Best thread pitch for scope mounts etc

[silva]

This might be the year I learn to tap threads.
Just wondering what would be a suitable thread pitch for securing scope mounts, aperture sights etc, Bearing in mind that there is only a few mm of metal on the cylinder.
1.25 pitch bolts and taps seem to be readily availiable in the M sizes. Would this be a good choice, or is there recommended gunsmithing thread pitches.

Also are there any drill guides availiable in the UK to make sure the drill bit is 90 degrees to the cylinder and the holes line up.

Cheers

[edbear2]

Hi mate.......Forget 1.25mm pitch! that is roughly 20 TPI (teeth per inch)..I would recommend at least 32 or 40 tpi, which means unf...ie 10-32 or 10-40 etc. or metric equivalent.....do a google for "model engineers taps and dies"...lots of good sites out there, and when you decide on a size of thread to use, you can also get the correct drill size, and technique the same way. As for a drill guide, you could get someone to bore a piece of bar the same size as your cylinder diameter, (the bar need to be 6-8 mm bigger than your cylinder) then drill it in a vee block to ensure the drill hole is correctly in line with the center, then bandsaw it in half.....you will then have a "saddle" to sit on the cylinder......either that or use vee blocks and a piller drill to drill the cylinder...

http://www.efunda.com/DesignStandard...ws/unified.cfm

and metric (divide 25.4 by the pitch of the screw....ie. 0.7 in 4mm, to get a TPI equivalent)

http://www.efunda.com/DesignStandard...ewm_coarse.cfm

What you are looking for as an ideal is the same as when you put a nut on a bolt........if you think about it, once the nut is tightened, any thread left sticking out does nothing!....so the ideal is to have a depth of tapped thread at least the diameter of the bolt/screw....now obviously a 4mm screw is going to be bigger than the the thickness of the wall of the air rifle cylinder, so then you need a fine thread for more security.........a 4mm screw works out at 36 tpi (ish) .....It all depends as well on what you are mounting to, you could use a coarser threadform on say, mounting a peepsight to an airsporter trigger block where there is more "meat".....

[silva]

Hi Ed,

Thanks for your reply, I've got the use of a pillar drill and clamp.

What does the 10 part stand for in 10-32 ?

I reckon about a 4mm wide bolt would be the right size, as thats roughly what Parker Hale and Weaver used to supply with their mounting blocks. What would be the right thread be called with a 32 or 40 pitch, approx 4mm wide ?
Sorry if its a stupid question
Cheers

[edbear2]

HI mate....if you read the links I have put (the second one) you will see a standard 4mm thread is about right, I have used 4mm screws to mount the scope on my BSA standard, and it seems fine (I also used locktite)........you need a 3.4mm drill to tap a 4mm thread......buy decent stuff, dormer or similar, as the cheap s*** goes oversize!........tapping drill chart here;

http://brassmein.com/tech/print/pf007.htm

Taps come in sets of three.....A tapered "no1, a less tapered second cut, and a full formed "plug".......the intention being to allow you to tap full size all the way to the bottom of a blind hole if you use all three........In a sheet metal application, which a cylinder is basically, you can use just the number one, as it will be full size about a third or less in.......Also, once you have drilled your hole, do not move the job, replace the drill in the chuck with the tap, and use your hand to turn the chuck and start the tap in the job....that ensures it is at 90 degrees..Then carefully release the chuck and finish with a tap wrench.....wind it about 180 degrees in at a time, then back a bit to release the swarf, .....plus and VERY important....use a quality, specific tap and die lube like rocal RTD or similar (any engineering merchant will have this)....you must use a correct high pressure lube, not engine oil, 3 in 1 and the like, to get the best job!........I can sort you out a bit of this if you get stuck. Ditto with screws, only use quality fasteners, available from local engineering places as well....

P.S.......I think the 6, 8, and 10 in the smaller unf series are just screw sizes......ie. 1/4x28 means what it says, never given it a thought before!......But I DO know that three turns of a screw into material are needed to get at least an engineering standard "hold"........so measure your wall thickness in mm (or inches).....look at the charts for the pitch, and do the maths to see how many pitches fit in the thickness you have........does that make sense?

Regards, Ed

[silva]

Hi Ed,

I was going to be mounting a vintage steel parker hale scope mount or blocks to fit a period 7/8" scope to a prewar Bsa underlever and possibly a PH16 rear sight to a cadet trigger block.

Cheers

[harvey_s]

I would have thought BA threads would be more in keeping than UNF (American) or Metric (too modern).
3 BA is around 4.1 outside dia. and nearly 35 tpi.

Thread data here : http://www.sizes.com/tools/thread_BA.htm

Everything else as Ed said.....

[edbear2]

Hi again, just had a thought about drilling the cylinder......once you have decided on, and marked the hole positions on the cylinder, it would be wise to lightly center punch the hole positions. I used a piece of steel bar about a foot and a half long, wrapped in masking tape, slightly smaller than the piston size, held in a vise. I then slid the cylinder over this, and center punched the hole positions (this supports the cylinder from the back and also makes all easier).......If you have a piller drill/vee blocks...that's excellent, but I would also pilot the holes with a very small center drill (they are much less likely to "skid" off)..and another, very usefull, cheap gizmo to buy is a tube/bar center finder.....you put the shank in the drill chuck, then move the job around until the line on the body lines up with the line on the pivoted part....a lot more accurate than you would imagine by looking at one........not at work this week, so found this pic of a cheapy (£4) one online.....

http://www.lathemaster.com/Images%5C...RFINDER_22.JPG

[magicniner]

The standard thread for Leupold Turn-In and Quick-Release bases is 6-48.
It's a non-standard gunsmith's thread OD is about 3.6mm with 48 TPI. The hardened, deep head Torx Leupold screws are the reason I'm using the supplied fasteners, I've had to order taps from the USA through Midway UK,
Regards,
Nick