Flathead oil gallery

[Jacques Reed]

Thought this might be of interest:

As discussed, the orifice size on a bypass filter, or the orifice size in a grub screw when used in a semi full-flow filter, has to be very small to maintain oil pressure within the system.

I head read somewhere what the orifice size was on the bypass filter but cannot find it again. Not even sure if the size was factory info or someone's best measurement.

Thought I would see what it was on a spare bypass filter body I had laying around.

I used a set of numbered drills to check the hole size and my best estimate is a No.56 drill size which is 0.0465" diameter.

Unfortunately my No.55 drill is missing but a No.54 drill shank is too large and does not enter the hole so it could also be a No.55 which is 0.0520".

The difference between a No.56 and a No.55 drill is only 0.0055 so No.56 could still be the size as there is almost no gap around the drill in the photo. Whether that small difference would be critical is another matter.

In simple metric terms the hole is between 1.2 mm and 1.3 mm so any enlargement of it is not good for oil pressure.

Cheers,