By, Jack Davis
There are two oil pressure ports on the W-670 engine. One on the right of the accessory case and one on the left. Although I have seen original engines that use the port on the left, and then route the oil pressure line back across the firewall so that the line goes down the right side of the fuselage, this seems like a strange approach. Using the port on the right allows a neater routing to the right side of the instrument panels.
In any event there is quite a length of 1/4 inch tubing and a number of fittings leading to each oil pressure gauge.
If a break occurs anywhere in this line or in the gauge itself, oil will come pouring out. With 60 to 90 psi behind it, and a 1/4 inch tube, it will take a shorter time than anyone would wish for, to have most of the engine oil residing on the inside bottom of the fuselage.
However, that mess is the least of the worries.
Although I haven’t seen it in any of the Stearman or W-670 drawings (that doesn’t mean it isn’t there), many people use a method to limit the oil flow in the oil pressure line
This involves putting a flow restrictor in the fitting coming out of the accessory case.
This restriction is in the form of a #42 drill hole in the inlet side of the fitting.
Coming out of the accessory case is a hydraulic fitting with 1/8 in pipe thread on one end and a 1/4 inch flare on the other. This can be a “straight”, but a “45 degree” gives a better routing for the attaching oil line.
One method of modifying the fitting is to run a 1/4-28 tap in the pipe thread end. It turns out to be just the right size. Just tap part way down. Then thread a bolt into the hole until it is good and tight, and cut it off flush with the fitting. For a brass fitting, use a brass bolt, and for an aluminum fitting get a 1/4-28 aluminum bolt. (you knew that).
Now dress up the end, center punch it and drill a #42 hole through the bolt end.
Now when the fitting is screwed into the accessory case, the small hole provides considerable reduced oil flow in the event of a break downstream.
The restriction does not affect the reading on the oil pressure gauge, since there is no flow in normal pressure sensing. What this does is give some more time before a critical amount of oil could be lost. Time that could come in right handy.