RE: Valve Clearances

["Retherford, Martin" <MRetherford@xxxxxxxxxxx>]

Eric,
	This would tend to suggest that the seats are moving in the head,
right?
	If the valve is loosing clearance then the seat must be moving
further toward the cam.  If that is the case then the head itself is the
problem.  Loosing seats was a notorious problem on some factory heads for VW
air cooled motors especially type 4 motors.  It is a bad casting if this is
the case.  That makes the fix more expensive by a whole bunch.
	If the seat is moving then re shimming isn't going to fix the
problem only prolong it.  Some cases I have seen heads with not enough meat
(metal) holding the seat in place and this causes the seat to recess into
the port (intake or exhaust) in other cases I have seen bad castings with
really ridiculously soft material.  In either case a fix for the problem on
these VW heads was to put in an oversized seat.
	A seat is generally just a hardened ring of steel that looks like a
piece of cut pipe.  Some seats are only, say, 5mm in wall thickness while
the oversized seats I refer to were on the order, say, 15mm.  I use these
values because I forget what they really were, it has been a few years.  In
any case the larger outside diameter gave it a better hold in the head and
thus allow you to get the use out of the head.
	Needless to say this is not the ideal situation to have because it
requires rebuilding the head.  On a gloomy side note since those type 4
heads had problems with there exhaust seats mostly, during the rebuild
process it was common to find any number of through the casting thus
requiring more extensive work (heli-arching) or new heads which
unfortunately begins the whole ugly process again.

Fun conversation for a Friday huh?

Martin

		
		Yes, the gap will get bigger if the shim is worn, The early
bikes tended to
		tighten up the clearance and cause the valves to burn. I
checked mine at 
		14,000
		KM and some of the exhaust valves had .004mm. This is only a
couple of thou 
		and
		would have been partly open when the motor heated up.



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