Dealership Tool Kit: Ford Rotunda Pinion Depth Gauge

New guy with a contribution that I hope some others find interesting. It's mostly an OT vehicle tool so OT is where I went with this. I'm getting ready to change out the 3.00 gears in my Ford 8 inch rear for a set of Strange 3.55 gears. I'm doing so because I swapped out a C4 auto for a T5 with a 2.95 first gear and a .63 OD. When doing a gear change I reassemble using the same pinion shim it came with, run a pattern check and then adjust from that if needed. I had planned to do the same with this gear change, but...


While rooting around on ebay looking at Ford related things to waste money on I came across a real good deal on the Rotunda T79P-4020-A dealership tool set that was used to set the pinion depth on several Ford rear axle types. I decided to buy it it to see if it could be used with aftermarket gears because why not? The good news is that I did not waste my money as it can be used with non-OEM gears! The only thing that has to be done is to measure between the bearing surface at the base of the pinion crown (rear tapered pinion bearing seat) and the end of the pinion bearing stub (in the case of the 8 or 9 inch diffs) and add that number to the aftermarket pinion gear depth number on the end of the pinion for total pinion depth for the gear set you are working with.

For example, the tool dimensions for the Ford 8 inch are:

.820" Gauge Disc + 1.7" Gauge Block = 2.520" Pinion Tool length

2.520" (from above) + 1.446" (1/2 of Gauge Tube diameter) = 3.966" total tool length

The shim chart in the pinion tool case calls for shim to be added to the gap that is measured with the tool installed so that it totals .040". For example a measured gap of .024" between the gauge block and the gauge tube would call for a .016" shim (.024 + .016). Adding .040" to the 3.966" tool dimension gives a total pinion depth for Ford 8 inch pinions of 4.006". Measuring the pinion stub to crown base on my Strange pinion I get 2.822". Adding 2.822" to the pinion depth that Strange wrote on the end of the pinion (1.184"), I get a 4.006" pinion depth which is the same as Ford 8 inch factory spec. If the number had come up any different then using that number I would measure the gap, add it to 3.966 and shim to that number.

A nice bonus that I discovered when the kit arrived is that while this kit precedes the 8.8 inch rear end by a few years, the Alignment Disc, Gauge Disc and the Gauge Tube used for the late solid tube 8.5 inch rear end are the same as used for the 8.8. So this kit covers the solid tube 8.8 inch rear up to 2014. Add in the T93P-4020-A Gauge Tube and this kit will also cover the 8.8 inch IRS rear. The 8.5 inch is also a Salisbury-style rear end like the 8.8, so it makes sense that the kit fits it.

The following images are of the instructions on the inside the cover of the tool kit so others can see the details and how the tools are used. I'm going to use the tool to set up my Strange gears and will post pictures of the tool in use. A nice extra was that the tool kit included the T79P-4676-A pinion seal "replacer" that is used on all models. I thought to post about this here as I have seen little mention of this elsewhere online other than here and there, nothing as detailed as what I am posting here.

 

Math is hard.

 

It takes practice. For example, when I see the right kind of rear end I always think about practicing multiplication.