Advice needed ... bolt opens on firing!

[browningautorifle]

remove some of the bolthead face by grinding

I've never heard of headspace being adjusted that way...

[Bruce_in_Oz]

"Increasing" headspace by removing metal from the front of the bolthead can introduce another "issue": excessive striker protrusion.

If you ever get to measure a bunch of different "number" No4 bolt heads, you will find that the overall length is VERY similar across the board.

This is because striker protrusion is set by the relative lengths of the front-end of the striker (forward of the collar) and the OVERALL length of the bolthead.

Given that SMLE and No4 boltheads, (and their associated bolt bodies), have indexed threads, the mathematics and sequence of manufacture must have been quite interesting.

The "back-thrust" of the cartridge is NOT supposed to be transferred via the threads, but via the tiny flat surfaces on the back of the main body of the bolthead and on the front of the bolt body. Extreme caution is advised before fooling with either of these surfaces. The bolthead thread is "beefy" because it takes ALL of the tensile load during extraction, especially primary extraction. A delicate "UNEF" pattern would not last very long in service. (And coarse threads are harder to "cross-thread' than fine ones: i.e. more "soldier-proof").

[spentprimer]

I've never heard of headspace being adjusted that way...

My friend is a retired mechanical engineer that has a complete machine shop in his basement. I knew he had this grinding attachment for his lathe and it works quite well. I figured it would be best if the surface was square with the world and flat as well. A small collar was made to hold the bolthead and the rest was quite easy. On the first pass of the grinder we found that the face of the bolthead was not very flat. Predictable if it was sanded down by hand. The grinder was removing about a thou per pass while the bolthead rotated quite slowly in the lathe. I will try to post pictures of the boltface this week.

When he lived here, it was his equipment and skills we used to build the replica of the Selous elephant gun some ten or twelve years ago, a four bore firing a one inch round ball with a four ounce black powder charge and a musket sized cap to ignite.

[spentprimer]

"Increasing" headspace by removing metal from the front of the bolthead can introduce another "issue": excessive striker protrusion.

If you ever get to measure a bunch of different "number" No4 bolt heads, you will find that the overall length is VERY similar across the board.

This is because striker protrusion is set by the relative lengths of the front-end of the striker (forward of the collar) and the OVERALL length of the bolthead.

Given that SMLE and No4 boltheads, (and their associated bolt bodies), have indexed threads, the mathematics and sequence of manufacture must have been quite interesting.

The "back-thrust" of the cartridge is NOT supposed to be transferred via the threads, but via the tiny flat surfaces on the back of the main body of the bolthead and on the front of the bolt body. Extreme caution is advised before fooling with either of these surfaces. The bolthead thread is "beefy" because it takes ALL of the tensile load during extraction, especially primary extraction. A delicate "UNEF" pattern would not last very long in service. (And coarse threads are harder to "cross-thread' than fine ones: i.e. more "soldier-proof").

I measured the firing pin (oops, striker) protrution and it is .044", my Winchester Signal Cannon extends .047" and will occasionally pierce a primer. I don't know what the spec. is on the firing pin protrution, but, I would rather make it shorter if I need to before the string goes on the trigger.

The bolthead measured .636" before removing about .005".

This has sure been a learning experience and I really appreciate everyone's input and insight on this project.

[Dad]

If I remember correctly the striker protrusion specifications are:
0.040" minimum
0.042" maximum

D.

[Bruce_in_Oz]

From:

Inspectorate
Provisional Standard

REME Central
File No: 7/IW/Gen
Issue 1, 1957

Striker protrusion:

Gauge, Armourers, No. SM 143

In post WW2 Oz, this (or its locally made equivalent) had the NSN of 5220-99-961-9944.

Max: 0.050",Min 0.040"

Viz:

"f. Striker protrusion is within the limits of 0.050 inch high, 0.040 inch low and the point is radiused 0.038 inch.

Note: Adjust by stoning the point of striker if too high, ensuring radius of 0.038 inch is maintained. Stone tenon of bolt head if low, ensuring face of tenon is kept square and that “lift” of cocking piece is within limits of sub-paragraph 2.e, otherwise, exchange striker."

Note the second sentence of that "Note":

If you are going to adjust anything, stone the REAR of the threaded tenon, NOT the face of the bolthead. There are two reasons for this:

Firstly, removing metal from the front face WILL enlarge the headspace.

Secondly, because the mating hole for the striker is TAPERED; "adjusting" the face will effectively enlarge the hole by a whisker AND it will remove some of the VERY thin hard skin.

The finished radius of the tip of the striker is important. You do not want a "truncated cone" like the tip of an SKS striker, unless you have a fetish for ruptured primers.

The bit about maintaining the "lift of the cocking piece" is kinda important, too

[Bruce_in_Oz]

But then from:

"Instructions for Armourers",
Published by the War office 30 Sept, 1931. Document No. 40/W.O./8743
Section 4.— Repairs, Modifications and Adjustments,

7. To fit new bolt-head to Rifles No. l.—(Spare part bolt- heads, marked “S” on the top, are longer at the front.) Assemble the bolt-head to the bolt, insert it in the body, and test with .064-inch No. 1 gauge; should the bolt not close over the gauge, remove the bolt-head from the bolt, and having placed a piece of emery cloth (No. F) on a flat surface, rub the face of the bolt-head on the emery cloth, maintaining a circular motion in order to preserve a flat surface, until sufficient metal has been removed to enable the assembled bolt to close over the gauge. The bolt should not close over the .074-inch No. 1 gauge. Care should be taken to keep the-face of the bolt-head flat and square. After fitting and adjusting, the top front edge of the face of the bolt-head is to be rounded to a radius not exceeding .02-inch.

Note.—When it is found that the bolts of several rifles turn over the 074-inch No. 1 gauge, the bolt-heads should be ex- changed among such rifles, as, owing to the varying lengths of bodies and bolts, bolt-heads which are too short in one rifle may be serviceable in another. Bolt-heads that have been replaced in rifles by longer ones, should be kept by the armourer and used whenever possible in rifles requiring the bolt-head replaced, so as to avoid unnecessary use of new spare part bolt-heads. Part-worn bolt-heads held as required by armourers in accordance with the foregoing need not be accounted for as part of the annual allowance of new spare parts.

Soooo, does anyone have any tech data on the "S" marked replacement bolt heads that can be "adjusted" for headspace at the front end???? Lithgow made a run of specially marked No1 type boltheads that came in incremental lengths like the No4 series.

They are marked thusly:


DSN Length of Bolt Head Size No.
1005-66-056-1292 0.635 in 5
1005-66-056-1293 0.636 in 6
1005-66-056-1294 0.637 in 7
1005-66-056-1295 0.639 in 9
1005-66-056-1296 0.640 in 0

A "standard" bolthead has the following critical dimensions:

Length of main body: 0.635" Accept, 0.632" Reject.
Length of threaded tenon: 0.694" Accept, 0.692" Reject.
Diameter of striker hole: 0.077" Accept, 0.078" Reject.
Diameter of main body (front bit): 0.585" Accept, 0.582" Reject

"The face and "hook" of the bolthead are to be case-hardened to a depth of 0.010"."

See also:
A.I.D 1529 and R.S.A.F. 3095(1)

Cheers!

[Dad]

Bruce,
Question, I have this tool and I thought that the .04 and .042 were to measure the min and max striker protrusion!
What does the tool measure?
Is the rounded recess to the right of the .042 used to check the striker profile or what?
Any info you have will be greatly appreciated!

D.

[ssj]

Bruce,
Question, I have this tool 8><---

Does it have any other part numbers on the other side? Ive never seen this before or any pics....interesting...Nagant's have such a tool but simpler. Is it an armourer's tool? or third party?

40 to 50thou is the protrusion spec for a no4 anyway, though 38 to 56thou seem OK...my no4s all vary a bit....

regards

[Peter Laidler]

On the introduction of Mk7 ammo, the FPP for ALL .303" rifles and MG's was set at .040 - .050", regardless. This meant that even if your particular unit or station was using up stocks of Mk6, the FPP was standardised thereafter for all .303" firing guns