Like John, I have added a breather to my swinging arm cases. I use a $4 Yamaha 90 degree plastic adaptor which is a push fit into an 8mm hole in the rear chain case, in line with and just below the magneto.
A common leak is from the the oil level and the oil drain screws. I use the standard slotted screws to fasten the chain case, except for the oil level and drain screws. I change these for BSW SHCS, and make sure they are not threaded along the length (they are plain 1/4" OD for at least 5/16" behind the head).
It is difficult (unlikely) to get the standard fibre washer to seal on the ID because (a) it has clearance on the thread and (b) the thread is an oil leak path.
Hence use a plain shouldered fastener.
Then make a seal to replace the fibre washer. Purchase a small billet of nylon or teflon solid stock (say 1/2" OD) from your local bearing supplier, and machine a replacement washer.
Drill the centre hole 1/4", or marginally under (say 6.2mm), and the nylon washer will seal on the (smooth) ID of the fastener. Machine the OD to the same size as the head of the SHCS. Thickness can be say 2mm.
When parting off, push a 1/4" bolt into the nylon and let the parting tool touch the bolt; this will remove the plastic flashing on the ID. Use high speed steel, sharpened without rake; it will cut the nylon like butter.
Give both sides a couple of passes over wet & dry on glass or flat face (press with finger).
Next, check the length of the SHCS; that it does not bottom out in the casing thread.
The SHCS can be done up tight, and you have a visual indication which fasteners have sealing washers behind them. The nylon will firmly grip the fastener on the ID when tightened.
Job well done; no more leaks.
Richard
