For what it's worth, I was curious about a more scientific answer about thread engagement length and did some searching.
Most machinist handbook suggest anywhere from 1.5 to 3 X the bolt diameter assuming like material tensile strengths. The intent and general good design results in the screw or bolt failing before the female thread.
This link provides some calculators and additional design information on thread engagement:
http://www.engineersedge.com/thread_...engagement.htm
It also notes the above assumes the tensile strength of the bolt/hole materials are equal. Unfortunately in regard to brass/acetal we don't have that, so you
should adjust it even longer by the stress ratio.
Yield Tensile Strength of Acetal = 8-10,000psi
Yield Tensile Strength of Brass = 19,600psi
So...technically you should actually increase that thread length by about a factor of 2
Regardless I have yet to find something to suggest much less than about 1.5X the barb diameter and we have a worst case scenario mixing acetal and brass where the female thread actually has about 1/2 the material strength of the brass barb. Looking at a typical barb, I see many with only a measly 2-3 threads or about .4X. Granted the barb itself is also compromised with it's reduced x-sectional area being drilled out to extremes, but the numbers seems to go far below typical machining standards.
I personally have stripped a block or two in my day of wrenching on standard barbs so it does appear even with the full engagement of the barb that we are already dealing with what most machinists would call a bad threading design in our barbs thread lengths. It has been a copied and accepted compromise made to reduce materials costs and why most users limit torque to hand tightening methods to mitigate the poor design. Unfortunately most barbs still today do not provide any torque guidance so stripping has been and will continue to be a problem for new users even with plenty of block threads to match that of the substandard barb thread depth.
Bottom line, barbs themselves are already poorly designed to handle normal torque limits possible using tools and should include some sort of torque guidance. If a block design utilizes even fewer threads they should seriously consider providing some torque guidance. Similar to that of a spark plug or as Swiftech has done, tell users they should tighten to "X" turns after o-ring contact. Without that critical guidance, it's anybody's best guess as to what is enough or too much...my 2c...never liked accepting hand tightening of barbs as it is and one of the reasons I've always favored brass tops where you get twice the strength.
As far as testing on multiple chips goes, don't burn yourself out. Every chip out there will produce a slightly different heat signature and produce completely different results, but nobody is a machine so cut it off clean with your current platform and let someone else slave away at it later..:)
