How does one determine a bolts size?

[RichardH]

I am trying to match up some rifles screws for replacement with allen bolts, is there some secret engineers way to determine thread size rather than simply glancing at it? Perhaps a thread gauge or suchlike?

If a gauge is available where would I buy one?

Alternatively does anyone know the stock screw dimensions for the following -

AA Camargue and Khamsin
BSA Airsporter
Sterling HR81


Thanks

Richard

[Saddle Tramp]

Most any Tool Hire or such as Machine Mart, can and will sell you a thread gauge. BUT that ONLY determins what type of thread it IS. Best use a micrometer or other width/depth gauge to measure the diameter of the bolt. Better yet, happen you know the make of the rifle, call them up and ask them what type of bolt, its diameter & thread type is. That's by far and away the CHEAPEST way to do it.

Saddle Tramp.

PS, I've still got all my thread gauges, 10 in all, but there again, back when I began as an aprentice, there were a whole heap MORE thread types than are in use today. AND mine were ONLY used in heavy engineering and thers was and still is gauges for BA types???????????????????????????

PPS. You COULD take the stock bolt's you have with you, roughtly measure with a ruler, then look for same diameter and length, then try fitting present bolt's thread into replacement bolts threads, if the fit and the lengths right, your on a winner!

[Davestate]

You need a thread gauge, micrometer/vernier caliper and a Zeus data reference book. Simply use the gauge to determine how many TPI (teeth per inch) there are, measure the diameter and look up the relevant info in the Zeus.

[Davestate]

Zeus data reference book is £6.50 and a thread gauge is the same and measures both imperial & metric screws. You can get plastic verniers for a couple of quid.

[Sam]

Works for me as well, there are thread charts on the net for reference though, save you splashing out on those as well.

A micrometer, thread gauge and good eyesight will do the job, I hold the screw & gauge upto the light to check, as some of the finer threads have a very similar pitch.

[bobkat98]

Am I missing something? Why not just telephone the manufacture and ask them what the existing threads are?

[Vic Thompson]

You can match threads by comparison. Measure diameter using a micrometer or vernier caliper, then compare pitch by holding threads together against a sheet of white paper or similar. Align peaks of thread at one end of bolt and make sure that they are still aligned at other end. By counting difference over a measured length you can determine pitch/TPI of unknown thread provided that you know the TPI of the reference thread. I use a vernier and thread gauge myself if I can, but the above works with a few limitations. However, no one has explained to me yet how to measure shaft diameter and pitch of tapped holes without using reference threads.

Vic T

[krazy_horse]

Am i missing the point here?

Why not get a selection of known thread nuts and simply try the bolt in each of the nuts!?!

JB