S1 Publishing(Oxford)©20-07

World of motorcycleS
S1 Publishing(Oxford)© 1999-07
 Maintenance - Engines
Valves & Tappets
What are Rocker Arms & what do they do?
The rocker is a small  part of the engine, between the cam and the top of the valves in the "head" of the engine, when the cam (see cams) runs over them, it opens the valves
A rocker arm for an over head cam 16 valve
Even crank cams have rockers, and usually have tappets to check and set at the head of an engine, these are pushed by push rods
No Photo as yet

What are tappets and what do they do?:
 Tappets there is a small, but essential part of the engine, it is the screw & nut, which runs through the right hand side of the rocker (see above).
 The amount of thread, showing below the rocker must be very precise, as this is where you get the valve tolerance.
 Too tight and the rocker will snap, or the valve can either hit the piston, or you will lose compression. You want to avoid both!
Circled in white: the locking screw

circled in white: the locking nut
Both together the comprise the tappet.
SEE ALSO Bucket and Shims - page not yet built.

What are Valves & what do they do?:
When valves are closed, They have to be air tight when the piston comes to Top Dead Center, so that compression is kept. when the piston on it's compression stroke (coming up when both valves closed), containing the fuel/air mixture, which you want for better bangs, you want all that bang contained in the chamber, not leaking anywhere, so the all the force is transferred into pushing the piston back down.
 You can use compression due to leaking head gaskets, valve seats, valves them selves as well as loose fitting spark plugs, not just through bores, piston rings pistons.
The top two exhaust valves (closed)

the lower two inlet valves (closed)
When the valves are pushed down, on an inlet stroke,  they open to allow fuel get into the combustion chamber. 
 At this time, the pistons are going down the bore and sucking the air/fuel mixture
See also carburetors
No Photo's as yet
When the valves are pushed down, on an exhaust stroke, they allow exhaust gasses out through to the exhaust.
 At this time the piston(s) are coming up the bore and pushing the exhaust gasses out of the combustion changer, when it reaches just before top dead center, the valve closes, before going down, the inlet valve opens and the cycle starts again
No Photo's as yet

This is just a general guide.
If you are about to do your own maintenance, using these pages, I take no responsibility if you get things wrong!
Modified for 2009