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Valve rotation

J

jcd06

Just got it firing!
Joined
Jun 6, 2014
Messages
5
Hi,
I'm in the middle of a LM-1 rebuild and I was wondering about a technical detail.
What is causing the valves to rotate on these engines?
The contact surface of the rocker will probably be under a slight angle with respect to the valve stem upper surface.
Is this done by a conscious choise in design of some engine part or is it left to the whims of the famous "Italian machining tolerances"?
And to be a complete prick: is it factual knowledge or opinion? :devil:

Cheers,

Jean-Claude
 
Hi mate,

Meanwhile I could find out that the contact surface of the rocker is parallel to the valve stem upper surface.
Also I got some info from a German Guzzi workshop that the valves do not rotate under 5000rpm.
In a next chat I will ask him how he found out.

When I get hold of a scratched-beyond-repair rocker cover I'll see if we can have it opened up and a plexiglass window mounted somehow.
I remember in the past there were some Ducatis with plexiglass windows on the heads bevel drive cover.
These were more cosmetic than science :)
 
This one didn't. (From my 1975 850T)

I never heard a comment like that 5000rpm idea. What exactly is stopping it from rotating at 4500?

Some valve brands have keepers that grip the stem, so no rotation. But I think it's well established that rotating is a good thing, no? The valves on BMW airheads certainly never show linear wear such as this.

IMG 1650
 
Does it really matter if the valve rotates so long as the engine runs? I've had customer bikes with well over 100K miles on them and never had to touch a valve on them. This includes 850 Eldorados which I think is pictured above.
 
Maybe not. I could see it as one of those factors that, strictly speaking, is proper engineering, but real-world is not that big of a deal.

And I’m actually wondering if it mattered more with leaded gas. Back then, we were constantly building up deposits on the seats, and rotation would help even it out, right?

And maybe the wear on my valve stems was due to a previous low oil, or overheating episode. I’m not going to dwell on that prospect...
 
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