I'm swapping to a th350 from a 700r4. As many of you may be aware, when you perform the swap you will reuse your old tailhousing from the 700r4. After swapping in the output shaft and putting on my new speedo drive gear, I bolted everything in the car. My new driven gear (for a th350) doesn't fit in the driven gear carrier for the 700r4 housing (that which is also supposed to be compatible for the th350 non-chevy) because it is too short. Does anyone know of a fix for this? Unfortunately, I have a really steep gear ratio that I can't compensate for with a 700r4 driven gear (believe me, I checked). Also, for those who have done this swap, how did you modify the mounting bracket for the shifter linkage to get it to bolt to the transmission pan?

Thanks,
Mike

 

I may be the odd guy out here, but I've never heard of transferring the tailshaft housing. I've never compared them, but I'm pretty sure the length is wrong. I can see you would think to do so for the torque arm mount, but that's why both B&M and Hurst offer kits.
Use the kit and the TH350 tail housing, and TH350 gears will work just fine.
Older TH350s used way fewer teeth on both gears of the speedo, while later TH350s used the same gears as the 700R-4.

 

They have a kit out now that allows you to reuse your old 700r4 tailshaft housing so you can use the original torque arm mount on that housing and your original speedometer gear housing. The housing for the 700r4 (I believe) is much shorter and they correct the problem by giving you a spacer block to place in between the back of the transmission and the housing. You also replace the output shaft and it moves the speedometer gear back so everything lines up. I guess my problem is slightly different in that it isn't a problem with that kit, but the driven gear. Because I have 4.10 gears I saw that PATC has recently come out with a TH350 drive/driven gear combo that corrects for a 26" tall tire and 4.10 gears. I assumed that looking at their website the original 700r4 speedo housing (which they say is compatible with olds/buick/pontiac th350 tailshafts) accepts the same driven gear as the chevy "bullet" style driven gear housing. Apparently that is not the case because the shaft on the new driven gear I received is about an inch shorter. I figured maybe someone new of a way to correct that issue. Anyone?

Also, anyone know how I'm supposed to mount the shift linkage? Do I have to create a plate to get the shift linkage to bolt to the trans pan? My holes on my shift bracket definately do not line up with the holes in the pan.

Thanks,
Mike

 

Okay, now I'm clear on what's going on. If you want to calibrate your speedo using 700R-4 gears, it can't be done. If you want to calibrate with earlier TH350 gears, then you will need a TH350 tailshaft housing. At this point, the easiest route is to get a speedometer correction box. It's a small thing that attaches to the speedometer unit in place of the cable, and the cable attaches to this box.
Now, this little box is offered with just about any correction ratio you want. So, get these gears:
GM part # 8640517 red 17 tooth drive gear
for pre-89, # 9775187 light blue 45 tooth driven gear
for '90-up, #25513052 light blue 45 tooth driven gear
this will calibrate the trans for a 26.03" tire and a 3.42:1 axle. That's easy to correct for, because 4.10000000 divided by 3.4166667 is 1.20:1, and done the other way, 3.416666667 divided by 4.10000 is 5/6ths, or 0.833333:1
So, you tell the box maker you swapped from 3.41667s to 4.10s.