Free printableMeasure partsTSP-ME-22
Scooter mirror thread size and the reverse-thread trap
Two common sizes, and one trap that catches everybody. A five-second test settles which you have.
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TSP-ME-22 · version v1.0 · September 2026. Opens in your browser's PDF viewer, where you can print directly.
The problem it solves
Mirrors are cheap, break often, and are sold in two thread sizes that look similar in a listing photograph. Worse, some mounts are reverse-threaded, so somebody replacing a mirror tightens the new one anticlockwise, feels it go slack, tightens harder the other way and strips the mount.
This is a small sheet for a small part, and it is here because the failure mode is out of proportion to the part's value. A stripped mirror mount on a handlebar clamp or a brake lever perch turns a few pounds of mirror into a new lever assembly.
Two thread diameters cover almost everything on these machines, and telling them apart by eye is genuinely hard. Standing the stud on a true-size circle is not.
The reverse thread is the part worth reading carefully. It exists on some machines, and you will find confident statements online about which side it is on. Those come from very few observations and we are not repeating them as a rule. The test below settles it on whichever machine is actually in front of you, and takes five seconds.
The mistakes this sheet exists to prevent
- Forcing a mirror that is going slack rather than tight, which strips the mount.
- Assuming both sides of one machine are threaded the same way.
- Trusting an online claim about which side is reverse-threaded. Those trace back to very few observations.
- Buying an 8-to-10 adapter without first establishing which way round you need it.
- Overtightening into an alloy perch, which strips even a correct thread.
Who it is for
- Anyone replacing a broken or missing mirror
- Owners whose mirror will not tighten no matter how hard they turn it
- Anyone who has already stripped one mount and does not want to strip the other
What is on it
- True-size circles for the two common thread diameters
- The five-second reverse-thread test
- What to record before ordering
- Adapters, and which way round they go
How to use it
- 01Check the calibration rule before using the circles.
- 02Stand the mirror's stud on the circles to find its diameter.
- 03Do the reverse-thread test on each side separately before fitting anything.
- 04Never force a mirror that feels like it is loosening as you tighten.
Use it with the registry
Other sheets you may want
- Chinese scooter bolt, thread and spanner cardWhy an 8mm bolt sometimes takes a 12mm spanner and sometimes a 13mm, how to identify a thread without a gauge, and a true-size thread pitch scale.
- Mystery part measurement sheetYou are holding a part. Nobody knows what it is called. This sheet records everything a supplier needs in order to find it for you.
Free to print, copy, and share — including in forums, clubs and workshops. If you find a mistake on this sheet, tell us and it gets corrected. Quote TSP-ME-22 and the version so we know exactly which sheet you mean. All printables.