700R4 to TH350 Swap



Or, Reverse Engineering for a 700r4

So that 700r4 is starting to act up, the TV cable is properly adjusted, and its living with a fresh transfusion of Dexron, but it still just doesn't operate right? Or, you've swapped in a fresh 350 cubic incher with practical thought out hot rod goodies, but it runs like your grandmas stock 305 Caprice? Whats wrong!? The "wilder" 350 in your Nova runs better as a stump puller than the engine you've intended to be a stump puller!
In my case the problem spot was the pesky 700r4, I never liked the unit from the start. 1st gear shifted too soon, and when 2nd gear came along, the engine felt like it was wheezing just to twist through 2nd. Yes, some of the problems I experienced could have been addressed with a shift kit, but that goes against my frugal nature. With regular maintanence, and religious inspection/adjustment of the TV cable, I could get some more life out of the unit and save up for the inevitable.
Anyhow, the day came when the tired 305 drank too much oil, got into the heavier beverages, like 20w50, became incontinent, and took up a nasty smoking habit. It was retired and a 350 cubic incher with a touch of the aftermarket wand found its way into my trucks engine bay, and a mere two months later the 700r4 pined for its lost brother the 305 and started throwing temper tantrums at the demands the 350 put on it. Time came for the 700r4 to be revived, or replaced.
You've done your homework, a plain jane 700r4 will cost you more than a built th350, and the mere mention of beefing up modifications to the 700r4 has dollor bills springing from your wallet faster than electrons spew through a vacuum tube, you decide on a good 'ole non-lockup th350. It has an excellent track record, and has always done you well in the past. On to the swap!


TH350 & 700R4 Basic Facts

Since the th350 (w/ 9-1/4" tailshaft housing) is only 1/8" shorter than the 700r4, you can reuse the existing driveshaft and yoke.
The transmount interchanges, in my case the original 700r4 transmount is a simple "puck". Two bolts attach it to the transmision, and a single stud extends to attach to the trans crossmember. If you have one of those multiple bolt mounts, you might consider getting a "700r4 trans mount for a 1983 full size chevrolet c10, 2 wheel drive pickup truck" for simplicity.
You will have to relocate the trans crossmember approx. two inches forward. Not a major undertaking in my truck, since all holes existed in the passenger side frame rail, and its was a small matter to set it in to drill the neccessary holes in the driverside frame rail.

The Extra's You'll Need

If you are aquiring pieces hap-hazzard, there are two types of dipstick tube seals, O-ring and Umbrella seal. Get both types of tubes, or wait for your th350 to see which type of seal it uses.
The "new style" kickdown cable looks identical to the 700r4 TV cable. The difference? The flange on the transmission end. So, we've got two identical cables, but the 700r4 cable is now junk since it can't seal to the tranny case.
You'll need to run a steel line to the vacuum modulator. Tee it off from the manifold vacuum source found on the intake manifold behind the carb. Why steel line you ask? Becuase of the distance. Rubber vacuum hose will colapse (same reason those springs are found in better lower rad hoses.)
And most important of all, the speedo driven gear assembly! I discovered this one at 11:30am saturday...wrecking yards close at 12:00! This assembly is held into the th350 with a single bolt and retainer, like a distributor. You'll need all these pieces, bolt, retainer, sleeve, and driven gear. If possible, remove the speedo drive gear (remove tailshaft housing, remove clip and pull(or light tapping will get it out) gear from tailshaft--it might come in handy for speedo calibration.) Obviously if you get a "running" th350 it will have this.

Some of the Process

Basics: Disconnect TV cable, undo any wiring, unbolt oil cooler lines, and anything else that might be attached to the transmission.
Whichever is easier, in vehicle, or on the garage floor, remove the 700r4 tailshaft housing and remove the speedo drive gear. *Count the teeth on this gear* If you don't you'll be removing the tailshaft from your new th350 more than once. If you aquired a complete "running" th350 (w/ 9-1/4" tailshaft housing (notice a trend here yet?)) pull the tailshaft housing and count the teeth on the speedo drive gear. Now that that is done, pull out the driven gear, *count the teeth on this gear*.
If you got a rebuilt th350 it will not have a speedo drive gear, or driven gear assembly, thats what the 700r4 drive gear is for, and the wrecking yard driven gear assembly is for. After you've got the th350 in place, the oil cooler lines will have to be slightly bent to line up. The speedometer cable will screw right in. Run the new kickdown cable, utilizing the stock bracketry. It is setup just like the 700r4 TV cable, relax the cable, (engine off) give it full throttle. You'll hear it ratchet itself to posistion.
Run the steel vacuum line similar to a stock setup, tee into manifold vacuum, and attach other end to the vacuum modulator on the trans with a short length of rubber vacuum line. Hose clamps on the rubber hose will ensure positive fit and retention to the steel line and vacuum modulator.
Adjust linkage as per normal. Loosen adjustor, set column to neutral (by detent) cycle tranny selector to the neutral posistion, tighten adjustor. P-R-N-D will all line up...the 1st two gears will endup somewhere between the 3-2-1.
A trip to the autowreckers and appropriate vehicle with a three speed tranny will net an easy to remove and replace gear indicator plate & pointer.
Don't forget to clean up those left-over wires.

Speedo Calibration

Welcome to the tricky part. A bit of experimentation is in order, and some luck of the blind monkey won't hurt either. The 700r4 driven gear is the same as a th400, which means if you re-use the speedo drive gear your speedo will read faster. In my case I lucked out, my 700r4 drive gear (18 teeth) coupled with the randomnly selected th350 driven gear (19 teeth) showed exactly twice actual road speed. So, to reduce speedo speed by half, I had to turn the driven gear half as quickly, so, a 9 tooth drive gear would do it. A trip to the dealership...fingers crossed, I hope its one of the helpfull guys behind the counter, not one of those, "if you don't know the part number screw off" types. Luck was with me, got some info, and found out a 9tooth gear is available. WooHoo! And only CDN$4. I believe they went up to 29 teeth but not every possible count in between. Check with your dealer for available gears. Another tidbit the parts guy shared with me (I wish I knew his name to give him credit, as there are not enough helpfull dealship parts counter guys out there.) For every tooth difference in speedo driven gear, there will be approx. a 5mph change in speedometer reading. This will be usefull if changing the drive gear gets you close to actual road speed and some fine tuning is required.
At best, all I can advise is make sure you know the tooth count of the two gears installed, find someone who still has stock sized tires, or a cop doing radar willing to help out, to give you your actual road speed, and do some math from there to determine what you'll need to do with either one or both of the speedo gears.
A tach would be usefull to give you ballpark figures, but from my own experience, a bit of a wind, or a bit of a grade will throw the tach reading far enough out to not be accurate enough to set up the speedo.


Speedo Drive Gear : Nylon gear found on tailshaft.
Speedo Driven Gear: Nylon gear found on the side of the tailshaft, directly coupled to the speedo cable.


Remember to employ all safety precautions when working on any vehicle.
This information is only intended as an overview and may not include all the necessary information, data, or facts to complete the swap.
Remember, every vehicle is unique, and research for your particular vehicle is recomended.


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This Page Created By Alexander M. Bilan (gmguy89@netscape.net)
All Rights Reserved
December 26, 1996
Revision 2.0 May 2, 1997