Radial Engine lubrication

Basic question, how were Radial engines lubricated at start-up? I can picture the oil pumps working when the engine was running, but for the initial starting, how were the upper pistons, and valves lubricated?

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Hi,

Just like almost all piston engines, the Radials mostly relied on building oil pressure only as the engine started. However, there were mitigating procedures. Usually, engines were turned slowly (usually by hand) before a start attempt to test for hydraulic lock, to help with priming and this did turn the oil pumps as well. Additionally, many engines with electric starters have a count of blade turns before the ignition is turned on, and this helps. Engines that have stood for a considerable time have procedures for anti-deterioration that include priming with oil at a special adapter in the oil system. Some engines have pre-oiling, whereby an additional, usually electric, pump can pressurise the oil system before a start.
Taken together, these procedures are effective on engines. However, on very low utilisation engines, such as some vintage types, the original methods of anti-deterioration may not be sufficient.

Eng

When you build up ANY WWII aero engine from scratch, radial or inline, you "pre-oil" it before first startup. I have first-hand experience with the Allison V-1710.

Basically, we leave the valve covers off and get a propane tank, fill it with oil about 3/4 full, pressurize it, and put oil in from the bottom of the engine. When it starts to leak out in the valve lifter area, it is pre-oiled. Then you can put the valve covers on, bolt the engine into a stand if it isn't aleady, connect everything, and start it for bench run-in. There is a definite procedure for bench run-in. WE always ran ours until the exhause manifolds turned from dark black (oil) to light gray (normal color). When that happens, you have seated the rings and the engine is ready to fly for break-in.

In my experience, most but not all radial engine fighters have a pre-oil pump. When you go to start it up after some time, say several days to a week, you have to run the pre-oil pump for awhile (unsure about duration, it varies from engine to engine). Then, before you try to start it, you have to pull the propeller blades through at least one revolution to see if the bottom cylinders are full of oil. If so, the prop stops because you can't compress oil. If it stops, you have to remove the spark plugs from the bottom cylinders to drain the oil.

Once drained, you can re-install the cowling and start the engine. To start, you leave the mixture at idle cuttoff and run the starter through 3 complete revolutions. For a 4-blade prop, like a Bearcat, that's 12 blades. Once you turn it through 12 blades with the starter, you can put the mixture to full rich and start the engine normally. Some radials need a shot of prime and some don't. The engine will belch and smoke until the accumulated oil in the bottom cylinders has burned off and it will then settle into a normal idle. By that time, it will be warm enough to taxi.

What RPM during ring seating on the test stand/

Edit: Made a mistake. Changed Tightbond to the correct TightSeal.

For the Allison, we ran it at idle until warm, then changed the oil and cleaned the cuno screens. We did that maybe 3 times and then ran the engine slowly up from idle to 1500 - 1800 rpm slowly up and slowly down. Aftrer some 2 - 3 hours, we'd run it up to 2500 rpm briefly and then back down to 1500 - 1800 rpm over several hours, cleaning the screens and changing the oil by our own schedule.

The engines were assembled with TightSeal per WWII manuals and it would run through the engine to the oil screens. When the screens were clean after 2 hours of running, we'd assume the loose TightSeal was out of the engine and then run that oil until the rings seated. After the rings were seated, Joe always ran it up to 2850 rpm very briefly, just to check that it was good at rated power. After that, we'd put the engine on a stand and crate it for shipping, together with a sheet of instructions for mounting and connecting the engine, and a caveat that if the engine sat for longer than 2 months before running again, it should be pre-oiled again before startup.

That was at Joe Yancey's shop. During the time I worked there, we did maybe 12 Allisons. When I say "we," I mean Joe and his wife Pat, I helped out as they allowed, but Joe did the vast majority of tasks on the V-1710s. Pat did all the ignition harnesses. He and she just retired and are no longer in the business. He kept a left-hand turn Allison on a stand that runs.

May I assume Tightbond is not the popular wood glue?
Your run in is very reasonable to the way I ran in auto engines 50s 60s 70s. We never ran the car at higher RPM until about 1500 miles. The first 500 was the first oil change. Road mileage was not at a set speed, but gentle accel and gentle decel to seat the rings evenly. Granted, this was with old style oil (Quaker State or Pennziol) and much lower temps than today.
I was curious because TV performance auto engine programs show a new engine in the test cell run up to max RPM just after assembly. I usually can't watch to end.

pinehilljoe said:

Basic question, how were Radial engines lubricated at start-up? I can picture the oil pumps working when the engine was running, but for the initial starting, how were the upper pistons, and valves lubricated?

If this is off topic, moderators please move to the right forum.

Click to expand...

Hi!
I work on Russian radial engines (ASh-21 and ASh-82) which are based on the Wright Cyclone.
Neither engine produce any oil pressure until they are running which always gives you some heart-stopping seconds until pressure shows on the gauge.
Most rotating parts are lubricated via oil galleries but some are splash lubricated as are the pistons and con rod pins so rely on the engine to be running to be oiled.
You can pre-oil the engines which is a good idea if it's the first run after an oil change or if they haven't run for a while. I use an old fire extinguisher which I fitted a Schrader tyre valve to so I put some engine oil in the extinguisher, blow it up to 60psi and force some oil into the oil galleries. But this only circulates oil to certain parts, it does nothing for the cylinders. The valves are in the circuit so would get some of the oil that drips out of the rockers.

special ed said:

May I assume Tightbond is not the popular wood glue?
Your run in is very reasonable to the way I ran in auto engines 50s 60s 70s. We never ran the car at higher RPM until about 1500 miles. The first 500 was the first oil change. Road mileage was not at a set speed, but gentle accel and gentle decel to seat the rings evenly. Granted, this was with old style oil (Quaker State or Pennziol) and much lower temps than today.
I was curious because TV performance auto engine programs show a new engine in the test cell run up to max RPM just after assembly. I usually can't watch to end.

Click to expand...

Sorry, my mistake. It's TightSeal! Will change it above. Duhhhh .....

If the aircraft sat for any length of time, you had to do a "pull-through" (turning the prop by hand) before start up, as the oil in a radial will settle in the lower cylinders and cause a "hydraulic lock" if the engine were started on the spot.

GregP said:

Sorry, my mistake. It's TightSeal! Will change it above. Duhhhh .....

Click to expand...

It was a test, right?

In my experience, most but not all radial engine fighters have a pre-oil pump

Click to expand...

Hi Greg,

Pre oil pumps are an addition made by owners fastidious about their expensive machinery, they were never a feature of production aircraft that I'm aware of.

In cold climates aircraft may be fitted with an oil dilution function, fuel is injected into the oil to decrease viscosity, as engine warms up the fuel evaporates off. C-47 and C-54 had the system fitted as standard during USAAF/USAF service.

Once you turn it through 12 blades with the starter, you can put the mixture to full rich and start the engine normally. Some radials need a shot of prime and some don't

Click to expand...

All the radials I'm familiar with and flown are started with prime if cold (Mixture at idle cut off). Once running on prime the mixture is brought up to rich and finger is taken off the prime when the carburettor has taken the load. The Bearcat manual dictates the method, see attached..

Could be about aftermarket pre-oil pump.

I've seen several planes with, essentially, a pre-oil pump. It's sort of like when I used to race cars, and I disconnected the ignition from the starter and made them two switches. That way, you can run the starter until you see some oil pressure, then switch on the ignition to start it. If you do that, then you have a lot less wear on your expensive race engine! .. particularly a built Mazda Rotary engine (a 12A or 13B, bridgeported) That means fewer bucks spent on rebuilding race engines, which is usualkly a good thing for self-sponsored people.

I've seen several aero engine run-in stands including one on a 3/4-ton Ford dually, and all have an external oil pump switch. But that's engine builder stands used for break-in, not a flying aircraft.

As a reminder, there's a dedicated "engines" forum in the technical section, perfectly suited to this kind of discussion.

On the subject, it must be said that engines equipped with pre-oiling systems are not the majority, but rather the exception.

For the other ones, the vast majority whose oil pumps are driven solely by the crankshaft rotation, would these engines be radial, inline, V, inverted, or whatever..., they ALL operate without any pressure in the first few seconds. Lubrication then relies solely on the oil film that was deposited on the parts during the last use.

There is nothing special about radial engines, apart from the risk (also present in inverted engines) of hydraulic lock in the lower cylinders - but obviously, that's not your question.

Last reminder, until the 1930s, many engines had open-air valvetrains that did not receive pressure lubrication. They worked though !