Taps with plus sizes?

I was browsing eBay looking for a new tap and noticed a few 18mm x 1.5 taps with + 004” and + 0.24 after the size

i must confess i had not seen such before

why have an oversize tap?...only seemed to be for 18mm that I’ve seen

I think it's for galvanised bolts etc?

And are galvanised bolts common in m18?

They are used on steel that has to be case hardened to allow for hole shrinkage/distortion. Colin

Good engineering practice is to plug tapped holes so the threads remain soft during the case hardening process or you can carburise the part then drill & tap the required holes then finally harden.

Tony

Oversized taps do not only come in m18.

I have cyanide case hardened threads to about .005 deep on tooling that requires a wear resistant tapped hole. It gives long life and greater strength. The taps used were known as "B" taps. Colin

Pre plating and anodising taps will be different oversizes depending on the exact conditions. At the museum we have a large number of taps, mostly given to us and I get rid of the oversize ones before they cause problems.

Old Mart you are correct. I used to source pre plating taps, which are slightly oversize for the tapped hole in a component. After plating the tapped hole is to the correct size.

This is common in industry.

Alan.

Edited By alan ord 2 on 31/03/2021 20:33:34

Edited By alan ord 2 on 31/03/2021 20:34:18

M18 x 1.5 = sparking plug size before WWII* - just in case it helps.

* except model T Fords, of course.

Tim

Didn’t know anodising changed dimension, mine doesn’t,!!

Model T was/is 14 TPI 1/2" American Pipe thread IIRC...

Model "A" Ford and early Chevrolet used 7/8" x 18 UNS spark plugs, took a while for the world to arrive at "standard" spark plug threads.

Not wishing to swerve away from the original thread, but there are currently 4 'standard' plug threads, as well as the piddly little things used in ic engine models.

Tim