Flatheads Forever! Lubrication for the CJ flathead

Oil slinger

Pump pressure

The flathead uses a combination of pressurized oil from the pump, oil slinger shields (plates), centrifugal force and gravity to get the job done! In the purest sense of the term, it is not a splash oiling system. The crankshaft does not go into the oil sump and throw oil. There is a separator plate between the crankcase and oil sump at maximum oil level on the dipstick and the oil does not attain a level higher than the separator plate.

Oil is fed from the sump through the pump to a channel that feeds the crankshaft and bearings. The oil slinger plates use centrifugal force to displace the oil fed to the bearings outward in a circular pattern towards the cylinders and lifter cavities. Two drain holes feed oil to the tops of the pistons from the lifter cavities. The dipstick side has a small oil bleed to the bottom of the jug (as long as you get a good gasket set with the hole in the right place and you put it on right.) But on the opposite side—sorry, no help—only slinger oil and drain-back from the lifter cavity depending on the production year

And you may find a passage all but blocked.

A high-volume oil pump will help extend the amount of spray oil from the slinger plates and with enough volume, a spray tube to the side opposite the oil pump could be fabricated. A deep sump oil pan is a good idea but somewhat worthless without a deep pick-up and a filter to bring the cooler oil off of the bottom of the pan adjacent to the cooling fins. With a stock oil pan or deep sump without an extended pick-up, once your oil level goes below the lower oil pump steel plate you are done for. All that extra oil is there but the pump cannot pick it up!!

This is 1937 technology. It is not a tri-metal bearing type of crankshaft support that requires a film of oil at the correct viscosity to lift the crankshaft as used in most vehicles! Using deep ball bearings and slinger oil is very forgiving of oil viscosity so a straight weight oil with a lot of wear inhibitors like zink is the way to go with this engine. Multi-viscosity range oils in general are more prone to viscosity and thermal breakdown due to the high polymer content. It is the oil that lubricates, not the additives (excluding synthetic multi-viscosity)

I see a lot of discussion about piston problems and CJs. 99% of the autopsies that I have done on failed CJ pistons are caused by fuel mixture and carburetor problems, not oil! Oil will cause the piston to transfer aluminum from the piston to the cylinder and cause an engine to sieze. It will in itself not cause a piston to melt a hole in the top. Only detonation will cause the kind of combustion temperature needed for that to happen.

I am lucky enough to have use of equipment that can map the combustion chamber of a flathead for pressure and temperature. It is truly a nightmare! Again, 1937 technology—a non-vortex type of chamber with no EGR for cooling pushes the piston to its design limits!

In the summertime the stock carburetors percolate (boil the fuel in the float bowl) so badly that the fuel mixture is forever lean. In China, carburetors are cheap. I have sometimes had to buy four sets before I could find two that I could rework to the proper float level, idle mixture and main metering fuel mixture! CJs have fuel supply problems also through the fuel valve and tee-fitting and with rust and paint from inside the fuel tank often causing a lean mixture in one or both cylinders.

In an L-head (flathead) engine most of the heat is developed in the cylinder (jug). Ever wonder why the early V8 Fords had two water pumps?

The stock cylinder head gasket for the flathead CJ is made of asbestos covered with thin aluminum foil. It is an insulator and will not let the heat flow through to the cylinder head fins. A solid copper head gasket is the answer but be careful. There are some on the market that are a rip-off as they are far to thin and increase the compression ratio which can cause far greater problems with detonation and hot spots.

If you look at the restrictions in the exhaust outlet and mufflers, some of the reasons for high combustion temperatures and detonation are easy to see. Restricted exhaust will increase low-end torque but with the gearing of the CJ why would you need to? More than likely it was done to keep the engine temperature within a workable range in the Russian winter.

Don't get me wrong. I love the CJ but want to evolve it to a higher level of dependability through sound engineering.

Motorcycles are like people. They all have personalities, strengths and weaknesses. Correct the CJ's weaknesses and it becomes one beautiful machine.

Cajun Mike

Beijing 2003