[Vwdiesel] Help - messed in the head...Tolerances my boy...

Mark Shepherd mark at shepher.fsnet.co.uk
Sun Apr 17 03:55:44 EDT 2005

I'm in agreement with you James...
Here's a senario:
If we're only talking about a few thou gap and head skimming
reduces that...
How much can someone turn up the boost before the sudden
acceleration of the crank causes enough delay in the timing
belt to cause Le Crunch?

What I'm getting at is that the valves move closer to the
piston... reducing
the margin of error that the valves (cam) can be  out of
perfect time and
still not tickle the piston.  I've never delved into the cam
timing events
of the vw diesel, but the longer duration cams I work with
do not allow much
room for error- the exhaust valve is just going closed as
the piston is
reaching TDC of the exhaust stroke, and the intake is
starting to open... so
if the whole head assembly moves closer to the piston, it
follows that the
early opening of the intake (for scavenging) might result in
tapping the
piston when the piston is at the top of the exhaust stroke.
What's in my head is that you are moving the valve face
closer to the piston
when you surface the head... since the seat which is
recessed, is coming
closer to the machined surface of the head by the thickness
of the amount of
material removed,  the valve will be closer to the piston
with all other
things being equal- unless the seats were machined deeper to
compensate. The
piston protrusion measurement is assuming a stock head with
stock distance
from valve seat to machined surface of head... by planing
the head, you are
reducing this dimension, hence the valves are closer to the
piston. From
what Loren sez, I guess it works, but I would think the
tolerance for valve
timing error diminishes greatly.

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