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
.295	M7.5	7.5
.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