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prepare
04-02-21, 04:58
Looking for an image of a BCG cut-away showing the 3 bore.
Wondering how critical is the bolt tail seal?
Ned Christiansen
04-02-21, 07:30
Looking for an image of a BCG cut-away showing the 3 bore.
Wondering how critical is the bolt tail seal?

The fit there is held pretty tight; if we talk about it as a gap all the way around it's held to min. .0004 per side to max .0012. I have seen all of one that, between wear and/or parts that were maybe out of tolerance to start with, it leaked enough gas there to be a stand-alone problem.
prepare
04-02-21, 09:23
The fit there is held pretty tight; if we talk about it as a gap all the way around it's held to min. .0004 per side to max .0012. I have seen all of one that, between wear and/or parts that were maybe out of tolerance to start with, it leaked enough gas there to be a stand-alone problem.

Thanks for the clarification.
This provides some context as to why it’s not a good idea to aggressively over clean the bolt tail area.
Kadelic
04-02-21, 10:48
This is a crop from an old blueprint for the carrier. You can see the tight tolerances down to +/- .0003 in addition to the true position callouts and surface finish requirements.

Interestingly, the .5315 diameter calls out a .0010 tolerance, but the tolerance block on the drawing doesn't have a value for a four place decimal. Gotta love engineers.


65526
Clint
04-02-21, 11:38
That bolt tail seal is probably the most critical dimension on the whole carrier.

It's also the worst offender area for out of spec with lower quality units.


Thanks for the clarification.
This provides some context as to why it’s not a good idea to aggressively over clean the bolt tail area.
Stickman
04-03-21, 11:21
That bolt tail seal is probably the most critical dimension on the whole carrier.

It's also the worst offender area for out of spec with lower quality units.


Some people need to read that twice and let it sink in.
markm
04-03-21, 14:42
This provides some context as to why it’s not a good idea to aggressively over clean the bolt tail area.

Interesting. I've never given bolt tail seal much thought. I've seen that the metal doesn't foul there however. I used to clean my bolt tails very aggressively and never had an issue.
gas1
04-11-21, 16:27
Check out the sotar school of the American rifle on u-tube for great gauge ideas