why did they cut the threads

[ActionYobbo]

what is the purpose of this cut in the threads (see red arrow)

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[Woodsy]

It is a place for crud to ooze into (as shown in your photo) and allow to bolt to go into battery when the rifle is dirty and circumstances prevent you from cleaning it (like when someone is shooting at you!).

[Peter Laidler]

I've got a funny feeling that that is the starter for the cutter as it cuts the extractor way. The extractor way can't be cut from the rear so has to be cut from the front as the body tilts to accommodate the shape. Just thinking on my feet but that's why a rifle is ZF when it is damaged externally at that point.

[ActionYobbo]

I've got a funny feeling that that is the starter for the cutter as it cuts the extractor way. The extractor way can't be cut from the rear so has to be cut from the front as the body tilts to accommodate the shape. Just thinking on my feet but that's why a rifle is ZF when it is damaged externally at that point.

that would be logical if the extractor cut was not on the other side

[Lee Enfield]

It's a fairly common artifact in (all types of- Marlin lever actions for instance-) receivers, usually from a broaching operation.

[Woodsy]

I have just examined 6 FAL bodies I have lying around. The groove aligns perfectly with the upper left corner (looking forward) of the squared bolt recess in the receiver ring, so it has clearly been formed as part of the broaching process to form the bolt recess. There is also another similar groove on the opposite side. The extractor recess has clearly been cut from the rear of the action with a special end mill as it comes to a square blind end before the barrel thread, and the axis of this cut is angled upwards towards the rear by about 10 degrees. This extractor cut could only be made by a special fixture and custom tooling in a milling machine.

[ActionYobbo]

I have just examined 6 FAL bodies I have lying around. The groove aligns perfectly with the upper left corner (looking forward) of the squared bolt recess in the receiver ring, so it has clearly been formed as part of the broaching process to form the bolt recess. There is also another similar groove on the opposite side. The extractor recess has clearly been cut from the rear of the action with a special end mill as it comes to a square blind end before the barrel thread, and the axis of this cut is angled upwards towards the rear by about 10 degrees. This extractor cut could only be made by a special fixture and custom tooling in a milling machine.

by your description I assume these are not Lithgow produced bodies you have. I have several lithgows and they dont have any groove in the threads on the extractor side but the imbel's and FN's do (must be a metric thing).

I wondered if it was a safety feature like a built in fail point because there is really no other practical reason for it to be there. That part of the bolt recess can be completed from the rear just like the extractor recess was done. To me it makes no sense to do the extractor side from the rear and then just finish off that corner from the front and run into the threads while doing it.

[browningautorifle]

I wondered if it was a safety feature like a built in fail point because there is really no other practical reason for it to be there.

It's just the way the machines did it, that's the direction it approached. The thread is not needed at that point. Do you sit up at night and think of things to worry about? Look up the sequence of machine work on their receivers and you'll find the answer...

[ActionYobbo]

Do you sit up at night and think of things to worry about? Look up the sequence of machine work on their receivers and you'll find the answer...

my next step in machining is to do this area and I can find no information on these on the www. I dont want to waste time and effort making a fixture to set up to do that cut if its just window dressing but if it is a safety feature then I have to do it

[Bruce_in_Oz]

No Mister E. what you have there; a good start!

May the swarf be with you!