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Larry Wolyniec is owner of Competition Motorsports in specialize in building NMCA type cars. If you have a chassis question, please email to chassis@racingnetsource.com.

Larry,

I am in the process of building a 1962 Chevy II. My question is how to correctly determine the width of a nine-inch rear end that I plan to install. Where do I measure from on the housing to get the right cut? Do I have to leave the brake assembly on the housing when measuring? Also, must I know the exact tire and wheel size first?

This is my first time at doing something like this and I greatly appreciate the opportunity to ask someone knowledgeable like yourself for advice.

Thank you,
John Johnson
Millville, Calif.


John,

First and most important is to have the wheels and tires that you are going to use and mount, then inflate them to the proper pressure. One of the easiest ways to start this process is to remove the old rear end and housing from the car. Now place the new wheels and tires in the proper place and lower the car to ride height.

Set dimensions from sidewall of tire to the quarter panel. It is best to have 1 to 1-1/2 inch clearance in this area. Then, simply measure the inside face of each wheel. That will give you the axle flange to axle flange dimension.

Then add whatever brake drums or rotor clearance is necessary. Next, determine your pinion offset. The easiest way to accomplish this is to run a string line from the center of transmission to the rear of car, measuring off the rockers on either side to square the string line. Measure from the established axle flange dimension to the string line and the difference is the pinion offset.

Now you have just established the two most important parts - the length of the rear end and the pinion offset. The only other thing you need to establish is the axle standout. For this, it is best to work with the axle manufacturer to get these numbers correct. It is based upon the brakes you are using as well as the axle.

Here is a diagram to help make this easier to understand. Note: the numbers on the diagram are just an example. You need to determine your own numbers.

 

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