Before you buy a bolt, a tap or a thread insert, you must know the thread exactly: the diameter, the pitch (the distance from one thread peak to the next, or the number of threads per inch) and the standard. The thread information sheet in the course shows that thread standards differ in the angle of the thread and in the shape of its tops and bottoms. Two bolts can look alike and still not fit. A wrong bolt forced into a hole strips it.
Common thread standards on motorcycles
| Standard | Where you find it | How it looks |
|---|---|---|
| ISO metric (also JIS on Japanese bikes) | Most bikes from Asia and Europe, all modern commuters | 60 degree thread, pitch in millimetres |
| American unified (UNC coarse, UNF fine) | American bikes and many parts made for them | 60 degree thread, threads per inch |
| British Whitworth (BSW, BSF) | Older British bikes | 55 degree thread with rounded tops and roots |
| British Association (BA) | Small screws on older British bikes and instruments | Small screws, rounded thread |
| Cycle thread (CEI, BSC) | Older British bikes and bicycles | Fine pitch to resist shaking loose |
Steps
- Clean the bolt thread with a wire brush.
- Measure the outside diameter of the thread with a caliper.
- Try metric and inch pitch gauges. The right leaf sits in the thread with no light showing along its whole length.
- Check if it is coarse or fine. Bolts into castings are normally coarse; a bolt or stud that takes a nut is normally fine. Many bikes break this rule, so measure.
- Note any marks: a head grade mark, a left-hand thread mark (a notch or "L"), or a maker mark: Identify bolt strength, thread and locking types.
- Write down diameter, pitch and standard, for example "metric 6 mm, 1.0 pitch". Take the old bolt to the shop.
Good to know
- Metric and inch bolts can start into each other's holes for a turn or two and then jam. If a bolt goes stiff after a few turns, stop.
- On an old British bike, keep all bolts sorted by place. Whitworth spanners are sized by the bolt, not across the flats, so they do not match inch or metric spanners.
- Some Japanese screws have JIS cross heads. Use the right screwdriver: Fit the right screwdriver to JIS screws.
Check your work
- The new bolt or tap screws into the old nut or hole by hand, smoothly, to full depth.
If it did not fix the problem
- Hole already damaged: Repair a stripped thread in a bolt hole.