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Fabricating a dust cover
I made a simple little dust cover from 26 gauge sheet metal. The purpose is to keep the majority of dust and water off the pulley and axle. Time will tell, if my design is successful. After one summer and one fall of city commuting and riding I disassembled the entire unit, and found it as clean as the day I installed it -- so far, so good.
I used my sheet metal snips to cut out the simple pattern. See diagram below. Then I used 'duck bill' type welding pliers to sharply bend the sides to form the box. Once I was happy with my shape, I soldered the corner joints. Tip: you could use JBWeld or other strong glue if you prefer.
Next, I needed to create space in the dust cover for the axle and bushing -- sticking up from the framework-plate surface. I inserted the assembled TPS module, small axle end first, into the cover-box and pressed it against the box's bottom, so as to allow the axle end to scratch the inside of the cover. Then I removed the TPS module, and used a dome punch, aligned to the scratch mark, to make a bump-out (a dent) in the dust cover to give the TPS axle room to rotate freely. Tip: If you don't have a dome punch, you could use a small ballpeen hammer (the ball end) or anything to create the dent. Just be careful not to tear through the thin sheet metal.
Then I placed my TPS module into the box, and checked the depth. I wanted the dust cover box to be just slightly shallower than the depth of the TPS module, so I used a scribe (or permanent marker) to mark the inside of the box to the TPS module's depth.
Next, I ground and filed the box just below the line, and check the fit again.
Then I drilled holes for mounting the dust cover box to the TPS module. To do this, I inserted the TPS module, small axle end first, into the box and pressed it against the dust cover, so as to allow the 4 framework-plate screws to scratch the inside of the cover. Then I removed the TPS module, and drilled holes, aligned to the scratch marks.
The only remaining step was to cut the throttle cable slots -- which I did.
Note: My dust cover wouldn't go on my TPS module while mounted on the bike. The fit was just too tight. And I couldn't put the dust cover on, with the module off the bike, because that would inhibit installation of the throttle cables... so I trimmed (cut away one corner of) the dust cover enough to be able to put it on after the TPS module was mounted, even though this compromised some of the cover's ability to keep dust out. See photos below.