View Full Version : LMT undercut lugs
I've often read here about LMT's bolts being easily identifiable due to their undercut lugs. However, I've never been able to find a pic of what is meant by "undercut," and how it compares with a non-LMT, non-undercut bolt. Can anyone throw a handy pic up of what we're talking about here? Thanks in advance.
Parabellum9x19mm
05-09-09, 12:52
LMT Enhanced Bolt
https://www.mapartsinc.com/products/1190693623_000000000000008%20001.jpg
tweakmeister
05-09-09, 13:02
The right angle, where the main shaft of the bolt meets the lugs, there is an undercut (like a trench) that runs the circumference of the bolt.
http://www.hogens.com/pics/bolts.jpg
Ahh, got it. Kind of like a groove where an o-ring might sit. Clear. Thanks.
Ahh, got it. Kind of like a groove where an o-ring might sit. Clear. Thanks.
Shit! Don't say that! You'll start getting people wanting to put an o-ring there too. :p
Shit! Don't say that! You'll start getting people wanting to put an o-ring there too. :p
Touche! I forgot o-rings can be sensitive topics around these parts. :p Me and my silly way of synthesizing thoughts...
I can hear it now, "Why won't my rifle return to battery???" :D
Thomas M-4
05-09-09, 15:50
Shit! Don't say that! You'll start getting people wanting to put an o-ring there too. :p
Hey i didnt know i needed o-rings around my bolt :eek:Who sells lmt bolt O-RINGS.
By the way is that a KAC E3 bolt in the pic I notice the dual extractor springs and the cam pin looks a little different?
tweakmeister
05-09-09, 16:13
Yes, it's an E3:
-- Larger cam bolt hole (thus smaller cam pin)
-- Dual spring extractor (longer springs last longer)
-- Larger extractor pin that is captured by a small spring bar in the extractor
-- Fully supported bolt face
-- Bigger lugs, more radiused
-- Probably more I'm missing...
Regarding the LMT:
The undercut appears to allow a little flex, which would better distribute the forces the bolt experiences. There are some good topics in the history of this forum about this and I recommend you searching more if you're interested (including bolts made with nicer 9310 steel, etc.)
AISI 9130 versus AISI 8620. Now that is interesting... :cool:
tweakmeister
05-11-09, 21:50
Anvil's new products include a 9310 offering, as do the http://www.leitner-wise.com/hpb.html
https://www.m4carbine.net/showthread.php?t=30091
SAE 9310 seems to be tougher than SAE 8620. I think that may be one way to improve the durability of the AR-15 - especially if the bolt is an enhanced one.
Cool.
Now for the "Carpenter 158" stuff...
Wow, this thread quickly went over my head. Not that it's that hard to do. :p
Good stuff.
SAE 9310 seems to be tougher than SAE 8620. I think that may be one way to improve the durability of the AR-15 - especially if the bolt is an enhanced one.
Cool.
Now for the "Carpenter 158" stuff...
Im not going to pretend like i know the difference between any of this stuff but here is a question.
If we are going to make our bolts harder, how will these bolts wear with the chamber?
Will we see reduced chamber life by a harder bolt?
I wouldn't think one would see shorter chamber life. If anything, I could see shorter barrel extension (lugs) and carrier life. But I really think it would all be a non-issue.
Blankwaffe
05-14-09, 16:49
There some really good information provided here:
https://www.m4carbine.net/showthread.php?t=25471&page=4
It addressed some questions I had on both Armalite and LMT bolt design.
If you do an advanced search for bolt and use Bill Alexander as the author you can find alot of technical informtion that he has provided on the subject.
One his posts in another thread talks about the overall quality of the BCG.As such describes how the carrier should be propery machined and tolerances etc..Which made me look at certain areas on my BCG's.
For instance the first port in the carrier in which the bolt rides.The bolt itself has a bearing band about half way down its length that rides in the front port in the carrier.The other area is the rear of the port in the carrier in which the tail of the bolt rides in,which also acts as a gas seal.
Which seems to vary wildly in the LMT carriers I have as far as fit/tolerance.
Caused me to wonder just how tight or how much play the bolt should have in the carrier.Some of the carreir ports I have for instance,fit a bolt tightly with a very small amount of bolt wobble while others fit the bolts loose and allow more wobble of the bolt in the carrier..
If a bolt fits loose in the carrier and has alot of end play what effect does this have?
On the other hand if a bolt fits in a carrier very tightly what effect could this have?
Ive also seen great amounts of tolerance in the cam pin hole of the bolts.Some of my bolts fit a cam pin tight,others are loose.Seems this is a critical area to look at as far as life of the bolt is concerned.
Definitely taking things to a higher level. Bill seems to be a very bright guy. This is all actually fascinating stuff, but I would add a couple of observations.
A. For the average shooter, what really matters is what comes from the jargon, much like the recommendation Bill made to replace the cam pin after certain criteria are observed. Smart guys do the homework, so the knuckle draggers can take the info, choose a reliable platform and maintain it in a manner commensurate with its use.
B. For the industry professional, including MFRs AND dealers, this is where the men and boys separate. I for one am thankful that some have stood up to ask the hard questions, test, research, test, research, ask more questions and further develop build their products in a manner that benefits us all. Plus 1 for the dealers who really know the products they sell and pass that info onto the end user. Matter-of-factly, a dealer's stock says a lot about that knowledge.
M4C is a great throughput for this flow. :cool:
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