Flathead filter

[Bruce Lancaster]

"there is a plug in the hole you speak..."
I know the plug well. Look again...the vertical stack is the feed from the pump.
At the rear main, oils has two holes seen right and left in the pic. Oil going RIGHT enters the rear main, oil going LEFT enters the filter. From this point, oil has to go through the filter (the plug's purpose) except for the oil that turned right into the RB, then it reenters the feed stack ABOVE the plug and proceeds upwards to the other passages feeding all other bearings and the valve train.
That lower passage is the filter entrance.
The oil returns from the filter right above the plug, and heads on up to oil everything else.
In an engine without the plug (a common mishap amongst inexperienced people assembly a freshly cleaned block), the filter can fill up but has no flow, so the whole engine would be getting unfiltered oil.
One of Smokey Yunick's books has a long essay on ways to rearrange things to get the rear main oil filtered, but concludes that there is no benefit to justify the trouble.

[Ronnie]

Originally Posted by Bruce Lancaster

"there is a plug in the hole you speak..."
I know the plug well. Look again...the vertical stack is the feed from the pump.
At the rear main, oils has two holes seen right and left in the pic. Oil going RIGHT enters the rear main, oil going LEFT enters the filter. From this point, oil has to go through the filter (the plug's purpose) except for the oil that turned right into the RB, then it reenters the feed stack ABOVE the plug and proceeds upwards to the other passages feeding all other bearings and the valve train.
That lower passage is the filter entrance.
The oil returns from the filter right above the plug, and heads on up to oil everything else.
In an engine without the plug (a common mishap amongst inexperienced people assembly a freshly cleaned block), the filter can fill up but has no flow, so the whole engine would be getting unfiltered oil.
One of Smokey Yunick's books has a long essay on ways to rearrange things to get the rear main oil filtered, but concludes that there is no benefit to justify the trouble.

Bruce it easy to see that the oil in hole goes to the right but that is where it is stopped. The large hole can be seen in the main saddle in the pic just under the bearing tang slot.This large hole is covered by the main bearing when it is installed.There is no corresponding oil hole there in the bearing. Lower in the picture you can see the oil hole in the main web that corresponds with the oil hole and groove in the bearing when it is installed. The large hole in the web is left behind when they machine the passage that intercects with the filter passage.