I’m not sure the wisdom of adding a washer under an NPT-threaded fitting at all, nor under a straight threaded fitting not originally specified to have a washer (especially in an aviation or other highly regulated application).
I wonder if you could get by in some of those applications by having the tag in a grease zerk cap instead (installed after and without disturbing the zerk torque/seal).
On my airplane, I rotate grease cap colors as a visual check that all fittings were serviced during the annual inspection. It’s a multi-day affair with more than one mechanic plus me plus interruptions, so it’s not difficult to miss some. But if the cap is blue and this year’s color is red, you find it at final walk-around.
For anybody who doesn't already know what NPT-threads are: it's a standard for tapered threads. You see it frequently on pipes, or anywhere you need the threads to seal up.
The point of it being tapered is that the fit between the male and female parts gets closer as you tighten the fitting. In domestic plumbing, you wrap the male threads with Teflon tape, and as you tighten, the tape gets squished into the very close fit and forms a seal.
The key point to understand before introducing a washer into the system is that there isn't a well-defined point where the system can be considered bottomed axially. It isn't like a nut and bolt where once the nut hits the bolt head, it isn't going any farther.
If you introduce a washer that doesn't allow the tapered thread to tighten to a sufficiently close fit, you've potentially lost your seal and might have a loose fitting. If your Zerk falls out, you risk loss of lube and/or contamination of the lube.
I'm sure they've given this some thought, but I would also be curious to hear how this works.
I think this is a very good idea and point. I raised the idea of GreaseBoss on the reddit aviation forum a few weeks ago and got shot down by all the questions about regulartory issues and approvals.
I wonder if you could get by in some of those applications by having the tag in a grease zerk cap instead (installed after and without disturbing the zerk torque/seal).
On my airplane, I rotate grease cap colors as a visual check that all fittings were serviced during the annual inspection. It’s a multi-day affair with more than one mechanic plus me plus interruptions, so it’s not difficult to miss some. But if the cap is blue and this year’s color is red, you find it at final walk-around.