Guys, you can't be successful in 45 years of aviation maintenance without learning something. That doesn't make us smart as much as experienced (you know the best experience sometimes comes from mistakes, right?).
I agree with Kirk, but Paul is correct, too. By nature of engine and pressure lubrication system design, internal combustion engines produce oil pressure fluctuations with each revolution of the crankshaft. For example, at 600 RPM, 600 oil pressure "pulses" are delivered to the guage or sender unit each minute. This rate is often too rapid to be seen on the guage, but the guage is still susceptible to wear from these pulses. An inline restriction acts as a buffer to reduce the amplitude of these pulses, reducing guage wear and delivering a stable indication (no needle jitters). What we read on the guage is actually a buffered average of high and low pressures between pulses.
There can be one or more inline restrictions to buffer the oil pressure pulses, including a restricted fitting at the engine or at the guage. Long lengths of small diameter oil pressure line (in our case 1/8" ID) also acts as a buffer.
The oil pressure guage used on most (not all) 150's and 152's incorporates a built-in restriction at the guage connection, as illustrated in the photo below of the guage for your 150M. So, you already have two buffers without worrying about the engine fitting.
The oil pump on the O-200 is not a piston or plunger type that would produce such pulses. It’s a
Gear Pump [
en.m.wikipedia.org], just like most car engines.
I KNOW you know the answer, Kirk. I concede that the presence of oil pressure pulses isn't a big deal and isn't noticeable without advanced equipment. But, you've missed my point, meaning I didn't explain myself well enough. I'll explain it for those who we've made curious.
3rd attempt at this post: I had hoped to not have to right a "lesson guide"!
Pumps (of any kind) create FLOW, not PRESSURE.Restrictions to flow create pressure!
Connect 2 garden hoses end to end with a T fitting between them and mount a pressure guage in the tee perpendicular to the hoses. Connect one hose to a faucet (pump) and leave the other hose open on the end. Open the faucet fully and water flows through the 1st hose to the tee, through the tee and out the 2nd hose (still open on the end). You obviously have flow, but register no pressure on the guage. Place an adjustable nozzle on the open hose end, and you can now vary the pressure on the guage by varying the nozzle's orifice size. The smaller the orifice, the higher the pressure, and vice versa.
Now replace the single nozzle with a Y adapter and 2 more hoses, left open on their ends. Open the faucet fully, and again there is flow through each hose, but no registered pressure on the guage.
Put a nozzle on each open hose, and open both orifices a small amount. Open the faucet again, and you have flow through both nozzles, and pressure registers on the guage. You can still vary the pressure by varying either nozzle orifice, but the important thing to learn here, is that the LARGER orifice determines MAXIMUM pressure available.
Our O-200's (and most other aircraft and auto engines) use a similar lubrication system to the water hose example above. The oil pump draws oil from the sump though the pickup tube and delivers oil flow to a large diameter oil gallery that runs the length of the crankcase. The pressure fitting (or electric sender) is tee'd into this gallery through the crankcase just past the pump. Along the gallery are connecting passages to supply oil to the main bearings and cam bearings (and elsewhere) within the engine. The bearings themselves are mounted stationary with the crankcase, with the crankshaft and camshaft rotating within the bearings. A single large hole in each bearing aligns with the oil passage in the crankcase, allowing oil flow to reach the crank and cam directly. The clearances between the crank, cam, and bearings is measured in a few thousandths of an inch, and
it is this combined clearance of all of the bearings which provides resistance to flow, creating oil pressure. Increase the total clearance of any or all bearings ANYWHERE in the system, and operating pressure drops (as does maximum available pressure).
So far, our system has no fluctuation, other than rising and falling with RPM, as oil flow changes with pump RPM. BUT, we haven't lubricated everything, yet.
The hardest working bearings in the engine are the rod bearings, mounted stationary in each connecting rod. The rods connect the pistons to the rotating crankshaft, and there is no direct way to lubricate them from the main oil gallery.
The simplest way to supply oil to the rod bearings is THROUGH the crankshaft to each set of rod bearings. Passages are drilled in each crankshaft "throw" (the offset journal on the crank where the rods connect). This passage aligns with the same crankcase oil passage that supplies oil to the crankshaft through the hole in the main bearings. Once each crankshaft revolution, this alignment of the crank throw's oil passage supplies oil flow to each rod bearing.
Fluctuating oil pressure? Yes! Each time each individual set of rod bearings receives oil flow through the crankshaft, their total rod bearing clearance is added to the rest of the bearing clearances, causing a small but very momentary drop in oil pressure, seen on a very sensitive guage as a "pulse" of oil pressure. On a single cylinder engine, this occurs once each crankshaft rotation. On a 4-cylinder engine, it happens 4 times each rotation. At 600 rpm, there would be 2,400 pulses a minute (I miscalculated earlier, counting only one cylinder). At max O-200 engine RPM of 2,750 x 4 cylinders, that's 8,250 pulses/minute. The pressure difference of each pulse in a healthy engine is only 2-3 PSI at best. To the naked eye, that's a steady flow at constant flow and pressure, but in slow motion, it's not!
Our gauges are not sensitive or fast enough to display these small pulses, but they are constantly there, beating on our gauges (or electric senders) without adequate buffering.
If these pulses did not exist, why put a buffering orifice at the guage (or sender) itself?
This is not a complete description of our oil systems (there is much more). We have not mentioned grooved main and rod bearings, which our O-200 engines do not (normally) have. But, we can apply what we've learned directly to oil pressure troubleshooting. That's another whole subject for another time.
(What'd I say about mistakes? "The bearings themselves are mounted stationary with the crankcase, with the crankshaft and camshaft rotating within the bearings." Not "with" the bearings. I corrected the text!) 
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