[eschaider]

Stephen,

You might want to take a step back before you begin your engine centering efforts and consider the following;

OEM rear axles use a pinion offset that differs by manufacturer and axle. Ford 9" rear ends use a 1 5/16" pinion offset from center, Ford 8.8" rear ends use a 9/16” offset from center and GM likes a ½" offset from center. The OEM manufacturers mount the engines in their chassis off center by the same amount the pinion is offset from the axle center.

If you use a stick axle the offset is baked in, so to speak. If you use an IRS center like the 8.8" T-Bird IRS, then you can easily center the pinion, although you will use different length left and right half shafts.

If you choose not to center your pinion you will need to offset the engine by an amount equal to the pinion offset for the rear axle you have chosen to use. If you do not then you will have your u-joints offset (and probably misaligned) in both the horizontal and vertical planes.

Most guys struggle with proper pinion angle positioning for just the vertical plane. If you center your engine without centering your pinion you will additionally be tasked with simultaneously aligning the pinion in the horizontal plane. The horizontal plane is every bit as demanding as the vertical plane because the transmission tail shaft mount is fixed and typically immovable.

My suggestion to you is determine your in car pinion offset (if any) and position your engine to match it. Alternatively, you can center your engine and cut your axle tubes to center the pinion in the chassis. If you go this route you might find you need to replace one axle tube to get the rear wheels where you want them.

Of course, you can ignore the horizontal plane alignment like some folks ignore the vertical plane alignment. The result is a driveshaft vibration that, depending upon the mismatch, varies from mild to impressive.



Ed