I ended up with 21 in-lbs preload and 0.009” backlash. The carrier is snug but not tight. Seems good so far. I’m not sure if the HD bearing on the inside of the pinion is the “trick”. It might be the great deal of time I spent making sure all the sparkly stuff was cleaned out.
For what\how you're using your steed, I think the FP 8.8" Hi TQ bearing kit is a must for you in order to get some kind of consistent longevity (to keep pinion angle true to carrier centerline......especially when you're lifting front tires on a 3,400-3,600 lb car running a PD blower) out of the bearings & gears.
The 21 in-lbs pinion preload is not bad in\of itself (Ford specs allow as much as 28 in-lbs preload) but IMHO the only reason to set this high is to seat\wear the pinion bearing rollers into the race when new to correct any slight deficiencies from manufacturing to achieve full surface bearing roller to race contact......back in the old days. Nowadays w\ the improvements in manufacturing, standardization of these parts & improvements in diff fluid lube performance I just can't see setting pinion preloads anymore past 20 in-lbs max regardless of whether new or not.
I went to 22 in-lbs preload @ 120 ft-lbs cause that's where the solid spacer's current setup allowed but I realized later after the fact that the solid spacer was actually set for preload AFTER final pinion inner bearing shim is set which will determine the final distance from the outer pinion bearing's inner race to the crush sleeve\solid spacer shoulder on shaft of pinion gear......I changed this inner pinion shim thickness from .011" to .029" which dictates a need to reshim the solid spacer (add shims back) to properly correct preload setting thru outer pinion bearing's inner race (make up for the .018").....which I didn't have (installer @ On3 didn't give me back the extra solid spacer shims not used w\ the JEGS 8.8" setup I provided) so I believe the preload I have set is most likely too tight on my used Koyo outer pinion bearing (I supplied a Motive Gear 8.8" install kit when the Richmond 3.89 gears were installed which used Koyo Hi-Cap pinion bearings) thus the advent of the harmonics being heard at a certain shaft RPM range, because outside of this, all else is good. I still have both Timken inner\outer pinion bearings & races from the FP 3.73 gear install (I also supplied the FP HD 8.8" install kit at that time which used Timken bearings) I had the dealership do some 4 yrs ago & they're in very good shape but I really don't think I'll need to change the bearings, just properly reset the pinion preload w\ the solid spacer. I'm intentionally keeping the miles off until I redo the pinion preload w\ my new JEGS solid spacer kit which will have all the necessary shims to allow setting this up using any Ford 8.8" gear set. I had a new crush sleeve on hand at the time, but I didn't want to use it so that's on me. I mic'd the new Ford 8.8" crush sleeve @ .478" tall then mic'd the old used crush sleeve from the 3.73 gear install @ .444" tall so only .034" diff between them thus not a lot of crush movement is necessary. Knowing this, I plan to mic the new JEGS kit which I believe will be taller than a new crush sleeve, reduce it down to this starting height to reduce the number of readjustments then mic the old Jegs kit currently installed to get the difference between the 2 then set the new spacer height slightly taller than the old 1 then install.......should get very close to my 14-15 in-lbs preload target @ 140 ft-lbs on the 1st try.
I do wish that Ford Performance did cut a 3.91 ratio 11 pinion\43 ring full hunting 2-cut (face hobbed) gear set but they don't & no one else does either.......aftermarket only 5-cut (face milled) & 2 brands only in full hunting 3.91 ratio (Motive & Nitro). I sized my final gear choice off a 4.10\27" dia effective gear ratio pattern using 26.1" dia tires (I run Conti Extreme Contact DWS06 295\35\18 rear tires on 07-09 GT500 SVT 18" x 9.5" wheels) & the closest gear set to match running 26.1" dia tires & keep the running engine RPMs in 5th .68 OD gear at 70 MPH under 2,500 is 3.90 gear ratio (about 1.4% faster than 4.10\27"......2,397 @ 3.91\26.1 vs 2,430 @ 4.10\27). This matched up to my 4.6L's current TQ curve w\ these Lunati VooDoo #21270700 cams perfectly thus is a very all-around fun car to drive.
What I wasn't aware of prior for a Ford 8.8" axle was 1.) there are 2 different gear cutting processes (2-cut face hobbed or 5-cut face milled) used w\ this axle thus use 2 different checking patterns & 2.) the pinion\ring tooth count determining the non-hunt, semi-hunt or full-hunt nature of a gear set concerning whine\noise.......but I am now.