Confusing Thread Callout

[fayettevillebill]

Hi! Me again. I'm presently making a clamp-in spindle as an accessory for an antique bit brace. I have it fitted to the brace's chuck body, no trouble there. I have a nice half-inch chuck that threads right up. Problem: The spindle was not drilled and tapped for the left hand screw that holds the chuck when running in reverse. I want to install one. I have three such screws from dead drills, and they all fit a Makita drill I have perfectly. I looked up the Makita drill and downloaded a parts list, and it listed the screw as being an "M5 x 18" screw. I have determined that this thread callout does not seem to connect up with anything on the internet. I am hoping to get a left-hand tap and a tap drill to drill and tap it myself, but I don't want to lay any money down until I know for certain what I want to buy. I'm wondering if the thread callout above is a hybrid of metric\English standards - or something. Attached is the "order image" for the screw. Can anyone un-riddle this for me? Thankee!

 

[Jasonb]

M5 is the thread form and size, the 18mm is the length.

You need a M5 left hand metric coarse tap. The pitch will be 0.8mm so take pitch from major dia to get a 4.2mm tapping drill

J

 

[TimTaylor]

If you can't find a 4.2mm, the equivalent inch size drill would be a #19

 

[fayettevillebill]

Thank you all so much! I'm conversant with American thread callouts, but the metric ones are a bit different. It never entered my mind that the length would be there. I guess I'm just old enough that I never used metric to amount to anything. Tinkering is just a hobby, though several summers I kept myself in movie money by bringing home lawnmowers from trash piles, repairing them, and selling them as used. All righty! Now to find me a tap. I'm sure I'll have more questions as more projects arise.