A Technician reports a Cessna 175C with a GO300E engine experienced a sudden drop of oil pressure. Pieces of rubber like gasket material were found mashed in the oil relief valve plunger head and oil galley area. A clapper valve type filter gasket and crossed oil lines are suspected. Three oil filter manufacturers supply two types of filters for the FAA/STC approved remote spin-on oil filter adapter.

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Reporter stated older engines that do not have oil filters can use an FAA STC approved method to install one. A STC held by ADC includes oil lines; a fixture that is attached to the firewall at the back of the engine to secure the oil lines and an adapter that allows what ADC calls; a remote spin-on oil filter to be installed to the adapter. The oil filters are manufactured by at least three different companies; Purolator; Autolite and Tempest. They were using a Tempest oil filter on the Continental GO300E engine. The engine has a gear reduction at the front of the engine for propeller speed.Reporter stated there are two types of remote spin-on filters; one using what is called a clapper valve; which is just a rubber type gasket material used inside the oil filter to prevent oil from draining back out of the filter housing through the peripheral outer holes in the filter head after an engine is shut-down. The gasket material acts as a one-way check valve. Reporter stated engines with an oil filter installed on top of an engine with the filter head facing down; generally have a clapper valve in the filter that keeps the oil from draining out of the filter. The other type oil filter does not have the clapper valve gasket material inside the filter. Both filters have different part numbers to identify them. The clapper valve gasket is NOT the outer sealing gasket most people are familiar with on a car's oil filter face/base. Reporter stated the ADC STC spin-on oil filter adapter has oil 'In' and 'Out' fittings on the adapter for oil lines to be connected to. The spin-on the oil filter is also attached to the adapter. Engine oil; under pressure; flows through the outer holes into the filter head (face); than forced through the paper filter membrane and into the center core and back to the engine or oil tank. Reporter stated he was informed by Tempest that either filter type can be used with the ADC remote spin-on oil filter; but correct installation of the oil lines to the ADC adapter is critical if using a filter with a clapper valve gasket. The oil pressure line from the engine must be connected to the 'In' connection on the ADC adapter; in order to allow proper oil flow through the filter's outer peripheral holes; into the paper filter element and out through the core hole to the 'out' adapter fitting. If the pressure line is connected to the 'out' port of the adapter; than oil pressure from the engine flows backwards through the filter's core hole; through the filter element and the oil backs-up against the clapper valve gasket which lays against the filter's peripheral holes; preventing the oil from leaving the filter. Reporter stated each Tempest oil filter can (housing) is tested to 600 LBS pressure; whereas maximum oil pressure from cold starting an engine could reach 220 LBS. Aviation oil filters use a paper filtering element material that is a finer mesh and the element has a stronger tie to the center core than car filters; that design helps prevent the filter element from separating from the center core even if the oil flow is reversed through the filter. They believe the oil lines were crossed and incorrectly connected to the ADC adapter. But if the oil lines were crossed than the problem must have happened prior to the last two oil filter changes because the rubber clapper gaskets in those two filters were intact.Reporter also stated there was no evidence the remote ADC oil filter lines were hooked-up incorrectly allowing for a reverse flow from the engine to the spin-on filter. He would recommend ADC manufacture oil lines with different size fittings to prevent cross-connections of the oil lines at the ADC adapter and clearer directions on the oil line connections. He also recommends the three manufacturers of the two different types of spin-on oil filters; add a warning with the filter assembly stating that reverse flow may cause engine contamination.