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Have the threads on the 3spoke been used recently? Might be a little dirty?
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Why doesnt my shiny new freewheel extender thread onto a specialized 3 spoke
Poor QC on the part of the manufacturer. They should test with a plug (SPNK) or ring (Specialized) gauge at least every tenth one off the lathe, and change the threading insert when they get out of spec. What they've actually done, in all probability, is not buy a gauge (they are expensive, even more so for cycle threads which aren't run of the mill for gauge makers), not change their threading insert often enough, and not check any of them. This approach is one you can get away with when doing short runs on free-cutting material with industrial grade thread fit tolerances. If you've been unlucky, you've got the last Specialized hub and the last SPNK adapter. The hubs will grow as the tool wears, and the adapter will shrink. Eventually you go past the tolerance band, and a little past that (about 0.0008" or 20μm past the ends of the standard tolerance bands for 1.375"×24TPI) you get zero clearance.
Either that or something is just damaged or contaminated and you can fix it by ordinary "preparation for assembly" processes like cleaning and deburring.
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boredom
gbj_tester
@svenhöek
Why doesnt my shiny new freewheel extender thread onto a specialized 3 spoke. It gets stuck after 1 turn. I dont want to force it.
Extender should be BRITISH THREAD 1.375"x24tpi : https://www.ebay.co.uk/itm/223104186048 Does this thread pitch standard exist?
sheldon says british thread is 1.370"x 24 tpi and iso is 1.375" x 24tpi.
and is this why it doesnt fit?