Measuring for a new drive shaft

Question:

In the March 2011 issue you had a couple of letters touching on transmissions and drive shafts and some of the parts companies that handle these kinds of problems. I have a similar problem and could use some help.

I am installing a Bill Mitchell 351 Ford Windsor that is bored and stroked to 427 cid (4 1/8” bore x 4” stroke). The Ford Cobra R bell housing has allowed the Tremec TKO 600R transmission to be a perfect fit.

What I need to know now is what length should the drive shaft be to fit correctly and what Ford cross member would work for a rear transmission mount? A friend was helping me with this but I haven’t been able to stay in touch with him regarding the project so perhaps you can help.

Answer:

Different driveshaft manufacturers may have their own preferred way of measuring, so your first course of action is to ask whoever will make your driveshaft how they want you to take measurements for it.

Having said that, I can give you some general instructions.

With the engine and transmission installed in your vehicle, raise the vehicle and support it under the wheels (such as with a drive-on lift) or support it under the rear axle and front suspension. The goal is to support the vehicle so that it is at its normal ride height, without the suspension either hanging or compressed.

Push the transmission’s output yoke (also sometimes called a slip yoke) into the transmission as far as it will go and then pull it out about one inch.

Precisely measure the distance from the centers of the transmission U-joint and rear axle U-joint. This length should enable the driveshaft company to make the correct length driveshaft.

I can’t recommend a specific Ford cross member to support the rear of the transmission because, among other things, you didn’t reveal what kind of vehicle you’re working on. That notwithstanding, if you’re not going to fabricate a custom cross member that fits your vehicle’s chassis and adequately supports the transmission at its rear mount spot, you are probably best off starting with your vehicle’s stock cross member and then modifying that as needed to fit the custom engine and transmission combination you’ve chosen. The position of the new transmission’s rear mount is almost always in the same vicinity as the stock transmission’s rear mount so a bit of cutting and welding or fabrication of a simple extension plate from 3/8-inch plate steel will get the job done.