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Bicycle engine clutch gear replacement (Part 4: Repairing gearbox and cover)

Supplies: epoxy putty, Goop, rubber gloves
Tools: flat file, die grinder or dremel, gasket scraper

If this were any other vehicle, most people wouldn't attempt to repair a gearbox in the state, they'd just order replacement parts. Well, the only replacement part for this problem is a new engine, and although I did tell you to buy a spare engine, this break can be repaired.

If you have a TIG welder you might be able to repair this. I would be surprised though, because the metal is of such poor quality, I think it would just evaporate.

I used a combination of epoxy putty and silicone adhesive. Epoxy putty comes in a stick and is sold in a tube under names like JB Weld. It costs about $5-7 a tube. If you chop off a chunk, it will look something like:

Epoxy putty

  1. Use the gasket scraper to remove all remnants. A screwdriver or razor blade can work in a pinch, but be careful. The steel in the screwdriver or blade is harder than the aluminum allow and can scratch or gouge it.
    Gasket scraping
  2. Time to put on the gloves.
  3. As you can see, the two parts of the epoxy are rolled together. To activate the epoxy you thoroughly mix the two parts by smooshing it like playdoh. After about a minute it will start to warm up and is ready to mould. The clock is ticking. In about 5 minutes it will be unworkable.
  4. I split up the putty and moulded it to repair both the gearbox and the cover. The cover worked pretty well, but unlike playdoh the epoxy is not sticky, it is quite dry. I gave up on having it stick in place and instead just made it fit perfectly, as if it were stuck in place. After a few minutes it was hard, and I had a nicely cast piece that wouldn't stay in place.
  5. Goop to the rescue! Goop is a silicone-based adhesive and it is amazing at bonding things that usually don't like to bond, like glass and metal. It comes in gel form in a tube like toothpaste. A tube is about $10 I think. I generously coated the cast gearbox piece in Goop and held it in place for about a minute. It cures quickly once exposed to air.
  6. When both parts were repaired it was time to touch-up the repair. I used a file to smooth the mating edges of both pieces so that they were flat in the same plane as the rest. I test-fitted the gears and there was a bit of interference in a high spot, so I ground it down using an air die grinder.
  7. A few blasts of compressed air cleaned up the job. If successful, the two pieces should mate perfectly (no gaps) and the gears should be able to turn freely.

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