The rotary breather valve is timed to open on the downstroke of the pistons. ¹⁾
This allows crankcase exhaust air pressure to expel the scavenge oil from the crankcase breather oil trap into the gearcase.
The breather valve closes on piston upstroke, creating vacuum in the crankcase.

The breather valve must be retimed if disengaged from the oil pump (spherical) drive gear on the pinion shaft.

Find the advance timing mark on the flywheel.
The single vertical line in most ironhead motors indicates the advance mark used for timing with a timing light. ²⁾
This line is also used for setting the breather and has nothing to do with pinion gear marks, cam marks or TDC of the pistons.

The FSM says to remove the timing plug, rotate the engine until the advance timing mark is centered in the timing hole.
However, any particular bike may not have the original flywheels. So it is most important to always do a simple check to verify which mark to use on the flywheel. ³⁾
Pull the front spark plug, and using a wooden dowel or a drinking straw, feel the piston come up as you rotate the engine. ⁴⁾
Before it gets to the top, the advance mark will show. As the piston hits the top of the stroke, the TDC mark will come up.
The first mark to come up makes the motor fire “earlier” than the second dot to come up.
Earlier, is advanced. So the first mark to come up, is the advance mark. Some later bikes are reversed, so the type of mark is not consistant, but the geometry is.
Normally. a single vertical line is the advance timing mark on earlier ironheads, not the dot.
Lock the engine when you've found the advance timing mark so the engine doesn't rotate and lose that position.

Remove the pinion shaft gear, and turn the engine so the mark on the oil pump drive on the shaft is pointing straight up.
The pump drive gear is a slip fit over splines on the pinion shaft.
A mark is cut on one side of it which should face outward against the pinion gear when installed.
Line up the timing hole on the breather sleeve gear on the oil pump to be in the notch (window) in the front. It may take a few tries to roll it into place.
Don't sweat it too much if you can't get the dot exactly centered in the notch.
It is not uncommon to have the DOT slightly to the left just entering the notch of the pump. ⁵⁾

Then reinstall the pinion gear. It should have one larger spline to engage the shaft and it can only go on one way and have the line facing out. ⁶⁾
You can heat the gear up in a grill, oven or with a propane torch just enough to make it expand enough to slide it back on the pinion shaft.

Note: Once the breather valve is correctly timed, the position of the flywheel and breather valve timing marks can be disregarded when installing timing gears (cams).

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