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"
|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:
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
- 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
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
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
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.