Easyriders January 2006
Issue 391
Pgs. 160-161
Product Review

Oil Down Your Leg? - John Sullivan

Harley's Twin Cam 88 is a great motor. In stock from they make half again as much power as the Evo did and there are off-the-shelf kits to build them up to 124 inches. They have only one area of concern, which involves the cylinder head breather system. It seems that some motors pump oil out of the cylinder head breather passages into the air filter or whatever they are connected to. If your motor's breathers are routed to the air filter back plate, you could wind-up with a bit of oil running down your right leg after a long ride.

This is not a new problem. We first saw it back in 1999 and there appears to be more than one reaon for it. These include combustion gases leaking past the piston rings, incorrect alignment of the oil pump, poor sealing of the oil system's O-rings, not enough volume of the scavenge side of the oil pump and inadequate oil separation in the breather valve. Not all of these items occur in every motor; in fact we would be surprised to find one motor with all of them.

We think that the two biggest reasons for excessive oil blow-by are incorrect alignment of the oil pump and poor oil separation in the cylinder heads' breather valves. Most of the oil pump alignment problems are caused by inexperienced technicians who are installing camshafts and do not follow the correct oil pump alignment procedure when the reinstall the cam plate. The poor oil separation up in the cylinder heads is caused by the stock breather valves being overwhelmed by the volume of oily/air mist that gets up to the valves during a really hard ride.

Tim Doherty of Doherty Machine has spend the past several years working on the oil separation problem in the cylinder heads and he's come up with a new breather valve design. After looking at his design, he is either very smart or an exfarmer. His Mystfree valve uses a two-chamber oil separator, whereas the factory breather valves have a single chamber separator that is fitted with a gauze element.

In the Mystfree design the first, or stock oil separation chamber, which is cast into the center of the rocker arm support, is augmented by a cylinder that protrudes into it from the bottom of the Mystree valve body. This cylinder has three openings spaced around its edge that cause the pressurized air- oil mist that is inside of the motor to change direction several times as it is pushed towards the exit in the breather valve.

As this mist moves up against the walls of the cylinder, a good deal of its oil content is stripped off and returned via the stock vleed holes in the bottom of the chamber. It's kind-of like when the wind pushes damp air through plants and their leaves catch the dew.

When the pressurized air passes through the six holes in the Mystfree valve, it hits the bottom of the neoprene valve cause it to rise up forming a second chamber. At this point any oil remaining in the air is removed by the aluminum valve seat and drains back through the lower chamber and into the rocker box.

The neoprene valve that Tim uses is of a smaller diameter than that of the stock umbrella valve, it thereform develops a higher vacuum in the second separation chamber, whcih helps to pull any oil back into the cylinder head's rocker box. The idea behind the Mystfree valve is sound and his machining is flawless.

Having seen how well the Mystfree valve worked in a 95-inch motor, we decided to use them in a high performance 103-incher we are building. The Doherty Mystfree breather valves are installed in the same manner as the stock breather valves and come with all needed gaskets and mounting bolts.

- John Sullivan
Easyriders