To start with what many of you no doubt already know, the oil pump in Atomic 4 engines is integrated into the rear main bearing cap. Oil from the pump is directed through a channel around the rear main bearing shell (just inches away) and from there to the rest of the oil swept system as per the following schematic.
Note that oil pressure is manually set by an adjustable spring loaded re-circulating valve in the rear corner of the block just behind the carburetor. Pressure is measured at the opposite end of the oil swept system, near the flywheel end of the block. Between the points where the pressure is set and where it is measured, oil flows through a generously sized oil galley cast into the side of the block. In this way, oil is provided to most of the oil swept system at a pressure close to the pressure indication in your cockpit. The only bearings downstream from the oil galley and the point where the pressure is measured are the front main bearing and the two rod bearings at the flywheel end of the engine.
This pressure distribution means that the actual pressure on these front-most bearings can be somewhat lower than what you’re seeing in the cockpit and will be the first bearings to suffer as oil pressure decays below recommended minimum pressure. This leads to a great Atomic 4 Trivial Pursuit question; “If you hear of a bearing failure in the aftermath of reported low oil pressure, which is the most likely bearing to have failed?” Answer: the second rod bearing from the flywheel end of the engine.
UNIVERSAL’S RECOMMENDED OIL PRESSURE FOR AN ATOMIC 4: Throughout most of the 1970s and later, Universal recommended any good quality detergent, straight viscosity, 30 weight or 10-30 multi-viscosity oil. If you operate your boat in colder climates (frequent drops below freezing), you can drop down one viscosity rating in each case. Factory recommendations for minimum oil pressure have been rather sparse over the years and limited to only a few RPM ranges. In our own MMI Service and Overhaul manual, we originally used Universal’s recommendation of 40 psi at 3000 RPM as a safe across the board oil pressure for continuous operation. However, we have come to learn over the years that many engines simply won’t maintain 40 psi at normal cruising power settings, and in some cases, setting the oil pressure to 40 psi at normal cruise will result in needlessly high pressure while the engine is cold.
To assist in extrapolating recommended oil pressure to other power settings, we constructed the following graph by plotting the (relatively few) specific oil pressure recommendations contained in five Universal Owner’s Manuals that we were able to collect from the 1970s.
From this graph, you can see that at Universal’s recommended cruising power setting of 2000 RPM, the minimum recommended oil pressure would extrapolate to 23 psi for continuous operation. For those of you given to fact checking, one of the 5 owner’s manuals we used as background in preparing this newsletter showed 3000 as the recommended cruising RPM. Since all the rest of the manuals were consistent in recommending 2000 RPM, we assumed the manual with the 3000 figure to be an outlier.
It’s important to note here that depending on the air and water temperature of the day, your cruising power setting, and of course the condition of your engine; oil pressure will usually continue to decay for several hours into your trip. Therefore, the most important time to note your oil pressure, and to make an adjustment if necessary, is when the oil pressure has leveled out at its lowest value while at your favorite cruising RPM. Again, this point will usually only be reached after a period of several hours due in large part to the heat sink provided by the large cast iron oil pan, and the cooling effect on the oil as it is continuously recirculated through the reversing gear and back to the oil pan.
We’re told that automotive engineers use a generic rule of thumb of 10 psi oil pressure for every 1000 RPM as a minimum pressure for continuous operation. We only mention this rule of thumb because it would give you another 2 to 3 psi to play with below Universal’s minimum recommendations in an emergency should you ever be faced with having to shut down your engine while in harm’s way.
The most important thing to remember about oil pressure is that as long as you maintain above the recommended minimum pressure for your RPM, all your bearings are receiving enough oil under pressure to provide a hydraulic barrier in all bearing clearances to keep them working indefinitely (at least in theory) assuming the oil is kept clean. When oil pressure decays below the recommended minimum, some accumulative permanent damage will be incurred by the bearing shells. As seen in the above flow schematic of the oil system if you press too long below the minimum oil pressure recommendations, the metallic clanking sound you will be hearing is most likely the #2 rod bearing starting to fail.
If a significant drop, or total loss of oil pressure occurs, serious damage to this part of the engine can happen between the times that you would normally be checking your oil pressure gauge which leads many folks to add some form of warning system , or diagnostic system, to back up their pressure gauge.
WHAT CAN BE DONE TO REMEDIATE LOW OIL PRESSURE (without disassembling the engine)?
- Given the seriousness of low oil pressure, a manual gauge should be installed to replace the oil pressure sending unit to verify the cockpit indications. A standard 0 to 100 PSI hardware store water gauge will suffice, but a small needle valve (like those used in a kitchen refrigerator ice maker line) should be installed between the engine block and the gauge to dampen the indications on the gauge for a more accurate reading.
- The obvious first step in remediating low oil pressure should be turning the regulating valve in a turn or two (in a clockwise direction). The regulating valve is not a very sensitive device. You can first try an arbitrary turn in a clockwise direction before even starting the engine and then checking for the desired result the next time you’re out.
- If you have a late model spring and ball type of regulating valve, and you’re unsuccessful in maintaining oil pressure within normal range, you might try an early model sliding plunger type of valve. (Our product number OOIL_06_122). These valves tend to be a bit more consistent in regulating pressure and sometimes a few more psi can be obtained than is possible with the spring and ball type of valve.
- You can dress the seat in the block very professionally with one of our regulating valve seat dressing tools (Product No. – TOOL_05_172 in the “Specialty Tool” section of our online catalog). It takes just a matter of seconds to create a very clean symmetrical seat in the block to line up with the centerline of the sliding part of the regulating valve.
You might try a heavier oil, but we don’t recommend going any heavier than 20-40 detergent oil. Heavier oil tends to give you a higher reading on your gauge, but (satisfying as that might be) the flow and pressure across the bearing clearances will actually decline so going to a heavier and heavier oil becomes counterproductive.
WHAT CAN BE DONE TO REMEDIATE LOW OIL PRESSURE DURING A REBUILD?
- Insure that main, rod, and cam bearings have proper clearances (main and rod bearings .001” – .0025”, and cam bearings .0015” to .0025”).
- Replace idler gear bushing if required to provide a clearance between .002” and .004”.
- The oil holes in late model idler spindles are approximately .090” in diameter. The oil holes on early model spindles were considerably smaller. It’s our understanding that the larger holes in late model spindles were to provide a bit more oil flow from the spindle to be flung over to the accessory drive catch basin to lubricate the accessory drive bearings. If you modified your accessory drive to raise the oil pool you could safely use an early model idler gear spindle (if you can find one) to give your oil pressure a little boost. Insert link to accessory drive modification tech tip
- Oil pump: If you know that an engine has had good oil pressure prior to a rebuild, the oil pump should be OK to reuse without any repair work. However, if oil pressure history is not known prior to a rebuild, or oil pressure was known to be low, inspect the oil pump thoroughly. Oil pumps that meet the established criteria in terms of maximum gear back lash can still be problematic. After we started testing oil pumps on our test stand in 2009, we discovered a wide range of performance on oil pumps coming to our shop for rebuilding. We discovered after considerable testing that an oil pump output less than .8 GPM (one gallon in 1 minute and 15 seconds) at 1050 RPM against a backpressure of 40 psi would not sustain adequate oil pressure in an engine on our dyno stand after several hours running at normal cruising power after oil pressure stabilized at its lowest value, even in engines with pristine bearing clearances throughout. Approximately half of the pumps tested from the engines going through our rebuilding operation fail our pass/fail criterion. Only rebuilding the oil pump, to include re-machining the gear cavity and gear shaft holes in the main bearing cap, and then installing oversized gears and shafts restored these pumps to reliable service. If you have any doubt regarding your oil pump’s condition, you can call our Sales and Service line (610-421-4436) to make arrangements to have us test it.
RELATED ITEMS OF INTEREST:
FULL FLOW OIL FILTRATION: Full flow oil filters are designed into an engine between the oil pump and all the rest of the oil swept system, including the important main, rod, and cam shaft bearings. For whatever reason, Universal never provided a full-flow oil filter capability for the Atomic 4. We’re guessing that cost was a big consideration. Full flow filtration is an obvious benefit on any engine and the Atomic 4 is no exception. In addition to the normal contamination that collects in engine oil, the Atomic 4 also picks up a bit of additional contamination due to the fact that the reversing gear is included in the oil swept system. The reversing gear has the capability of generating small amounts of debris including rusty particulates in the aftermath of water intrusion episodes.
With the advent of C&C machine technology, we have been able to modify our new engine blocks to add full flow filtration for not much more than the cost of an aftermarket by-pass type of system. At this point in time, it is not possible to add full flow filtration on used block engines.
AN UNUSUAL CASE HISTORY OF OIL SYSTEM FAILURE: The most unusual and interesting oil system failure to come to our attention in recent years involved one of our engine customers totally and instantly losing all oil pressure. On teardown, it was discovered that the tiny 1/8” roll pin that secures the oil pump drive gear to the pump shaft had failed. Closer examination of the failed roll pin revealed that it had been somewhat compromised by rust, no doubt from prior water damage during one or more water intrusions of the type we hear about when engine cooling water washes back in from the exhaust system. Since then, we’ve been replacing those little roll pins (and you can as well) during an overhaul. They’re available at any local hardware store for about ten cents. What made this case so interesting is that it occurred in an engine we had shipped to Sweden. In fact, we had barely stopped celebrating our first engine sale to Europe when the email arrived informing us of the oil pressure loss. I’ll let those of you more in touch with the Spiritual Realm to ponder why this unique failure had to happen in an engine we had shipped the greatest distance. It may be nothing more than confirmation of the famous quote attributed to General Patton at the end of the Second World War; “All Glory is short-lived”. As always, our newsletters are only intended to pass on what we think we know at any point in time along our own learning curve. In fact, much of what we do know we learned from you, so keep well and stay in touch.
The MMI Team
“If you hear of a bearing failure in the aftermath of reported low oil pressure, which is the most likely bearing to have failed?” Answer: the second rod bearing from the flywheel end of the engine.
“Therefore, the most important time to note your oil pressure, and to make an adjustment if necessary, is when the oil pressure has leveled out at its lowest value while at your favorite cruising RPM.”
“Approximately half of the pumps tested from the engines going through our rebuilding operation fail our pass/fail criterion.”
“Full flow filtration is an obvious benefit on any engine and the Atomic 4 is no exception. In addition to the normal contamination that collects in engine oil, the Atomic 4 also picks up a bit of additional contamination due to the fact that the transmission is included in the oil swept system.”
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