Re: [xj-s] Valve Rotation

[Jan Wikstrom <wickers@zip.com.au>]

At 11:40 28/07/1998 +0000, Kirby wrote:
<snippity-snip>> From:          "Richard F. Dixon" <rfdixon@chs.cummins.com>
>> On the subject of valve rotation, the collets that hold the valves
>> should not grip the valves. They should seat in the taper of the spring
>> retainers and the split ends butt together so that only the ring in the
>> retainer and the groove in the valve stem touch. Is that clear?
>
>It's clear, but it's not true on this engine.  The split ends of the 
>keepers do NOT butt together, a gap is always left.  The keepers are 
>wedged down on the diameter of the stem with the force of the spring 
>and are held VERY securely as a result, the valve will NOT be 
>rotating.

This has certainly been the case with all V12s I have worked on.

>> The lobes of the cam are ground with a slight taper to make the tappets
>> rotate and some of that motion should get transferred to the valve
>> stems.

Don't know what sort of cam followers the Cummins engines have, but the
Jaguar has inverted buckets. The interesting point about these is that the
ONLY contact between the bucket (which does rotate at some unknown rate, as
the cam doesn't wear a groove in it) and the valve is a 1/4in dia. patch in
the centre of the well-lubricated clearance adjusting shim on top of the
valve stem. Sorry, Dixon, but this is not capable of transmitting enough
torque to rotate the valve even if the spring retainers were mounted on
ball bearings. AND the adusting shim sits in a much larger friction surface
on the top spring retainer.

>> It
>> is indeed a tribute to the designers of the valve gear that these
>> engines can go the life of the engine without a valve adjustment. I know
>> of no other mechanical tappet engine that this is possible.

I do -- My Datsun 240Z was adjusted once after running-in and went 250,000
km over 21 years without further adjustment until I sold it. (SOHC with
single-arm rockers).

>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.

Seems a reasonable explanation to me.
  

- Jan


The Gum Tree Garage, where oil and beer mix just fine...
Visit the Gum Tree Garage at www.zip.com.au/~wickers

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References:

• [xj-s] Re: XJ-S Digest V2 #227
• From: "Richard F. Dixon" <rfdixon@chs.cummins.com>

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