Is this a knuckle thread?

[jccaclimber]

Trying to make a part that interfaces to something I already have. It's a low stress joint so just making something up would work, but I'd rather understand what's going on and learn something if I can.

Manufacturer is in Germany, so I'm guessing metric. My thread pitch gauge is inch and measures it as 6 TPI, but this is close to 4.25 mm pitch. Major over the threads is 157.96 mm = 6.219". I don't have the right pins to get a good minor diameter, but it's pretty close to 154 mm.

It isn't sharp enough to be a standard thread, isn't square enough to be acme. I found knuckle thread online, but this is the first I've heard of it. The closest I was able to find was Rd160x10 mm and Rd160 x 4 TPI, but the major diameter is too small and the pitch isn't right on either of those.

Ideas?

 

[sfriedberg]

Is this a plastic bottle or a garden hose fitting? Those both use industry-specific threads.

 

[jccaclimber]

Aquarium part, but not one that seals any pressure. There is an face sealing o-ring inside to keep the bubbles inside from seeping out.



Hose threads are too sharp, but bottle threads might be right. I've just found a standard going up to 48 mm for those, but I'll do a bit more searching.