For anyone interested in the subject, the two post below from a thread “Proper Liner-Lock Engagement?” are both interesting & informative reading.
The two posts are by Joe Talmadge. Many, many more than a few people consider him an expert……
Proper Liner-Lock Engagement?
by Joe Talmadge
Okay, I think the very question doesn’t make sense, because by design liner locks are meant to change position over time. It would be very odd if a lock that was purpose-designed to change position, really only had one position that was “perfect”. Why design it to change positions then?
The liner is going to wear, period. As it wears, it will move over to the right. At the point that it hits the far scales, there is no more room to wear, so when the liner inevitably wears just a little more, the lockup becomes suspect and blade play gets introduced. At this point, the liner lock is technically worn out, but there are ways to fix it, like introducing a bigger stop pin, which will send the liner back to the left again.
Overall, IMO the answer to the question of “proper proportion of engagement to blade” is, provided the lockup is secure and not susceptible to accidental failure, the entire range of liner-to-the-left to liner-to-the-right is “proper”. Obviously, on a new knife, it’s desireable to see the liner a bit more to the left, since that gives it lots of room to wear, and a longer lifetime. Beyond that, I have seen liner locks that are susceptible to failure even though the lock is way far to the right, or in the middle, or anywhere else. There is absolutely no particular lockup position that will guarantee you best lock safety. I’ve seen liner locks with the liners hanging precipitously off the blade tang way to the left, that I couldn’t fail no matter what. The challenge of liner locks is getting the geometry right across the entire lockup range, not trying to find one perfect position, because none exists.
Another of many variables here is the tang ramp. The smaller the angle on the ramp, the tighter the lockup (to the point that it could take two hands to unlock) but the more quickly the lock wears. The bigger the angle, the slower the wear, but the more critical the lockup geometry. As usual, there’s no clear right answer: just like choice of blade steel and method of heat treatment, it’s up to the knifemaker to make the right tradeoffs for his customers. 7 degrees-ish is popular.
Joe
[ edited to change “pivot pin” to “stop pin” (oops!) thanks to cpirtle for pointing that out]
……
One last note. The liner locks on new knives often have a “break in” period where they move to the right very quickly. So for me, the optimum position of the liner for a brand new knife is hanging very slightly off the left side of the liner. Then I open in 25 times with moderate force, and the liner will quickly move right and engage across its entire face on the left side. But I’m picking nits here, as long as the liner starts off to the left, I’m happy.
Joe