Setting rear axle bearing clearance

by John Grady

This issue is REALLY important. That is what happened to that F that stranded me in downtown Chicago 30 years ago! That car had been reassembled with no shims. Usually you cannot put it together with no shims since it will be too tight by a lot. That someone was able to assemble it with no shims says the little spacer pins I have written about fell out of the Sure Grip center block. You MUST have factory specified axle clearance (.013” to .018”) side to side, Sure Grip or not. They are Timken conical roller bearings, not ball bearings. They have to be loaded against each other like the carrier bearings in middle of a rear end.

This incorrect rear axle clearance is a common problem that has happened to me and many others. Later Mopar are ball bearings and this clearance does not matter as the clamped outer race of the ball bearing locates with a press fit on axle. Each side is located by itself like that. The method the factory used for our Letter Cars is way better but takes a shim pack and more assembly line labor. The shim is supposed to be the same amount on each side too in order to keep stuff in the middle (see FSM drawings of thrust block).

The regular differential uses a solid thrust block whereas the Sure Grip uses two "axle shaft thrust spacers" as shown here. They look like little nails. When those spacers fall out of the thrust block, the spacer head thickness (maybe .030” or .040” each) is lost side to side and the axles are then way too loose. The shim pack will not bring axle end play to the .013” to .018” specification so the mechanic leaves the shims out and the axle is then way too loose with the spacers are sitting in bottom of pumpkin! Sometimes you can retrieve the spacers with a magnet through the fill hole.

To see if the spacers are in the center block, you need to pull axles on both sides, shine a bright flashlight light in and you will either see a little round nail head in center, way in, or else a hole in a flat surface. The hole will be about 3/8” diameter and if you see the hole, the spacer pin is gone. Often the spacer pin sticks to end of axle when you pull it out first time -- due to thick grease/oil, then falls off once it has slid out of its hole in the center block. Some spacer pins have a little hole in the head on the nail too—do not get confused. The nail is either there or it is not. A trick old Mopar mechanics use was to weld two weld wires /rods end to end, put spacer pin on end with drop of glue, get it into the hole down the axle tube, (fun!) use another double weld wire bent up like a U paper clip at end around first one (think pool cue and table guide) to keep spacer pin in there and pull the first one out of the pin. This leaves the spacer pin in place. If not this method, and no spacer pin, you have to pull carrier / center out of axle housing. Some used to leave the spacers in bottom and just get new ones. Don’t do that.

If you do not fix it, the axle cones will ride up off center in outer cone race space. That causes the axle and brake drum to ride up onto the lower part of the brake shoes. The shoes drag on the drum and if you are driving at 60 MPH, before you know it the back brakes CATCH FIRE. This cost me $3,000 in 1970 dollars on the Dan Ryan expressway plus towing plus shipping the car home plus air fare to Boston for two people.

For reference, see also Sure Grip Spacer Pin