Hi All, (warning long post......) I recently ran into a gear whine issue when I had a set of Ford Performance 3.73" gears replaced w\ a set of Richmond EXCEL 3.89" gears in my Stang. The shop manager that did the work is a very experienced guy that knows how to properly set up gears (as well as myself) & he knew the gears were whining before I picked up my car (he told me so up front as well) but he couldn't figure out why (he also told me up front that he would prefer me to use Ford Performance 4.10 gears instead but I insisted using these Richmond gears thus understand the risk of potential gear whine). So you now know the context up front. I have read so many accounts of folks saying that Richmond (or Motive, or US Gear, etc) gears always whine....also certain Ford OE & Ford Performance gears always whine....so it's always the gears are bad, rinse, repeat, etc. So I decided to research this issue as I believed that there was more to this than just a lot of references\speculation to specific brand names & a person's knowledge level\competence level in gear setup....what I found out is that, associated w\ the Ford 8.8" axle's appearance, Ford also introduced a new gear tooth cutting process (2-cut face hobbed process....is used to make all current Ford 8.8" OE\Ford Performance gear sets) to make the gear sets for this axle that is different geometrically from the prior 5-cut face milled process Ford used w\ the Ford OE 9" axles (all the aftermarket gear manuf's still use this 5-cut face milled process to make all current Ford-specific 8.8" gear sets) that gives a different pattern checking migration direction & tooth placement position when using gear marking compound to check the same pinion depth\backlash setting procedures used to align the pinion gear teeth chord line to the ring gear teeth chord line (or center if preferred) to achieve proper tooth contact thus mesh quietly under operation (no whining). So, unless a person knows which type of gear cut process was used on a particular set of gears being installed in the Ford 8.8" axle in particular, to then know which pattern checking migration & tooth placement position parameters to use w\ marking compound to properly gauge the pinion depth\backlash settings being applied, it will be very easy to make setup mistakes as well as being very hard\frustrating to set them up (think of a busted clock getting the time right 2 times a day scenario.....you'll eventually get there but not w\o a lot of effort & confusion) due to which type gear cut pattern you got used to setting up 1st prior to handling the 2nd type. With the Ford 9" axles this isn't an issue as all Ford 9" gear sets whether OE or aftermarket are made using the same 5-cut face milled gear cutting process so there is no pattern checking parameter differences to account for.....thus much easier & simpler to do. This has been the case until recently when US Gear (Moser) has started making Ford 9" gear sets using the newer 2-cut face hobbed process instead of the older 5-cut face milled process so all you Ford 9" users be warned............. The cause is then an identification issue w\ the Ford 8.8" axle of how to recognize which type of gear cut process you're looking at to then know which checking pattern to use to properly interpret the pinion depth\backlash setting movements to properly align the pinion depth\backlash to properly center the pinion to the ring gear to hit the pinion gear\ring gear chord lines so they mesh right thus not whine! This is the reason why most proficient shops will have either the high dollar, very accurate pinion depth setting tool (this tool eliminates the gear tooth cut process being a factor in setting the pinion depth.....but the only way it gets you out of the checking pattern issue is if you hit the pinion checking number exactly or within the gear manuf's recommended variation window used to center the pinion gear to the ring gear during QA\QC prior sale) and\or they know how to ID which gear tooth cut process they're working with to then know which pattern checking parameters to use when setting the pinion depth\backlash. So from checking this forum & finding nothing to help all us DIY'ers to know\be aware of this concerning the Ford 8.8" axle, here is a link provided to a very good article that explains this w\ very good pictures of these differences showing the pattern migrations from center for both processes when setting pinion depth (called PMD or pinion mounting distance....same thing) or backlash after correct pinion depth is achieved : DIY Ford - Ford Axle Ring and Pinion Assembly How to Guide as well as provided below a .pdf of a Dana Gear Type Interpretation Chart that shows how to ID which type of gear cut process (whether 5-cut face milled or 2-cut face hobbed) was used to make the gears as well as the associated pattern checking parameters to use for both processes so this is not brand dependent (by now I believe most OE gears are being cut using the newer 2-cut face hobbed dry process which is much faster & simpler thus cheaper to make a gear set with than the older 5-cut face milled wet process but most aftermarket manuf's can't afford the expense of a complete equipment swapout to change over based on their existing profit margins so they continue to use the old 5-cut face milled process w\ their Ford 8.8" gears that they sell.....). When I showed all this to the shop manager that set up my gears, he admitted that he didn't know this was a thing (I admit I didn't either until I did all the research)....he was always setting up Ford Performance 8.8" gears (his preference & sets them using the old school setting procedures....) so he had unknowingly gotten used to looking at a 2-cut face hobbed process thus gotten familiar w\ the associated pattern checking parameters for a 2-cut face hobbed gear set. The Richmond gears (aftermarket) were cut using the older 5-cut face milled process (but assumed that all gear sets are made the same way.....sound familiar?) which the pattern checking parameters he was accustomed to using w\ the Ford Performance gear sets (2-cut face hobbed) won't respond the same way w\ this Richmond gear set (5-cut face milled).....so he made simple interpretation mistakes due to natural (not willful) ignorance which caused him a lot of headaches trying to get them right (took over 4 days) & still got them wrong in the end (whining). 1 of my strengths in the profession that I worked in before retirement (Production Specialist for a major oil company) was ability to perform solid & accurate root cause\effect analysis to effectively solve operational issues............IOW's, fixing broke stuff. When we got together in person & I showed him the proper pattern checking parameters for a 5-cut face milled gear set (Richmond is an aftermarket gear....1st clue) & used it to compare w\ the pictures of the final gear setting compound pattern markings he sent me, he quickly then recognized that somehow he had lucked out & got the pinion depth set correct (the part that takes the longest time & effort to get right w\o using a pinion depth setting tool) thru a lot of excess time wasted (which I paid for) but had set the final backlash way too loose from trying to match up the pattern shape on both the drive\coast sides from using the wrong 2-cut face hobbed pattern checking parameters which put the pinion tooth chord line high heel on both drive & coast side thus completely off the chord line (true center of teeth or ease-off if you prefer) of the ring gear teeth thus whining on both accel\decel....not because he didn't know HOW TO SET UP GEARS or the BRAND of gear set used, he didn't know the actual gear tooth cut process used w\ the actual gear set he was working with to then know which pattern checking parameters to use to guide him properly w\ their setup! So in the end I get a free fix as from the proper comparison using the proper checking parameters that matched the gear set's tooth cutting process in question all we need to do in the end is to just reset the final backlash to the manuf's recommended backlash variation range (we're gonna shoot for the tighter side of the range on purpose) to make it all line up right & I got a big thank you from the shop manager for taking the time to work w\ him in a respectful manner to help him to learn something he didn't know\was unaware of that will help him out going forward (I gave him all of the research materials I had printed out in a packet to have so he could refer back to all this on future jobs) & not being a dick customer blaming him\degrading him for his "lack" of competence in doing his work. I'll wager this is a MUCH, MUCH more common issue (on both sides of the aisle) than most would admit concerning setting up gears in the Ford 8.8" axles.... Hope this helps.