THM400 question

Hi all,
I have a 1981 Chevy G30 van that has a THM400 tranny in it. I want to upgrade to a 4 speed tranny and i'm told that a 700R4 tranny will bolt right in, but i have to shorten the driveshaft by 3 inches? Where can i get it shortened at, or where can i find a driveshaft that will fit if i do this? Also, my tranny has a speedo cable directly to the speedometer. does the 700r4 have the same thing?

Bob

A 700 will bolt right in. The difference in driveshaft length will be more like 2-3/8" but that should be determined by measuring. The 700 has the same speedo cable hookup as your 400, but you'll have to figure out what gears to use. The output shaft of the tranny has gears which mesh with the speedo gears. Selecting the correct speedo gear is what makes your speedo accurate.

The other thing you'll have to address is the tranny mount. The 700's mount is further forward than the 400's... by a difference of almost 4.5 inches.

Its been done a thousand times so don't think its impossible. In fact, what I would do is find a light-duty van with a 700 in it and get the crossmember, tranny, and all associated parts. Getting the column shift will make the transition easier since you need a shifter indicator with the four positions.

You should be able to find a driveshaft place in your phone book. Look under automotive and transmission/driveline. I found three in my little town of 25,000 people so they are around.

One thing to consider; There's a reason they didn't put 700s in heavy duty vans. They are not nearly as strong a tranny. If you just use your van for highway cruising and don't tow much or haul heavy things, go ahead. Otherwise just know that the 700 is not nearly as strong as the 400.