Motorcycle Repair: Breather Valves & Bore, evolution engine, breather valves

Question
Hello,

 Can you tell me the pros and cons of both plastic and steel breather valves in a 1998 1340cc Evolution engine? How close is the tolerance between the valve and the case and how critical is it? Can you also explain the operation and function of it in the Evolution engine? Thanks

Answer
The metal breather will hold its shape better than the plastic. Plastic, under heat has a tendency to warp.
There really is no tolereance except for end play. The end play should be .001-.011. When its removed check for scoring or deep scratches in the case. Other than that you will be fine. If its gouged deeply then you can get an oversized breather valve, but you will have to have it bored out. Should be around .002 spacing.
The peration of hte breather valve is to (1) vent the crankcase when the piston is making its downward stroke and (2) oil the gears and bearings in the cam area.If you didnt have it you would build up pressure in the bottom end of the bike.
hope this helps
Mike