3.91 to 3.55 in 8-3/4″ Rear

Tech Question


ron stahl, millersburg, PA, 1969 dodge dart 440

My question is: I plan on changing my 8-3/4” gear (489 case) from 3.91to 3.55. I have Yukon 3.91 in now, new gears are Strange US gear. Do you think Iwill need a shim kit? Or, will they match up? Also I have an Auburn SG unit. Whatgear oil is the best? Should I us the oil for Sure-grip?




The pinion bearing spacer is a crapshoot – there are 19 different thicknesses so the odds are stacked against you. If you want to try this w/o a proper pinion setter tool kit, you have a few options:

Compare the depth markings on the pinion head and do the math, then install the calculated-correct spacer, see if it is close (within 0.001”) of the one you have.. (Aftermarket gears use a different marking system then Mopar gears, I am not an expert on them). This, of course, assumes the current gears are installed correctly.

Or, get 2 new Timken rear pinion bearings, hone one out so it is an easy slip fit, then trial/error by looking at the gear patterns. When you get it right, install that spacer with the un-honed bearing.

Either way, you’ll need a new collapsible collar (“crush sleeve”) for SURE, unless you opt for the solid collar and bearing preload shim kit (which basically converts the 489 case to the 742 case procedure).

One tip not to be overlooked: Any gear pattern that extends over the edge of any tooth equals near-instant destruction!


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