Hi Petar,

Oil pressure sending units are famous for being un-reliable. I suggest that you go to your local auto parts
store and purchase a direct read oil pressure gage for the same fitting as the electrical sensor. Then put
on the direct read (or attach a TEE fitting and attach both the direct read and the electrial) and verify
that the oil pressure is indeed reading ZERO. I think that your oil pressure sending unit is bad. It also
seems to me that you wouldn't be replacing it, unless you already suspected some sort of oil pressure
problem. The rattling suggests a deeper problem (prop).

Next, check the compression by putting a compression gage into the spark plug holes and ensure that
you have good compression. Good is above 90. Check all 4 cylinders. A blown head gasket or bad
rings will give you about 60 or less. I suspect that your compression is good.

Its my opinion that you have bad bearings. I have bad bearings in my atomic-4, and it also would seem
to rattle, almost like a loose mechanical linkage, at irregular but frequent intervals. My oil pressure
would start high (I adjusted it to start around 65 PSI), but would quickly fall to 17-18PSI as the
engine warmed up. I finally pulled the engine and when I put it on a engine stand and removed the
bottom, I found lots of aluminum shavings all throughout the engine. It was bearing material. When
my mechanic buddy grabbed the #3 connecting rod, he could wiggle it around on the crankshaft. This
was the occassional clunk I was hearing. The oil under pressure is squirted through holes in the
bearings, so when they wear, your oil pressure reads low.

Oh, and the reason I said "prop" is the problem, is on my Catalina 27, it came from the factory with a
12 inch by 9 pitch prop, which is way over proped for the atomic-4. This means that my engine would
bearly get about 1/3rd throttle and be at crusing. Overproping lugs the engine, causes high compression
loads on the pistons, and wears the bearings. So if your like me, your prop is the source of the
engine problems. Pull the engine, fix it or have it fixed, and re-prop.