Thread sizes:
86-90: 5/16“x18 ¹⁾
91-Up: 1/2”x13

91-03 breather bolts.
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04+ breather bolts.
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Here is the OEM ham can without the filter.
The two bolts on either side of it are the breather bolts that go into the heads.
(and where the oil mist comes out).
With the stock configuration, the mist is vented back into the intake.

Backing plate (29367-07A)7)

SCREAMIN’ EAGLE® PRO HIGH-FLOW AIR CLEANER KIT FOR SPORTSTER® MODELS. ⁸⁾
(For race application only)
This air cleaner kit provides increased air flow when used with stock, or Screamin’ Eagle carburetors.
It includes a composite backplate, an air cleaner cover adapter, and a high flow, washable element that does not require oiling (K&N).
The breather system also has been redesigned to route breather gases directly into the inlet of the carb to minimize excess oil in the A/C assembly.

29042-04A:
Fits ’04-later XL models with stock air cleaner cover.
Harley-Davidson® Air Cleaner Cover P/N 29742-04 requires separate purchase of adapter kit P/N 29848-04. $109.95 29066-03:
Fits ’88-’03 XL models. $109.95

SE Air Cleaner (29066-03) 9)