Having spent 30 years with Japanese bikes, I'm struggling with all these different threads. I'm looking to buy a tap and die for the engine and gearbox mounts but don't know what to get. One bolt is 1/2 inch 20tpi and the other is 7/16 20tpi - I thought cycle was 26 tpi but some places are also listing 20tpi as cycle, arghh!
Are these cycle or something completely different?

email (option): lee@twowheelstested.co.uk

Unfortunately it's not that simple, it depends on the diameter of the bolt. The smaller bolts have closer threads in each group. You need a chart, have a look at this;

http://www.classicmotorcycles.org.uk/technical/technical_thread_size_chart.htm

email (option): horror@blueyonder.co.uk

i have a 2009 triumph bonneville which i thought would be metric threads but some of them are not i went to convert the handle bars used a metric thread bolt the original nut would'nt fit even the nuts sizes are wrong a 10mm will not fit the nut but a 11mm is very loose

hiRoger,triumph may or may not be using the iso metric system
i was going to answer before i saw 2009.
simplest way to identify any thread,you will need three items
1,a screw thread gauge,available at machinery mart (draper)
2 a 0-1''micrometer,or vernier caliper,available at car boots ,
auto-jumbles etc,you dont need spend a fortune.
3 a reference booklet,ie (zeus) or print off the info,online.
then by simply measuring the outside diameter of the bolt or screw,checking the pitch with your thread gauge,and reading off a comparison on your chart,you will never struggle again.
hope this helps cheers rick

email (option): richardholt@rocketmail.com

Thanks for the info, emails & charts. This could be a stupid question, but does that mean that the above size/ threads are the same for both UNF and BSC?

email (option): lee@twowheelstested.co.uk

Yes, there are threads that are the same in different groups. That is a bit confusing that there's 7/16ths and 1/2 inch 20 & 26 TPI in BSC. The 20 TPI in these sizes is the same as UNF

email (option): horror@blueyonder.co.uk

email (option): lee@twowheelstested.co.uk