Oil Pressure Gauge

[EricOlson]

I just fired my newly rebuilt 1940 UA flathead motor for the first time and the oil pressure light immediately came on. I let it run for a few minutes to see if it would build pressure, but the light never went out. I pulled the return oil line and verified that oil was circulating back into the tank, and it was flowing good. I checked the wiring to the pressure switch and it all seemed correct, and since the wiring was working before the engine rebuild I don't suspect it suddenly went bad. The pressure sending unit switch was also working fine before the rebuild, but it is about the only thing I can think of that might have gone bad and I don't really have a good way to bench test it.

At this point I'd like to install an actual pressure gauge to verify if I have any pressure. I can find an adapter to convert the 3/8-24 male threads to 1/8" NPT female threads so I can install a gauge (https://aaok.com/index.php/extension...1958-1984.html), but what I'd really like to do is install a T fitting so I can permanently run both the stock pressure switch and a pressure gauge. Anyone know of T type fitting that would be 3/8-24 male on one side, 3/8-24 female on the opposite side, and 1/8" NPT in the middle?

Also, just for informational purposes I replaced both the feed and scavenger pumps during the rebuild because the old ones had issues. I also replaced the cams with a set of Andrews, and we verified the cam to feed pump fit was good. As I said earlier, I know oil is flowing because I pulled the return line and watched it pump out. I just don't know how much pressure I'm building.

[T. Cotten]

Is your sending unit aftermarket, Eric?

Our twins need flow, not pressure, as they are basically a 'splash motor'. (The pinion feed to the crank is a pump in itself while running.)
Pressure only results from resistance to flow, so gauge readings are only an indication of how the motor is warming up.

OEM sending units opened at four psi or less!
When I tested DIXIE repops, they varied from around seven to eleven...

SENDTEST.jpg
.....Cotten

[EricOlson]

Is your sending unit aftermarket, Eric?

I think it's aftermarket, but I'm not 100% sure. Many of the parts on the bike were aftermarket when I got it. I may have to try a setup like yours to test it's functionality. Thanks for the input Cotten.

[EricOlson]

Turns out the sending unit was fine, but I had a couple problems with the feed pump. First off I bought a new, repop feed pump and the seats for the check valves were non-existent. Looks like they just drilled the passages and left the raw ends from the drilling where the ball would need to seat. When my engine builder went to cut in good seats he found that the passage was not concentric with the outer bore, so he couldn't use the seat cutter that he had. We ended up pulling the guts (rotor, balls, springs, etc.) from the repop pump and hunting down a decent OEM pump housing that we could make work. Lapped in the seats and no more wet sumping. But we still had no pressure. Turns out the rotor sat way too deep in the repop housing so oil was just flowing past, and it still sat a little deep in the new OEM housing. We tried several gaskets trying to find the right thickness to make the rotor stand out approximately 0.005", which is what appeared to be the correct amount. In the end we had to make our own gasket, but now I have good pressure and the light goes out right after start. Took a fair amount of asking around and measuring parts to finally figure out why we had no pressure, so I thought I would share just to close the loop in case anyone reads this post down the road.

[T. Cotten]

Nobody's surprised, Eric,...

That a repop pump has issues!

As to the check seats, I have never had the opportunity to inspect an OEM NOS pump, so I still cannot be certain what the Factory did, if anything. The seats we see, most often quite worn, are obviously not as they were produced.

My studies of repairing the seats showed that only a flat-grind to remove the wear wouldn't make things worse, because of the very concentricity issue your builder found. Any attempt to dress the sharp edge only made leakage increase, as shown by bubble-testing at 3~4 psi.

CHEKTEST.jpg

All attempts to cut, grind, burnish, lap, or beat with a drift proved futile, .

It very well could be that the Factory started out with a simple hole, but the OEM casting was far more flat!

....Cotten