thread size identification

need help
its a male thread
largest diameter is 9.95 mm
pitch size is smaller than 1mm but larger than 0.9

according to my cheapo thread gauge

it should be m10 fine -1mm pitch as its French built 1960's but the 1mm gauge definitely is not a perfect fit
id guess at 0.97mm
nearer 1mm than 0.9mm

or could it just be shitty tolerances?

its the thread on the end of the crank that holds the magneto on. could the thread be deliberately undersize a tad so the nut is extra tight?

Could be a 26 tpi thread which is almost exactly 0.97mm...if it is an imperial thread the size could be 3/8” (9.525mm)... or 7/16” (11.11mm)... have a look at some imperial thread tables, bsw, bsf, unf, unc, ba..... available on line. The o/d of a screw is normally slightly below the nominal size...

its too big for 3/8ths
if I squeeze the thumbscrew on the caliper I can get it to 9.93

and its an expensive caliper not the cheapo black ones so im sure its M10
but the pitch seems to be like you say rob 26 (or 27) tpi.

strange
someone said it could be bsc cycle thread but its the 10mm or 9.97 that's throwing me

would it strip if I tried putting an M10 extra fine 1mm pitch nut on?

Look at this it’ll be 3/8 or 7/16 bsc.... if its old it could be worn down from 7/16
https://britishfasteners.com/threads/bsc.html

Slightly out of pitch wont strip but it will be fooking tight if its a deep threaded area...if its just a few mm deep it wont make much difference...just a bit tight maybe

I looked at that exact page today

ive just bought a new thread gauge and ill measure with the caliper the exact size of say 10 threads then ill know the exact pitch

no way it can have worn down 1.1mm from 11mm diameter to 9.97

it will have only been taken off a dozen or so times at the very most

Don't forget that the outside diameter of a thread is always below the nominal size even when brand new.... the only way to measure accurately is to use thread wires but that is not easy as you need the correct dia wires to start with.... Can you buy an M10 nut and try it, if it fits use it.... or buy a BSC nut and try that....don't waste money on calipers and gauges

it was as I suspected M10 fine 1mm pitch, I know m6 is always 5.96mm or summat after years of mneasuring bolts to see what thread they are. , like you say they always end up a tad smaller.

the problem was the very inner most thread (furthest from the end) was finer than 1mm
and the last 2/3 were also finer

I bought a thread file for £12 off ebay got it today and 15 mins careful filing and the M10 nut screws on fine.

now my problem is a std m10 fine nut is 17mm socket size
that's too big to get even I light box spanner on cos the hole is too small!!!
I think the std nuts were 16mm

so how the hell do I find a smaller M10 nut?

ive found  some reduced size/japanese style nuts only 14mm width but they come with a warning that they aren't anywhere near as strong
ive also found flanged nuts 15mm width but the flange would need machining down as its too big

but no 16mm!!!


edit im lying it has to be 15mm width as ive just checked with a box spanner and I see flanged nuts are usually 15mm wide so I just machine the flange down
I used to be good with flange in my youth

unless I got an extra long one that sticks out a la Boadicea (17mm nut)

Not much you can say about that Al.... you certainly take time to make sure your nuts are well screwed

that's why I have a high pitched voice....