drill & tap
Is there any general guideline for how deep a tapped hole for scope mounts can be in a rifle barrel, with respect to how much metal remains between the bottom of the hole and the bore ?
As my eyes age, I need to switch from iron sights to scopes on my Model 1885 Winchester. It is .32-20 caliber, number 1 octagon barrel, made in 1889, and the holes will be # 6-48 thread for the Unertl style bases. At the location of the front mount, the barrel measures .850" across the flats.
Thanks for any suggestions !
As my eyes age, I need to switch from iron sights to scopes on my Model 1885 Winchester. It is .32-20 caliber, number 1 octagon barrel, made in 1889, and the holes will be # 6-48 thread for the Unertl style bases. At the location of the front mount, the barrel measures .850" across the flats.
Thanks for any suggestions !
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The general rule for thread engagement in steel is 1 to 1 1/2 times the diameter for depth of the threads.
If you use a 'bottoming tap' your hole should be a minimum of 0.1380" + 0.050" 0.042"to account for the 'lead' of the tap.
So, 0.1880" 0.180" depth would be the minimum.
Knowing the distance from the top of the bore to the top of the barrel would be useful info.
If you wanted to go for maximum thread engagement 0.260" 0.250" would be the depth of the hole (0.207" + 0.050" 0.042" rounded up to 0.260" 0.250")
If you use a different tap, your hole depth would be greater.
Edited: because I miscalculated the tap lead.
Edit2: Don't forget to account for the drill tip. Go slow, keep everything cool and use plenty of cutting fluid.0 -
No offense! But I definitely wouldn't be D & T'ed, a original Model 1885, for scope mounts. No matter it's present condition, it would have some collector value. Use the money, to buy one of recent Japanese replicas. I believe they already come, D & T'ed, for bases. 0 -
Thanks for the information. I have a bottoming tap. The dimension from the top of the barrel to the bore calculated to .270. If I drill the tap hole to .190, that would give a thickness of metal of .080 to hold the pressure. Not being an engineer, I wonder if that is a sufficient amount to be safe for .32-20 pressures ?quote:Originally posted by iceracerx
The general rule for thread engagement in steel is 1 to 1 1/2 times the diameter for depth of the threads.
If you use a 'bottoming tap' your hole should be a minimum of 0.1380" + 0.050" to account for the 'lead' of the tap.
So, 0.1880" depth would be the minimum.
Knowing the distance from the top of the bore to the top of the barrel would be useful info.
If you wanted to go for maximum thread engagement 0.260" would be the depth of the hole (0.207" + 0.050" rounded up to 0.260")
If you use a different tap, your hole depth would be greater.0 -
Trust me, I did not come to this decision without plenty of thought and "hand wringing". I belong to WACA and appreciate the desire to not mess with old original guns. However, this is a "shooter grade" gun with no special features or history, and .32WCF was among the most popular chamberings. Other than being an old, shootable gun, it has little collector value, and there are still lots of better examples around. Also, after I am gone, there will already be a scoped old '85 available for someone with old eyes. Your comments have caused me to think more about my decision.quote:Originally posted by rufe-snow
No offense! But I definitely wouldn't be D & T'ed, a original Model 1885, for scope mounts. No matter it's present condition, it would have some collector value. Use the money, to buy one of recent Japanese replicas. I believe they already come, D & T'ed, for bases.0 -
You don't need to go that deep IMHO. 2 or 3 threads will hold, more is better (depends on the steel). 48 TPI is 0.0208 so 6 threads is 1/8" 0.125 0 -
How about using a super glue such as Loctite 420 to glue a scope block{s} such as Unertl to the barrel and use Kelby scope rings. Blocks can later be removed using heat leaving the gun original 0 -
This looks like a good topic to throw in a opinion. On a firearm as such , with low
recoil and a octagon barrel, with potential collector value,, and need for better
sights there is another option. Magnets, don't laugh, I am talking modern
Neodymium magnets. Unbelievable power. They come in all shapes . What you
are after is rectangular, pre drilled ( can not drill a magnet) . Tap the bottom base
of the scope, attach magnets ( you can stack too) then on the barrel (flats) with
removable chip glue make a fore/aft position so scope will attach to the same
place. Now you actually have a detachable scope. you can attach magnets to
other scopes. On certain rifles you can use the rear sight or dovetail groove
as a fore/aft location point, or create a temp fixture. The magnets will hold a
reasonably weight scope , but you need repeatable location point.0
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