I have a 97 F150 that has the 9.75" - 12 bolt diff (semi floater). I erroneously purchased the 10.25" - 12 bolt cover (Mag-Hytec). I matched up the cover, without removing the original, and eveything looks like it will match up. I also Talked to Roy @ Mag-Hytec. He said they are different covers. I haven't had a chance to call Roy back yet to clarify if the differences are major or subtle. He also explained that he is in the process of developing a cover for the 9.75" - 12 bolt.

The sense of urgency is that I will be trailering my brother's '70 couger from Vegas to Phoenix (>100F heat) in a week, and trailering the girlfreinds stuff from Oklahoma to Phoenix in October and can't wait the 4-6 months until development is complete.
I probably won't have a problem, but I would also like to monitor the fluid temp (Autometer trans temp gauge switched between the diff and trans) and have a little peace of mind, since I have heard of a few failures, though not specifically with the 9.75. I would like to take (maybe more than reasonable) precautions to ensure I do not have a diff failure.
I had changed the gear oil at 40K mi. and sent a sample to Blackstone. Results: (metal reading/Avg.) Iron 511PPM/396PPM, Nickel 17/2, Manganese 15/8, Boron 136/117, Silicon 46/70, Sodium 26/18, Calcium 15/526, Phosphorous 2176/1126. Their comments (paraphrased), "..accumulated abrasion in the oil. The Iron and Nickel steel parts, probably gear teeth. Silicon may be abrasive dirt. No moisture found. ...Insolubles show the oil is oxidized... Wear accumulations are probably normal." I am assuming the high alloys and phosphorous counts are from the LS clutches.
I am likely to pull the original cover and check fit the Mag-Hytec over this weekend. I have taken some gross surface measurements. It looks like there is enough clearance. If it fits, I'll install it and rotate the diff (tires) by hand to check for interference.
Does anybody know conclusively if the 10.25" - 12 bolt cover will fit the 9.75" - 12 bolt diff?
Thanks in advance for the information.
Tony

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'97 F150 SC 5.4l/E4OD 3.55ls Tow Package, K&N, LARGE Trans Cooler, towing 18' Car Hauler

 

For what it's worth, the covers are different, by ~1 inch! The 9.75 Housing is slightly smaller.
So, I re-installed the original cover with one mod: added a temp sensor. At least it wasn't for nothing and I'll be able to track the stock gear oil temp.
Oh, by the way, i will NEVER return to ANY "Jiffy" or similar lube store. Three months ago, I had them suction out the old fluid and refill with new ford lube and FM that I supplied. All because I didn't have the time to pull the cover. I found the fill plug loose. Another bonus to checking the cover. And, this time it took the full 5.5pt.
If the temp runs high (>200F), I'm going to try the "LE" synthetic that Roy @ Mag Hytec sells.

Tony


__________________________________________________ __________________________________________________ '97 F150 SC 5.4l/E4OD 3.55ls Tow Package, K&N, LARGE Trans Cooler, towing 18' Car Hauler