1901-1953 Indian Motorcycle – Parts – Accessories

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  5. 98 – Dry Push Rods, Tappets and Valve Assembly 339


The crankcase vacuum plays a very important part in the lubrication of the assembly during the breaking in period of a motor and in City Police Departments, where the speeds are very seldom over 45 miles per hour, the crankcase vacuum is at its highest. This condition tends to drain the oil from the cam case into the crankcase, causing a drying up of the valve assembly.

This condition can be satisfactorily over come by drilling a 1/8″ hole in the crankcase breather disc. This will reduce the crankcase vacuum and will allow a dampness of oil to accumulate around the valve stems, tappets and push rods.

The following is a brief explanation of crankcase pressure and crankcase vacuum: Front and rear pistons travel up and down at the same time. On the downward stroke of the two pistons the breather valve opens and this causes the a1r pressure in the crankcase to pass out thru the breather valve. When the two pistons start up, this breather valve closes, and very little air is drawn back into the motor base. This creates a vacuum in the motor base and, tends to draft the oil away from the push rods. At higher speed. the volume of air in the crankcase tends to equalize itself with outside
atmospheric pressure. At the point when this happens this oil has a chance to travel from the crankcase to the cam case and up to the push rods, and into the valve assembly. Now, this condition is controlled entirely by the breather valve.

Until such a time as the cylinder and piston rings are worn to the extent of allowing a blowby on compression and firing strokes, at this time is when a pressure builds in the motor base and tends to blow the oil out thru both the breather and the valve mechanism and this oil blowing condition will be at all speeds of the motor. This is the warning that should tell you that it is time to service pistons and cylinders and rings. Oftentimes, piston rings alone will overcome this condition.

Now, it is desirable to have a certain amount of vacuum in the base at all times but where a machine is operated continually at the lower speeds, it will be necessary to reduce this vacuum by drilling the crankcase breather disc.


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