By Andy Kennedy
When it comes to the oiling systems of the Big Block Chevrolet marine engine applications, there seems to be a lot of confusion. This article will hopefully clear up some confusion that some members have encountered on the BBC Gen V/VI. Keep in mind that there are several manufacturers that are using the Big Block Chevy platform, and some parts numbers might have to be cross-referenced at different dealerships.
These blocks use two different bypass valves in various locations depending on the application. There are two types of bypass valves used, one is 11 PSI, and the other is 30 PSI. See the illustration shown below for details of the correct bypass to use and its location.
Bypass Valves
30 PSI Bypass Valve: OEM Part #25161284
11 PSI Bypass Valve: OEM Part #25013759
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Factory Bypass Valve Setup – Engines with Factory Oil Coolers:
The stock setup on these engines had two 11 PSI bypass valves installed in locations 3 and 4. The oil cooler lines are installed in locations 1 and 2, leaving passage 5 open. Passage 5 will always remain open.
Engines Without Factory Oil Cooler:
When working with an engine that will not use a factory oil cooler, the 11 PSI valve in location 4 will need to be removed. Removing this bypass eliminates 3 of the 4 right turns where the oil will flow to the main oil gallery. If the 11 PSI bypass valve is installed in the number 4 location without an oil cooler, it will still function, but low oil pressure can occur from the 4 right turns the oil will need to flow to the main oil gallery. Locations 1 and 2 in the pan rail will each require a block off plug after the oil cooler lines are removed. Passage 5 will be open.
Engines With Remote Oil Filter:
When working with an engine that uses a remote oil filter, a 30 PSI bypass valve will be required and installed in the #3 location. A plug will be installed in the #4 location to prevent oil from flowing through this passage. Oil will be returned to the block through a .375″ diameter hole just above the filter pad. The #5 location will remain open.
Now to explain more about the gallies and oil flow. It starts at the pump and goes to the oil filter housing. There is an O-ring under the main cap in the gallery that goes to the filter housing. It is critical that this O-ring gets replaced during reassembly. After the filter housing the oil then flows to the oil galleys. Oil will always flow to 1 and 2. (The oil cooler) When you plug 1 and 2 it will bypass the oil cooler. Not all the oil is filtered on the BBC. If you install a plug into 3 it will force all oil to go through the filter, and this can cause the engine to run out of oil at high RPM’s. Also, if the filter becomes partially plugged the oil will be reduced in flow if there is a plug installed in 3. Installing a 30psi valve in 3 will increase the amount of oil that goes through the filter. No matter what rate of oil you want filtered, always remove the bypass valves for block cleaning and if they are stuck from debris, they are normally stuck in the open position.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2025EPQ3/#p=66