Tony’s Train Exchange shows how they installed a KD #4 coupler on the pilot of a BLI heavy Mikado with a #080 or #1 screw and I’ve got 4 Mikados that need working front couplers.
First, I don’t think I’ve ever seen a #1 screw. Is it bigger or smaller than a #080?
Next, I’ve found some screws that will thread thru the shank of the #4, but I don’t know for sure what size they are. Without a Screw Checker, how can I tell? I have calipers if they’ll help.
Finally, what size tap do I use for a #080 or #1 screw? What size drill should be used before tapping?
You could Google drill sizes and find out the diameter of the drills that were suggested, then measure the diameter of the screws.
You could buy a 0-80 tap, tap a hole and then see if the screws thread into the hole.
You could order the tap and drills and while your at it spring for the extra couple of bucks and buy a package of 0-80 screws so you will know they fit.
Since you have a caliper, the clearance hole drill for the 0-80 screw is a number 51 which is .067 inch, so the body of the screw (on the outside of the threads) should be that diameter. The pilot hole (the size you drill before you tap the hole) is a number 55 which is .052 inch. The size of the screw, 0-80, normally is written with a dash in the middle to separate the two numbers. The “0” is the diameter of the screw and the number after the dash (80) is the number of threads per inch. Number 1 screws can be found with either 64 or 72 threads per inch. The body clearance drill for a number 1 screw with either thread is a number 47 which is .0785 inch.
Now if the screws you have are metric, I don’t know what to tell you.
Make sure you buy at least three 0-80 taps - one primary and two back ups. 0-80 threads are very small - i.e. 80 thread per inch. It doesn’t take much to break them if you are tapping into metal - even if you are careful.
If it’s a through hole, make sure you buy a tapered tap, NOT a bottoming tap. Bottoming taps are for blind holes. If it is a blind hole, you’ll need a tapered tap to start the hole and a bottoming tap to finish it.
Buy a tap handle to use with your tap.
As mentioned previously, use a lubricant on the tap before you start and while you are tapping. This keeps the tap cool and debris from clogging your tap.
When hand tapping, take it slow and take your time.
Always keep the tap perfectly vertical in hole and rotate ONLY your wrist. Your forearm should be in the same axis as your tap. This minimizes any rocking movement that could break the tap.
A good rule of thumb for small tap sizes in metal: Two 1/16ths of a turn CW at a time, then rotate the tap back and forth a few times to dislodge any packed debris. Repeat step.
If you feel the tap starting to bind, back it off gently and rotate the tap back and forth, as in Step 7.