Wednesday, May 25, 2016

Taps for inner cap extraction

Pulling inner caps can be tricky work, especially when the cap is celluloid (which shrinks with age) and the inner cap is hard rubber (which does not). I'll eventually get around to explaining the techniques I use, and particularly the ways I apply heat, but for now I'd like to share the chart below. If you decide to use taps as inner cap extractors, you'll want to make sure they are sized correctly. If you use a mix of metric and fractional inch taps, you should virtually always be able to thread a tap into the inner cap without having to cut too deeply into its inner surface, which risks both breakage and loss of inktightness. For the same reason, try to get taps with the finest threads you can for each given diameter.

OD (inches)
Tap size
OD (mm)
.1575
M4
4.0
.1772
M4.5
4.5
.1875
3/16”
4.76
.1969
M5
5.0
.216
12-24 or 12-28
5.49
.2362
M6
6.0
.250
¼”
6.35
.2756
M7
7.0
.2812
9/32”*
7.14
.3125 or .3150
5/16” or M8
7.94 or 8.0
.3438
11/32”
8.73
.3543
M9
9.0
.375
3/8”
9.53
.3937
M10
10.0
.4062
13/32”*
10.32
.4375 or .4331
7/16 or M11
11.1 or 11.0
.4688
15/32
11.9
.4724
M12
12.0

*Special thread – a bit harder to find

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