No.4 versus No.1 Barrel Threads

[Nailcreek]

Can a No.4 barrel be safely installed on a No.1 action, assuming correct indexing and chambering? I recall that the thread pitch is the same, but the thread angle is different.

Information

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[jmoore]

Barrel ends up about 180 degrees out of proper rotational position, IIRC. Been a while since I tried it. (loose barrels and idle hands, etc.)

[Bruce_in_Oz]

Pitch is the same: 14 TPI.

Form and start point are different;

No4 threads are essentially standard Whitwoth form: 55deg, nominally rounded roots and crests.

No 1 threads are the same 14 TPI .BUT, the form is an "Enfield special"; 49 degrees and some odd minutes included angle. Furthermore, the radius at the root is different from the crest.

Even if you fiddle with the tenon and shoulder to "adjust the indexing", you will end up with a thread union that does not have anything like optimal engagement area.

Final difference is that SMLE barrels "breech up" on the inner breech ring, like a Mauser 98 is supposed to, and No4 (and No5) barrels use the shoulder of the barrel against the front face of the receiver.

Final twist, so to speak; Mauser 98s use a true Whitworth, 55 deg breech thread, NOT a 60 deg "Metric" / "Unified" / "US" form. Oddly enough, several suppliers of "aftermarket" Mauser barrels seem to cheerfully cut 60 deg threads,

[Nailcreek]

Bruce, do you have, or can suggest a source for the correct tenon measurements? I'm trying to sort through rebarreling a couple of Lee Speed rifles with new barrels. Criterion will end up putting out a No.1 barrel at some point, but it likely won't show up for a few years.

[Bruce_in_Oz]

When I get home I will have a look in the "dungeon".

Are you talking about "original" (long) Lee type, Lee Speeds? The thread is the same as a SMLE, but the "Knox" form is quite different.

That is part of the catch: even your "basic" SMLE barrel has several features that are "indexed: the extractor slot, driving flat on the Knox form and the keyway for the frontsight band.

The earlier "long" barrels are a bit more complex at the "fat" end.

If you are doing your own machining, it is a basic measuring job on an "old" barrel or the the receiver itself, to find the required length.

[Bruce_in_Oz]

Here we go:

Tenon length:-

No.4: 0.693" MIN - 0.696" MAX

No.1: 0.6911" (Low)

No.4, 7.62mm, Enfield spec: 0.680" (Basic)

Depth of Breech thread in receiver (Front of receiver to inner breech ring):

No.1: 0.681" - 0.689"

No.4: 0.701" + 0.005"

Thus, it can easily be seen that the No. 1 barrel MUST breech on the inner breech ring, but the No4 MUST breech on the front face of the receiver.

The 7.62 NATO barrels for the No4 have a (Basic) tenon length of 0.680".

The tenon length for the "Magazine Lee Enfield Mk1*" barrel is also shown as: 0.6911" (Basic). Lithgow was making these barrels, complete with the "special" sight for the "Rifle, Charger-Loading M.L.E. Mk.1*, well into the 1920's, for rifle club use. The settling on the SMLE as THE "standard" brought about the "abbreviation" of a lot of these barrels (and fore-ends) to meet the new rules.

These rule changes also brought into being the Lithgow "H" barrel, so that SMLE shooters could have a barrel with the same diameter (and stiffness) of the "bobbed" MLEs.