Building transmissions, specifically ball diffs. They are like the heart of an RC car and I love the attention involved in assembling them correctly.
Least favorite:
Oil shocks. I used to enjoy doing them but nowadays I dread them. A lot of messy work and they rarely bleed properly the first time. It's a nice sense of accomplishment when I'm past that step in the manual. I have two new cars (a Worlds Car re-re and an Axial EXO Terra) that I've had for years and are still waiting on the shocks. Add that to the two sets of anodized 10T shocks and two sets of original gold buggy shocks from projects that still need to be disassembled, cleaned and rebuilt. Ugh.
What's yours?
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