Discussion in 'Gun Project Questions & Gunsmithing' started by Dave in WI, Jan 11, 2018.
Assuming a .250 recoil lug, would my calculations be correct?
Others may comment but I believe the counter bore cut calculation on this sheet is wrong. I believe it should be dimension A - C. Adding 0.010 will give 0.020 clearance since the tennon is already set back 0.010.
Ive only done a couple but made mine 0.007 from the lugs to tennon and bolt nose to counterbore.
This will be my first clambering job.
Does anyone suggest a different worksheet?
Rich S is correct. You are doubling the clearance. Cut the counterbore depth the actual amount and the tenon .005 short. .708 plus the lug thickness. That will give you .005 clearance on the bolt nose and lug. In this case, the counter bore would be .154 deep.
In general I think the work sheet is fine just the formula for the counter bore is wrong. I have seen recommendations for 0.005 - 0.010 clearance. Tighter is better for safety if a case would separate. I believe 0.005 is the low end to ensure there is no binding in the most inopportune time.
Not to hijack your thread, but I am interested in what radial clearance folks are using for bolt nose to counter bore. I've been using 0.010 which should give 0.005 clearance on all sides if the bolt is centered. Any change needed for the M16 or SAKO extractors?
The easy way to do it to measure to the front of your bolt lugs (including the recoil lug of course) and subtract .010 or whatever measurement you choose. Simply make the counterbore depth the same as the measurement from the bolt nose to the front of the bolt lug. That will give you the same clearance in the counterbore as the shank has. Simple as dirt. For what its worth, it's difficult to measure the exact headspace of an action that has in a non stressed condition. They always close up a bit when the barrel is torqued on. I have had excellent results finish chambers with .001 headspace on actions that do not use a recoil lug and .002 on those that have a lug. When the barrel comes tight those clearances close up and I nearly always have a nice feel on the gauge as the bolt closes.
Don't delete it. someone else might find this information beneficial.
Consider shortening the shank under the recoil lug. Some guys on here explained it gives you more room for setback if something happens when chambering. I’m doing my first real chamber tomorrow after some practice. Good luck and take your time.
Here's how I do it. It makes the work go faster and automatically adds the clearance in the counterbore. I'm not concerned about the distance to the lugs. Tenon length just has to be shorter.
In your example
HS .957" w/lug
Counter bore bolt nose .146"
I note 3 measurements in my log
HS .957" Stub .947" CB .146" That's all I need
Turn and thread tenon .947" long
cut the chamber HS. 957"
Touch the breech with a boring bar. Set zero on my DRO cut .146" deep X .710" That gives me .010" clearance.
Except for a chamfer or two I'm done.
Just a few things I do differently:
1. For bench rifles, I use .005" instead of .010" clearance.
2. For bench rifles, I bore .005" over the bolt nose diameter.
3. If I have the old barrel, I modify the numbers to set the headspace exactly like the old barrel, this avoids the owner having to adjust his dies for shoulder bump, or at least minimizes it. To do this change, you have to measure go gauge protrusion relative to the shoulder.
Shouldn't head space be -.002? Subtracting Dim. B and recoil lug from overall shank length?
Here's that calculation sheet if anyone would like a copy.
Somebody should clean up the math that is very poorly described in the notes. I couldn't translate it. --Jerry
A good example of NOT "over complicating"....
I still calculate my tenon length based on the front of the lugs instead of the boltface. The longer the tenon, the more gas control you have in the event of a failure. Not gonna say this way is wrong. As a nuclear engineer, I always take all the margin of safety I can get. --Jerry
I measure and record the distance to the front of the lugs minus .005" (tenon length), the distance to the bolt nose minus .005" (tenon minus this gives me CB depth). Counterbore diameter is bolt nose diameter plus .001 (BR) or .005 (hunting). I make the counterbore plus .005 if I am fitting it with an O-ring as well. The distance to the bolt face minus .002 is my headspace distance. To all of these dimensions, the thickness of the recoil lug is added, of course.
I write the dimensions on the wall behind the lathe or, if I'm feeling fastidious, on a Post-it which I stick on the wall behind the lathe. WH
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