[Royalista]

Been hopping through fields and woods today.
Inspection at home: found a bolt missing from the frame. M10

Have M10, nylon hex lock. Wouldn't fit.
Went out to buy M10 regular. Wouldn't fit.
Realized I had a M10 from the bike cannibalized on another job. That one would fit. Problem solved.
Difference isn't much, but fitting our M10 would require rethreading (that's why I had an original one left over), which isn't something to do at the roadside nor a quick fix as simply fitting a new bolt.

Here is the question:
Why do our M10 bolts not fit on Indian M10 thread?

[Arizoni]

It could be the thread pitch is different.

Checking my thread data there are several different thread pitches for a 10mm bolt/screw.

The course thread size is M10 x 1.5mm so the distance between threads is 1.5mm (.059 inches).

There are three (yes 3) different fine pitch metric threads listed for a 10mm bolt/screw.
M10 X 0.75mm (.0295 inches)
M10 X 1.0mm (.0394 inches)
M10 X 1.25mm (.0492 inches)

I didn't see any guidelines which would indicate which fine thread would be commonly used but based on the nearest Unified National Standard threads used in the USA it would be similar to a 3/8-24 UNF which put into metric numbers would be a M9.525 X 1.058mm.
That would indicate the M10 X 1mm would be similar.

[barenekd]

You really need to get a metric thread gauge for these bikes. Even the shop manual doesn't tell you what the thread pitch is. They should be easy enough at any hardware store. Shouldn't be very expensive. I have use mine on several occasions!
Bare

[Royalista]

Big thanks you people.
Completely forgot about pitch.
It yanked right at my face and I didn't see it.

It's going to be quite a hunt for bolts other than 1.5mm here. Very few remaining traditional hardware stores that still stock everything. Another nice project.
The ones on the bike that mattered right now are 1.25mm, figures.