Question on Pinion Depth Measurement Calculation - Ford 8.8.

Hi folks,

Im doing my first rearend - a Ford 8.8  off of a 2004 Mustang GT. The stock 3.27 gears and differential are being replaced by a Ford Racing 3.55 gear and Torsen T-2R differential. I've pulled the axle and am doing my work on a bench.

Based on all the moaning and gnashing of teeth about setting up gearing, I am going slowly and taking my time. However, I have a couple questions on setting up the pinion and the calculations to do so. I have created a 'set up' bearing with the old pinion bearing and am using the following Ratech tool:

The directions for the tool state the following:

1. Clean and remove any burrs or paint from the top of the pinion head.

2. Mount the head bearing on to the pinion shaft and install in the housing without the pinion seal or crushsleeve. Tighten the pinion nut enough to take up the slack in the bearings.

3. Place the pinion setting tool on top of the pinion head with the slot end of the tool in the carrier bearing journal. The end of the tool will rest against the end of axle tube surface.

4. With the tool being held by your left hand firmly against the top of the pinion head and the slotted end touching the face of the axle tube, insert the dial caliper with your right hand into the slot and position the depth rod so it touches the bottom of the carrier bearing journal. 

5. Take the measurement from the dial caliper and subtract it from the dimension printed on the tool. The answer is the current checking distance of the pinion gear. Add the appropriate amount of shims to arrive at the number marked on the end of the pinion gear by the manufacturer.

***8.8 Ford gear sets do not have a checking distance marked on the pinion head like aftermarket pinions. To use this tool you need to establish that number. It is a very simple two step procedure: 1. Measure the thickness of the pinion head. 2. Subtract that dimension from 4.420. The answer is the checking distance number needed. Proceed with procedures 1 through 5

So, here is what Im wondering - I did a test run with my old pinion (3.27) following the above directions to see what I got and the numbers confused me:

My pinion head thickness measured by calipers comes out to 1.875 in.

4.420 - 1.875 = 2.545 = checking distance

The dimension printed on the measuring tool is 3.028

With the pinion installed according to the instructions above, my caliper depth rod measurement (step 4) is 0.443.

3.028 - 0.443 = 2.585 = installed checking distance

Per the instructions, 2.545 - 2.585 = -.040 - this doesnt make sense - the pinion is installed naked, with no shims, I cant removed -.040. And if I am reading it wrong and I am actually supposed to ADD .040 in shims, it still doesnt make sense, because when I dissasembled the pinion, the factory shim was 0.254.

What am I missing?

I always start with a shim the same thickness as the old pinion had, and check the pattern on the teeth. Most of the time it is right or very close.

I would be surprised if a Ford Racing gear set would require a shim quite a bit different than was on the old gear set.

81cpcamaro said:

I always start with a shim the same thickness as the old pinion had, and check the pattern on the teeth. Most of the time it is right or very close.

I would be surprised if a Ford Racing gear set would require a shim quite a bit different than was on the old gear set.

I too was really doubtful that the OEM shim would be THAT far off the measurement. Im trying to measure things out to avoid multiple iterations of slamming the carrier in and pulling it out.

81cpcamaro said:

I always start with a shim the same thickness as the old pinion had, and check the pattern on the teeth. Most of the time it is right or very close.

I would be surprised if a Ford Racing gear set would require a shim quite a bit different than was on the old gear set.

I have never had to reshim a Ford Motorsports gearset when replacing OE gears.

The past few sets didn't even have a machined face to measure FROM.

Figured out what i was doing wrong 

I had put the practice pinion in with the factory shim and the whole time I was thinking that the pinion was naked. 

ALSO...

I wasnt exactly aligning the Ratech tool the right way. Once I corrected the above, my number started making a lot more sense:

So based on my measurements with the tool, the data shows that I should utilize a .026 shim to set Pinion depth. Well, the OEM shim is .0254 - only a .006 difference. I can chalk this up to variation in measurement and just go with the OEM shim. So with this in mind, I tested the fit of te new 3.55 pinion with the OEM .0254 shim, and the practice bearings and the tool said I was only .002 off - that is so minute and negligible that I am going to proceed to final assembly re-using the OEM .0254 shim. 

So all this to  essentially re-iterate what 81camaro and knurled said in this thread.

What are your plans for the 3.27s? I have 3.55s in my Fairmont right now, and I think  that 3.27s would be handy for giving me a little more headroom in second for faster autocross courses. 

snailmont5oh said:

What are your plans for the 3.27s? I have 3.55s in my Fairmont right now, and I think  that 3.27s would be handy for giving me a little more headroom in second for faster autocross courses. 

I was just going to hang on to them for spare parts. But if you are interested, Im sure we can make a deal that works for you. I wish I had known this before I bought my 3.55s - we could've just traded gears. . Shoot me an offer you think is fair.