How does NH350 engine breather work?

[Paul Gasson]

Can anyone help to explain how the engine breather in the mag. chain enclosure on my 1955 NH350 works? I have an elbow screwed into the case and held in position with a locknut. After removing the pipe by loosening the pipe fitting nut I found a ball bearing which is not on any parts list I have. This is loose inside the elbow and held by the pipe tapered fitting. The ball seems to seal against the pipe end fitting and this would happen with gravity and further pressure when the piston is descending, just when I would have thought the engine would need to release pressure? Is there a spring missing from in here? It would work better if held away from the pipe under light spring pressure that could be overcome by increased bottom end pressure.

The other curious thing I found was that the elbow screwed into the case has a cross drilled hole through it near the thread so it looks like pressure can be released here around the elbow rather than down the pipe.

Am I missing something in understanding the design or missing some parts?

Thanks,
Paul

[pete.collings]

The elbow contains a ball bearing that is held in place by a split pin pushed through the bottom threaded parts of the union, the parts that faces downwards and to which the breather piper is attached. The breather should work by allowing pressure out, the split pin preventing the ball from falling out, and negative pressure should suck the ball back upwards and block the central hole,thereby (hopefully) creating a negative pressure in the crankcase.

I know others prefer to remove the ball completely, especially if there is higher crankcase pressure (eg piston blowby), to allow the excess pressure to vent unhindered by having the flow around the ball bearing. I believe the ball bearing is 1/4" diameter, and hence a loose fit in the union, but I don't have a union to check this against.

The part that screws into the inner timing case has a much smaller thread than the outlet side, which has a standard (petrol/oil sized) thread, possibly BSP.

There are other brass elbow fittings that could be adapted to fit, it is common to shear the smaller thread, which should have a thin locknut, by overtighting or excess force if it corroded in place.

[Paul Gasson]

Thank you Pete, what you have described is perfectly understood and I now know how it should work. No wonder it leaked some oil last year; the valve was seated and sealing against the breather pipe when it needed to be open. My only problem now is that the cross drilled holes for the split pin are under the pipe fitting nut when it is fully tightened so no room for the ends of the split pin. I wonder if a short piece of steel rod shorter than the diameter of the fitting would work?

[pete.collings]

You are right that the ball retaining insert has to be slightly shorter than the thread size, to enable the breather extension ppe to be fitted.

The problem with using a straight piece of rod is that it can move (unless it is a tight fit, or loctited in place) and possible be lost at a future time.

I use a split pin cut to the appropriate length to allow fitting of the extension pipe, and fitted so the two halves are facing downwards. Then a careful bit of work with a fine screwdriver or scribe can prise the two pieces apart in the centre, so that they cannot move outwards. If you ever need to remove them, a pair of fine nosed pliers can squeeze then together again.