We have recently added a new housing calculator to our list of builder tools, making life a little bit easier while you’re working in the shop. The all-new Pro Series Rear Housing Calculator takes much of the guesswork out of building your Spider 9 rear end while using Pro Series Unit Bearings & 45 Spline Axle Shafts. Much like our other Ultimate Unit Bearing calculators, if you know your desired width and pinion offset, this Excel sheet will help calculate the needed housing tube lengths and axle shaft lengths to make it all work.

One NEW feature on this spreadsheet is the numeric entry for Locker Gap and the new selection for Pinion to Locker. Here’s a quick rundown of how these two new fields work.

Locker Gap: In previous spreadsheets, we allowed for two values for this field, .500 and 1.500. Since 45 spline (and 36 spline) axle shafts are designed to nearly contact each other inside the diff, we allow for any entry in this field with recommendations of “common” locker gaps or simply the gap between the passenger and driver shafts.

Pinion to Locker: The distance from pinion center line to “axle shaft centerline” is commonly 2.125″. This is straight out of the Ford OE spec. It’s a value that has been around for ages and is what we usually see. THAT SAID, when you upgrade to 45 spline (or 36 spline) axle shafts, you’re looking for anything BUT ordinary. For design reasons, this distance has changed through the years to minimize the deflection from ring gear to axle shaft. Some locker & spool manufacturers have moved this value, so the axle shaft centerline is more in-line with the ring gear. You will have to check with your locker manufacturer on what value is best to use here, but we now allow any entry in this field to work perfectly for your build.