R80 20 Rear Wheel Bearings

R80 20 Rear Wheel Bearings

No funny blog title today: this part is (more) serious.

Get the old races out—heat the hub, use the axle to hammer them out.

Cleaning off old, hardened (from the heating?) grease.

New bearing from FAG. I mangled the left one in my airhead infancy.

The rear wheel bearings had me worried. It’s press fit, requires heat and accurate sequence of parts and preload. If the engine was something I wanted to tackle on my next build (hence we farmed out the engine rebuild to Shail’s here in Vancouver, Canada for this, my first build), then the rear wheel is just challenging enough to have me worried. Doesn’t help I fucked up the left bearing removing it the wrong way at the start of this rebuild.

Powdercoat sneaking in on the brake surface. Note the marring on the inside of the hub flange from removing the old bearing incorrectly.

But I think it went well. Cleaned everything, in particular some sneaky powder coat spray that entered through the rear drum vents. New seal for the side I mangled, then carefully following a number of manuals and forums and another Boxer2Valve tutorial I did the bearing preload in the vice. It’s all about 0.1mm increments of one “wedding ring” spacer in the stack. It was a tad loose according to some things I read; after I shaved it down it was a tad tight. One way to measure this tightness is by the play of the outer sleeve. But if my experience with bicycle bearings helps, my reasoning is new bearing + new grease will pack down a bit, and the tightness in the sleeve will go to just the right amount of play. Also seemed to be differing opinions on how much play is OK. Again, another item to check after the first 1000km (or 100km).

A little photoshopping once or twice until I had the order down, after consulting peeps on Facebook.

Used the custom spacer for the front forks as a temporary preload spacer on the rear axle. You’re synching down items #4–11 as a test before you pressfit items #5–7 into the hub. You can’t check the preload by hand, accurately anyway, once those are in.

Looking back I think this was a good amount of play in the outer spacer, but some reports (like Boxer2Valve’s authoritative series) asked for a tighter preload.

Getting the order of the axle parts was a bit tricky—different nomenclature, German-to-English translations, etc. Finding the BMW Airheads forum on Facebook more helpful than “old fashioned” forums these days: better interface, more photos, faster responses, no membership fees. A few iterations of the layout and I confirmed first online I had it right, but really, on assembly most of the parts find a logical order based on their design.

Mechanic buddy taught me how to properly pack a bearing.

Tried a heat gun first but felt too little to me. Carefully, slowly heated, without damaging paint or stressing too many members.

The bearing pack fell in with a light tap of my palm, so likely need a little less heat next time.

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