Taps with plus sizes?

[Ian ParkinParticipant @ianparkin39383]

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

[Ian ParkinParticipant @ianparkin39383]

[Pete.Participant @pete-2]

I think it's for galvanised bolts etc?

[Ian ParkinParticipant @ianparkin39383]

And are galvanised bolts common in m18?

[colin hawesParticipant @colinhawes85982]

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

[Tony Pratt 1Participant @tonypratt1]

Posted by colin hawes on 31/03/2021 17:28:44:

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

[Pete.Participant @pete-2]

Oversized taps do not only come in m18.

[colin hawesParticipant @colinhawes85982]

Posted by Tony Pratt 1 on 31/03/2021 17:59:10:

Posted by colin hawes on 31/03/2021 17:28:44:

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

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

[old martParticipant @oldmart]

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.

[alan ord 2Participant @alanord2]

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

[Tim StevensParticipant @timstevens64731]

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

* except model T Fords, of course.

Tim

[bernard towersParticipant @bernardtowers37738]

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

[StueeeeParticipant @stueeee]

Posted by Tim Stevens on 31/03/2021 21:18:32:

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

* except model T Fords, of course.

Tim

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.

[Tim StevensParticipant @timstevens64731]

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

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