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Taps with plus sizes?

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Ian Parkin31/03/2021 16:58:24
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1016 forum posts
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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

Pete.31/03/2021 17:04:06
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794 forum posts
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I think it's for galvanised bolts etc?

Ian Parkin31/03/2021 17:18:45
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And are galvanised bolts common in m18?

colin hawes31/03/2021 17:28:44
557 forum posts
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They are used on steel that has to be case hardened to allow for hole shrinkage/distortion. Colin

Tony Pratt 131/03/2021 17:59:10
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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.31/03/2021 18:50:40
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Oversized taps do not only come in m18.

colin hawes31/03/2021 19:12:09
557 forum posts
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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 mart31/03/2021 19:48:22
3717 forum posts
233 photos

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 231/03/2021 20:32:28
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126 forum posts
35 photos

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 Stevens31/03/2021 21:18:32
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1584 forum posts

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

* except model T Fords, of course.

Tim

bernard towers31/03/2021 21:39:57
568 forum posts
109 photos

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

Stueeee31/03/2021 22:16:49
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121 forum posts
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 Stevens01/04/2021 09:40:12
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1584 forum posts

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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