH350 lens that had the extra L2, but still missed the overdrive symbol. I cleaned out all the letters so I can put the right ones on later in time. Because I had bought a tilt column that needed rebuilding anyway I could adjust the detent plate that is in de head of the column and controls the positions of the shift lever. I adjusted it to accommodate 7 positions (P, R, N, OD, D, L2 and L1) for the 700R4.

I found a shift linkage in the salvage yard. I had to switch the hookup on the tranny which rotated the position 90 degrees. I had to lengthen the PG bracket on the chassis with 1 iinch in order to keep the linkage straight while turning, as well as to keep it above the exhaust pipe. I also had to rotate the coupler 45 degrees.

Changing the transmission will probably also change the drive and driven gears inside the tailpiece of the transmission. My speedometer has been designed to add one more mile after 1001 revolutions of the cable it is driven by. Knowing the rear tire outside diameter (take deflection into account), the rear axle ratio you can determine the ratio you will need between the drive and driven gears inside the transmission. I got my information from a very informative page by Mike Ervin. I also used the spreadsheet designed by Ron Cook. Please e-mail me and I will send you a copy of this.

Because the crossmember has been moved to the rear, the intermediate emergency brake cable must be adjusted. It must be either shortened or you will have to create your own new guiding hooks to remove the slack.