I need to determine the size of a particular bolt in order to purchase the proper tap. I thought it was a 12MM bolt, but I am measuring 12.5MM with my caliper. Thread pitch is 1.25MM. Is 12.5MM outside the acceptable tolerance for the nominal diameter of this size bolt? 12MM is close to 1/2" and 1.25MM thread pitch is about 20/inch, but 1/2-20 is not a standard size. This is off a child's bicycle, most definitely made in China, so maybe it's some odd ball size?

 

Do you have a hardware store nearby that has a metric thread gauge ?

 

I measured the thread pitch with a gauge and got 1.25MM. Are you talking about the kind you can screw bolts into to check if it threads in properly?

 

Yes . . . . full service hardware stores will often have exposed metric nut and bolt threads for most popular sizes and pitches for their Customers to use before finalizing a sale.

 

Yes, they have one of those. I'll swing by this weekend. Thanks.

 

1/2- 20 is a standard SAE fine thread.

 

Hmm, okay. Maybe that's what I have then, because 12.5mm seems way out of acceptable tolerance for a 12mm bolt, don't you agree? Whereas I believe 0.492" is within acceptable tolerance for a 1/2" bolt.

 

It's a 1/2-20 as suggested. I was able to clean up the threads on the bike pedals using a 1/2-20 RH and LH nut and purchased 1/2-20 LH and RH taps to clean up the threads on the bike. All is good now