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KrisS

What dimensions/spacing for mont holes in the frame

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Hello

 

I am new owner of BUL SAS II Open pistol that was purchased without predrilled holes for the scope mount on dust cover.

I am looking for the information/advice what are the typical dimensions / location of the holes that are drilled and tapped in the frame/dustcover.

 

I've found some info in one of the threads and would like to verify if this is correct as I can drill the holes only once ;-)

 

I am looking for 5 holes pattern with distance between them 0.375 inch ( there will be optics mount + thumb rest on side of mount)

The distance from center of the slide stop hole to center of  first hole for optics mount : 0.5 inch

Distance of the holes from top of frame/dust cover  edge 0.135 inch

 

All distances above assume that I will use 5-40 screws. The issue is that I am in a "metric" country and the closest metric

screwes are M3 (which have slightly smaller diameter than 5-40) so I could use M4 but those are roughly 0,1575 in diameter

and seem to me too thick to make holse so close to dust cover edge (dust cover is stainelss steel)

 

Now the questions are:

 

1. If the dictances I provided above seem to be ok ?

2. If the M3 screws are not to weak (assuming I will be using 5 holes mount) ? Should I consider M4 screws? If that will influence/increase the required distance from dust cover edge ?

3. What should be the minimal clearance between the bottom of the mount shelf and the slide ? I am awaiting for the mount at the moment that will be used with micro red dot / RTS2 size. My concern are potential extraction/jams.

 

I would appreciate advice.

 

K

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M3 will b fine my sps Vista has that thread pitch with 5 holes has not come loose yet

 

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I have an STI MatchMaster with M4 threads put over original holes (by previous owner for Quinn mount extravaganza, I suppose). I use Cheely Custom offset RTS2 mount with enlarged holes instead. It is a backup gun with relatively low round count, but so far there were no problems with this setup.

 

A typical M4 screw head is larger than the mount chamfer so it looks a little bit steampunk. Since the picture was taken the screws were sandblasted to match the frame so the looks are ok now.

 

MatchMaster.jpg

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You could also try M3.5.  Good size. Not as common as M3, but should still be available.

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Thx for the practical advice on screws size. I will either pick good quality M3 or look for M3,5

 

As for holes placement if the dimensions I provided are ok or I just put the mount on the gun, see if it clears the slide and drill the holes without overcomplicating ?

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Well, ideally you would want to drill on a drill press or vertical mill with coordinates, so it would be precise.  If I were you, and I didn't have such a machine, I would let some machinist do it for me.  It is just way too easy to make it crooked by drilling by hand.  Any machinist would probably do it for a six-pack. 

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I have no intention to do it myself (although it seems a little bit tempting ?)

I have no skills, no tools and no stainleess steel drilling knowledge.

 

I am asking for specific dimensions as it can be drilled only once and I would prefer to have it done right.

Drilling will be done by the experienced gunsmith with proper tooling but he has no open guns specific knwoledge so that's the source of the questions.

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Note that metric flat heads are 90 degree csk and English are 82 degree. If mount is set up for 5-40 csk will have to be cut to 90 degree.

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Small update:

 

After finding the shop that had the proper skills and machines to drill, tap the frame (stainless steel drilling/tapping, us tap sizes) and modify the mount I decided to make the holes in "non-bul" 5 holes pattern (0,375 inch between the holes) and US size screws (for convinience and change flexibility in the future in case of mount change). The red dot I use is Holsoun 507C (with Trijicon RMR mounting pattern) - as I see it better than other RDs due to my astigmatism and it has dot and the ring.  For mount I've chosen JVIndustries mount for RMR to avoid making custom mount. It had some offset and was not exactly inline with barrel so side part of it had to be milled to center it (leaving around 0,6 mm clearance from the side of the slide) and side of the mount had to be milled to provide acess to RD's buttons. Finally after two weeks from collecting the parts I received today the parts and put everything together. Tomorrow I will be zeroing it ?
 

Once more thanx for advice it helped me a lot to decide

 

 

15AcYKs.jpgnlEcAjz.jpg

 

29OMs2X.jpgMMUrov5.jpg

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