[xj-s] Re: XJ-S Digest V2 #228

["Richard F. Dixon" <rfdixon@chs.cummins.com>]

>-- Kirbert
>True, valve wear is not an issue.  Why would that cause you to
>conclude that the valves are rotating?  I would consider it further
>evidence that the valves are NOT rotating, since rotation would add
>to wear, wouldn't it?

I'm not sure of the mechanism involved, but valve rotation reduces wear
and reduces the likelyhood of burned valves. Back in the 70's when
leaded fuels became unavailable, American car manufacturers started
putting valve rotators on the exhaust valves of the older engines to try
and make up for the loss of lubrication the lead would provide. Now
these were much heavier than standard retainers and significantly
lowered the RPM at which valve float would occur. Later they learned how
to make harder valve seats and some of them dropped the rotators. Now,
understand we are talking about rotation that is only few degrees each
time the valve opens at the most. (probably around 10 rpm?). I think
this works the same as a hunting gearset that reduces wear by
distributing it around the whole valve seat. As an aside, I put a
performance cam in an AMC 360 and took the rotators off because of
increased valve lift and the need to use different springs, and the
exhaust valves beat down into the head after about 30k miles. I had to
take the heads back off and have hardened valve seats installed.

>If I understand it correctly, the reason for the lack of regular
>adjustment requirement is not that there is no wear, but that the
>wear is balanced.  Wear at the valve seat causes clearances to close
>up.  Wear just about anywhere else -- cam lobes, tappets, valve stem
>tips -- causes clearances to open up.  Apparently, the two types
>counteract each other quite well and clearances remain constant.

I'm sure this is not by accident, and is the result of careful design.
My Japanese motorcycle, which represents 80's technology and is DOHC
with 16 valves, requires valve adjustments every 7500 miles! Of course
it has a 10,500 rpm redline and the cam followers are a LOT easier to
get to!

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