Valves in Oil Filters

Two types of valves are common in oil filters: check valves and relief valves.

The check valve - a rubber flap - keeps the oil in the filter. Thus, after a period of non-operation, the oil gets faster into the motor or - on older models with the filter in the return line - the pump and the gear case are not flooded by backflow.

The spring loaded relief valve opens every time the filter is clogged. The motor is still lubricated via the relief valve then, even if by non-filtered oil.

Oilfilterology

One could think all oilfilters are the same. Oil is pumped through them and there is something in them that skims all the grit out of the oil. Why then do not all Harleys have the same oil filter?

First, there are differences in the thread size that they are screwed on with. By choosing a certain thread a motorcycle manufacturer can keep his customers from using other similar looking filters by mistake. Through the small holes around the center thread the oil enters the filter, and through the center thread it comes out again. The thread size depends on space and on the entry-exit relation of the filter. There's no use in filling up the filter quickly through big holes, but having a small exit that does not pass oil quickly.

Next point are differences in length. This, too, depends sometimes simply on space. E.g. Buell BX models have 7.5 cm short filters, just because there is no more space. On the other hand, one has to achieve a certain filtering surface. If the filtering medium is already optimally pleated, you have to make the filter longer for more filtering surface. Longer filters mean more circulating oil, too. So, the Dyna models with Evolution motor, which are sporty bikes that are rather run in the higher rev range, have about 8-1/2 oz more oil than Evolution Softails. This helps keeping the oil cooler.

The standard filter medium is pleated paper. Paper can be had at reasonable prices and in different pore sizes, and - when pleated - you get a large filtering surface. Latge surfaces ar enot so easily clogged and pass more oil at the same time. Also, there is a reason for different pore sizes. The bigger the pore (i.e. the hole in the paper), the bigger the particles that pass the filter, and the worse the filter. The smaller the pores, the cleaner the oil. But the counter pressure of the filter is increased as well. Therefore fine filters only work for high pressure pumps (e.g. S&S oil pumps). Pore sizes are given in microns (1 μm = .001mm).

As any engineer knows: high revs and a long service life require an as clean as possible engine oil. For this reason and after countless Wrecking Crew test miles on street and track W&W manufacures PanAm® oil filters with a maximum pore size of 10µm.