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Re: [T3] Stretching valve stems?


On 14 Nov 2005 at 14:26, Matthew Jones wrote:

> Someone once told me the valve head was welded to the stem, and that's where
> they broke, because it was a weak point. Fits the symptoms but thinking about
> it they would never make them that way!

They actually do! If you take a magnet to one you will find that there are 3 or 
4 different "zones" from the head to the stem which have different magnetic 
properties. It's amazing, but the head materials are quite special, so they are 
attached somehow. As odd as this seems, I think it is a rather standard process 
for making valves.

> Would the exhaust tend to go first due to hot gas further eroding the brittle
> fatiguing zone ?

The exhausts always goe first because the hot gasses flow out around them. 
Intake valves are cooled by the incoming cold air. I don't think it's due to 
erosion, just heat, and stress from bending as Russ pointed out. The only place 
I've seen erosion around the margins of the head, where the sealing is supposed 
to occur.

-- 
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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