Hi all, hopefully a quick question here. I just replaced the oil pressure gauge on my 51 8N. On cold startup, it reads right around 20 PSI (give or take a couple), and the needle bounces/vibrates...never holds steady. The old gauge read about the same I believe, but it didn't bounce; it was steady. So my questions:

1- Do I have a faulty gauge, or would something else cause it to bounce?
2- What is the "range" for good/normal oil pressure? (10w30, clean and full oil (no gas/water in it), not sure when/if engine has been rebuilt; if it has it was a long time ago.)

Thanks in advance!

Oil gages usually have a steel pin located in the intake port that snubs the pulses from the oil pump and keeps the reading steady, if that is missing or stuck may be the bouncing issue. I had one gage where that pin came out and needle bounced.

Before connecting the new gauge, I would have started the engine briefly so all the accumulated sludge in the oil line can be flushed out.If the gauge still reads erratically, I would try another gauge. Your pressure is low, especially on a cold startup. I run 15w-40 in my 9N. It is a somewhat "tired" engine. My startup oil pressure is 40psi. After about a hard workout of mowing for 1.5-2 hours, my pressure falls to 15-20 psi hot.

Thanks Farmer Brown; I did forget to purge the air out. I disconnected the line and did that, and then re-connected. The needle still bounces, but not as bad; I can live with it for now.

I'm still getting around 20 PSI on startup, and warm, and when full throttle about 25 PSI. Next time I change the oil, maybe I'll give it some heavier weight oil. Is 20 ok to run it for now? How low is too low?

You should be fine with that oil and pressure. Thats a flathead and does not need a lot of pressure for upper end lube.

Dan

Thanks to both of you!!

I run Shell Rotella 15w-40 in my '52 and it keeps the oil pressure in the safe zone in hot Virginia summers, but I rarely use my 8N for long periods anymore.

That is too thin for summertime. In your case, I would run either 15w-40 or 20w-50. If 20-25psi is cold, I would pay attention to the pressure after the engine is very hot.The pressure could fall into the danger category.

As a rule, you want a minimum of 10 psi for every 1000 rpm of the engine to be safe. I think yours is too low.