Sam,

In normal operation of any piston engine, the valve train, driven by the
cam, opens and closes the valves very smoothly. There is never any
slamming of the valve against the seat. However, over time, at any point
in the life of the engine, sufficient crude can gather on the valve face
and/or seat to cock the valve off it's seat and cause some leakage.On
aircraft engines, if during a compression test a valve leaks and there
just doesn't seem to be any way to get the compression up, there is a
last action to take before resorting to tearing the engine down. It is
called "Staking the Valve".

 To stake the valve, the rocker on the valve to be staked must be
removed. Then a block of wood is placed against the end of the valve
stem and a sharp blow is struck against the wood opposite the valve
stem. This sudden sharp blow will momentarily drive the the valve off
it's seat and then the valve face  will slam back down again against the
seat.  That sudden, hard slam of the valve face against the seat is
usually sufficient to crush whatever may be on the seat or face, and
allow the valve face and seat to come together as they should - sealed
sufficiently tight to prevent compression leaking.

Hope this helps.

Regards,

John Rodgers
88 GL Driver
Sam Payne wrote:

>Good Men and Good Women of The List:
>
>     I went today to look at a 2.1 for sale, leftover from a conversion.
>It's a VW factory rebuilt engine with 30,000 miles on it, (it's out of the
>vehicle, so it was tested cold). The engine looked its age, the seller is
>a really nice guy (and a List member!) but the leakdown test showed 10%,
>10%, 7% and 35%. I did the test twice on all cylinders, valves closed @
>TDC etc. and got the same values. I retested the 35% cylinder 4 times
>(really liked the engine), the last time with a tablespoon of 20w-50 oil
>in the cylinder; still 35%. On the low cylinder only I heard obvious
>leakage at the intake manifold. So the leak is through the intake valve. I
>tried to backflush possible debris out of the valve with compressed air
>when it was open, but no luck.
>      Questions: Neither the seller nor I could understand why the INTAKE
>valve would fail after 30,000 miles only. The heads are AMC but the VW
>factory would replace the Spanish valves with German ones, right? Wouldn't
>they? Any ideas?
>      Also, crankshaft endplay *seemed* excessive (I didn't have a dial
>indicator). Bentley specifies 6 thousandths max, but I could see a tiny
>bit of play and hear a clunk when I pulled. It *felt* more like 10-15
>thou. If it's sloppy I'm not worried about shimming the flywheel but am
>concerned about damage done. ???....
>
>                      Thanks in advance for the help!
>                                                        Sam
>
>
>