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- Subject: Feeler Friction
- From: "Tom Brown" <tbrown@xxxxxxxxxxx>
- Date: Fri, 11 Nov 2005 14:35:15 -0600
With all this hubub about the spark plugs, it seems your urgent question
about valve clearance got lost in the shuffle.
Adjusting valves isn't difficult if you've done it before, but if you
haven't, this is a pretty expensive engine to experiment with. Since
you've opened it up and have changed at least one valve clearance, you'd
better get some good information quick.
First, your bike has to be stone cold when you measure valve clearance.
This is because the spec is given for a room temperature bike.
Make sure you're sticking the feeler in where it's supposed to be...that's
not where the eccentric is, it's below that, just above the spring between
two bits of metal.
The feeler should be able to slide between the two surfaces, but if you
twist the feeler slightly, you should not feel rocking. You should also not
feel like the feeler is being clamped by the two surfaces. If you can
satisfy all of those conditions, the clearance is probably right.
Sometimes I'll try letting go of the feeler. I like it when the handle will
drop down, but the feeler doesn't fall out, if that makes any sense to you.
Put all four gages in the four valves at the same time. Make sure you're
at top dead center for that cylinder. There will be valve clearance in all
the valve if you are.
You should have a shop manual or something to help you with this and you
should get someone to show you the first time. Looks like you're already
past that. Here's a link.
http://www.ibmwr.org/r-tech/oilheads/valves.shtml This one is pretty good,
but there is another that has great color photos and is almost as good as
having someone there teaching you. If anyone on this list knows the link,
please post it.
Take your time. This is not a simple procedure if you've never done it on
any engine before.
Here are some non-obvious conceps for you to digest. They'll help you gain
some "common sense" about the way valves work. This stuff always helps me.
The reason there is clearance in the valve train to begin with is that the
valves themselves get very hot, being right next to the combustion process.
When heat travels up the long valve stem, it gets longer. If the cam just
pushed directly on the valve stem (or the rocker etc.) with no clearance,
the valve would begin to not close more and more as the valve stem got
hotter and grew in length. Get it?
The valves are opened by the cam, but they are closed by a spring. If the
spring can't close the valve all the way (valve stem too long from heat
expansion and being held open because it's contacting the cam), very hot
gasses will leak through the space around the valve causing lots more heat
in the area around the edge of the valve and the part where this edge
touches the head (Valve seat). This will literally burn up the valve and
can burn or melt the valve seat and other parts in the area....Expensive.
That's why the valve clearance is there.
CONCEPT II: The Exhaust valves always have more clearance (30MM) than the
intake valves (15MM). The reason is that the exhaust valves get hotter
and expand more. Makes sense. A a good thing to keep in mind when you're
trying to decide which feeler goes on which valve. The ones near the
exhaust pipe always get more clearance.
CONCEPT III: When the engine turns, the valves are slapping against the
valve seat every time they close. This gradually wears the valve seat
down...especially when the bike is new and the two seat surface hasn't worn
to a perfect fit to the valve yet. So chances are, if your valves are out
of adjustment, they will be too tight, not too loose. This is why
clearance has to be adjusted periodically. If valve clearance gets too
tight, the valves don't close. (See Concept I). As the valve seat wears,
the valve stem comes up further toward the cam and closes up the clearance.
Corallary to Concept III: What can also happen later on is that the valve
seats can be fully mated to the profile of the valves and will begin to wear
more slowly. The cam or cam bearings may wear a bit faster. The effect of
this cam/bearing wear is to pull the cam surface farther from the valve
stem. In this case, the valves will be too loose when they're checked.
This probably will not happen until 30 to 50K miles if at all.
CONCEPT IV: It's important to adjust all the valves using the same feel.
You can adjust them a little loose or a little tight...a little tight, not a
lot. However you adjust them, decide and stick to it. Be as consistent as
you can for all 8 valves. You'll find that change the clearance of one
valve may affect the clearance of another. Be patient and get them all the
These boxer engines are very sensitive to valve adjustment. It affects the
throttle synch. If clearance is more on the left side than the right, you
will try and synch the bike, but you'll be compensating for this difference
in valve clearance with the throttle butterflies. Since the butterflies
don't let air in in the same ratio throughout the RPM range like the valves
do, your bike will feel smooth at one RPM and rough at another. Changing
the synch will just move the smooth spot. If you get a good valve
adjustment, the bike will be much more consistent all over.
Again, it doesn't matter if their .001" loose or .0005" tight so long as
they're the same amount loose or tight on all the valves.
So...the first thing you must never do is make the valves too tight. If
they're too loose, the bike may sound funny or not run smoothly, but it
won't really hurt anything.
End of oilheads-digest V2 #264