Confusing feed cylinder connections

[Mark Hennebury]

20200619_124309 (2).jpgHi, So am rebuilding a woodworking machine that has an air over oil feed cylinder, Air is the power, oil is the control.
So on the rod side of the cylinder is the air line and fitting, on the other end is the oil line and fitting.
Both fittings are for push-in tubes, BSPP thread (British Standard Pipe Parallel) with an o-ring boss seal.


The confusion is that the holes that they go into are tapered thread! R3/8" which is BSPT ( British Standard Pipe Taper)
The o-rings have been removed from the fittings and thread sealant has been put on the threads and in the o-ring groove.

Seems to have been done at the factory like this and has lasted 40 years. I have another machine and it appears to be done the same way.
Seems to be a strange way to do things.


Anyone have any idea why it would be done this way?


20200622_100527 (2).jpg 20200619_124551.jpg

[Bill Dufour]

Only thing I can think is it allows a good seal before it locks down the position. Thus you can angle the fitting to where you want. But both tapering would do the same I think.
Bil lD

[Mark Hennebury]

Bill, These are swivel elbow fittings, so they can rotate to any position after installation.

You can also purchase the swivel elbow fittings in a taper thread; so that's what i don't get, why wouldn't they just put the correct taper fitting with a bit of teflon tape or sealant on it.
It just seems strange to put a a straight fitting into a tapered hole, and to remove the o-rings.

Only thing I can think is it allows a good seal before it locks down the position. Thus you can angle the fitting to where you want. But both tapering would do the same I think.
Bil lD

[Wes Grass]

They bought the wrong fittings, and somebody on the shop floor figured out how to make them work?

Also could have been a torque and sealing issue with tapered fittings. Are they plastic?

I despise NPT. JIS rules.

[Mark Hennebury]

Hi Wes, The fittings are black plastic.( Acetal)
This is definitely not a cheap machine, it was made in Germany in the early 80's and considering that there are only a couple of fittings like this on the machine, it doesn't seem worth their while to go cheap on a couple of fittings. If the plastic fittings were a problem they could have thrown in a couple brass ones.
So, it just doesn't make much sense.
Maybe the sealant was better against the vibration, i really don't know the reason,
And maybe i am over thinking it, maybe something like you said ,they just ran out of tapered fittings, who knows.

They bought the wrong fittings, and somebody on the shop floor figured out how to make them work?

Also could have been a torque and sealing issue with tapered fittings. Are they plastic?

I despise NPT. JIS rules.