Central sight mod

R1200GS

R1200GS

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New Brunswick
Hi I'm looking for the thread size for a Gehman rear iris. I need a tap and tap drill. I want to modify a couple old Central sights by cutting off the old PH sized iris block and silver soldering a slightly larger piece in place then drill and tap for the Gehman. The adapter that I currently use ( purchased)creates a lot of overhang and strain on the sight. I may need a source for the above tools as well. Unfortunately my eyes now need the filters and polarizers offered by Gehman
Mike
 
I have done a few of these. The thread is not a standard size. When I did mine I had a friend that manufactures sights send me some threaded blocks. KBC seems to have a 10 x 1 tap which I think is the correct size but check your aperture to be sure. The KBC part is on page 102 of the catologue. I also suggest you make the blocks before you solder them, much easier and if you pooch one it's not a start over job.
 
The Gehman and Anshutz iris' are a weird thread, something like 9.5 X 1 mm pitch. One of these might be your best bet...
http://w ww.champchoice.com/prod-CC_SMALL_DIA_TO_LG_DIA_ADAPTER_BUSHING-1843.aspx

Lke you already know with adding filters and polarizers you are adding weight and length. So you might be better off using these on your regular iris
http://w ww.champchoice.com/prod-GEHMANN_SIX_COLOR_RING_AND_GLASS_FILTER_SET_FOR_REAR_IRISES-10229.aspx
 
I just checked with my sightmaker, they are 9.5x1 as per Maynard. He says special order tap from OSG about $150ea. Same price from all the makers apparently.
 
KBC has a 3/8-26 tap for $11. It is real close and since the threads on the eyepieces are fairly short you might get away with it or thread them on a lathe.
 
Thanks guys
I'll order a couple 3/8 26tpi taps and try it in a piece of 1/2" key stock. I haven't had much luck cutting an internal thread that small and I'm not sure the old Colchester will cut metric pitch but likely I could get close if it was bigger dia.
I have an adapter now and will order another as I may be able to bore a block and bush it with the large end of an adapter. Usually more than one way to skin a cat, thanks for the ideas.
 
You might want to use brass, much easier to tap a big hole in a relatively small part. I used soft solder on mine but they were for my use. Some of the 44 stuff from Brownells (probably about the same as lead free plumbing solder) would be a good in between requiring a lot less heat.
 
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