>On 14 Dec 1997, GuyotLeonF wrote: ...( about valve seats )...
>> they were professionally fitted by
>> heating the head casting and shrinking the seats in ? liquid
>nitrogen?,however
>> when the engine attained its' normal operating temperature,they fell out and
>> stuffed pistons etc...
>
In message , Egil Kvaleberg <egil@kvaleberg.no> writes
>Sorry, the above sentence contains a contradiction. If the seats fell out
>when reaching operating temperature, they could not possibly have been
>"professionally fitted".
>To conclude: If the seats fall out, you have been conned, nothing more,
>nothing less. Correctly fitted valve seats should not fall out. Period.
>
I've been talking to a machinst who deals solely with cylinder heads for
cars and motorcycles as I'm interested in having a valve job myself. While
Egil's statement is broadly true, the machinist I'm planning to use does not
view the matter as so clear cut.
This guy deals with all manner of interesting vehicles, dating from the 80's
through to the 30's. He has a good reputation for period 'performance'
vehicles. He told me that its basically all down to the material used for the
head. It *is* possible to make a reliable valve seat fitting for most types of
vehicle - such as on a head like my TR7's (aluminium) - but he would not
recommend fitting hardened seats into some other materials. He used a
40's Harley WL engine that he had in for repair as an example. The original
material is too brittle to stand the temperature extremes of the
heating/nitrogen process without stress-cracking, plus there is not enough
room to get a good fit. In this instance, there would appear to be no
'unleaded' alternative other than use a (IMO snake oil) additive or replace
the head with a modern one.
BTW, he said the material that's used in the valve guide is important too,
certain materials tend to distort during fitting, hence you can find that the
carefully measured clearance when new quickly erodes to give a sloppy
fitting.
C. R. Fenn '80 TR7 dhc London
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