For HammerHead® Differentials, you’re only likely to encountry two styles of transmission output shafts for measurements. One with a transmission slip yoke, and one with a flat plate on the transmission side. All HammerHead differentials (at this time) use 1350 U-Joints. See the diagrams below but please watch this video from our friends at Driveshaft Shop which explains the process a little better with real world visuals.

Measure from the edge of the seal (Not the output shaft) to the centerline of the rear u-joint.

**Please do this at ride height**
The car can be lifted under the differential but make sure it has the weight on the rear springs** To properly measure the rear end yoke, first determine if the yoke has (2) tabs that the rear joint will sit-in between or if there is not tabs (tabs are in the picture above) or if it has a flat plate.
Be very careful with this measurement. The size changes in increments of 1/16(1-1/16|1-1/8|1-3/26) Please Provide the Following Information:
1. Transmission Year/Make/Model (please advise spline count if possible)
2. Differential Info: What did it come from (did it have u-bolts or straps and (4) bolts or it was a flat plate)
3. What HP and torque does the engine currrently produce?
4. If the car is being built for competition, please give us a full run down of the car, weight , tire size and rear ratio all of this will help get you the correct shaft.

(Transmission Flange to Center of u-Joint) This is for vehicles with a flat flange (can be 3-bolt, 4-bolt or 6-bolt, etc.) on the transmission and a pinion yoke with u-bolts or straps on the differential (Ford 9″, GM 12-Bolt, etc.):

When Measuring:

If you have a solid or live rear axle, measurement MUST BE TAKEN WITH REAR AT RIDE HEIGHT (The car can be lifted under the differential but make sure it has weight on the rear springs)
– We prefer your measurement to be taken from the BOLT FACE on the transmission flange ( ANY RUBBER GUIBO COUPLERS MUST BE REMOVED BEFORE MEASURING)
– See below for information on measuring your pinion yoke and flange