[xj-s] Valve seats - OOPS

["Craig Sawyers" <craig@sawyers.demon.co.uk>]

It seems that I have perpetrated the controversy on seat tolerancing.
Kirkby and Roger Bywater are correct that the 2-6 thou tolerance refers to
the guide, although this is the only tolerance listed in the "Valve Guides
and Seats" sub-section of the "General Specification" part of the repair ops
manual (AKM9006, rev 5), and therefore is a figure not without ambiguity.

Roger is correct that there is then a specific tolerance on seat
interference in the body of the manual (operation 12.29.18, Cylinder head,
Overhaul) which specifies that the old seat be machined out, the seat recess
measured and the new seat ground down to be 3 thou oversize.  The head is
then heated to 150C in an oven (big one, presumably) and the seat fitted.

Jaguar, being Jaguar, don't seem to gather all the relevant data in on place
bless 'em.

Assuming that the temp difference between the seat and the head is 130C, the
differential contraction is 10ppm/C and the seat OD is 1.5 inches, there
will be a 1thou interference under these conditions.  If the seat was
chilled in a freezer to -40C, it would be a precise fit.  Cooled to liquid
nitrogen, it would just drop in.

Machining to give a greater interference than 3 thou may be
counterproductive, as the seat would have to be driven home, and shave the
aluminium on the way in, reducing the interference again (to about 3 thou?)

I can find nowhere what the tolerance on the 3 thou is.  It is probably in
the Rear Suspension section somewhere.

Craig Sawyers

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