"Jim Wilkins" fired this volley in news:l6suie$fts
:

What is the tolerance for hydraulic spool valves?

A few tenths, usually. And then, only that close when they pay really
close attention to the coefficients of expansion of the spool and the
frame.

Remember that there has to be enough room between parts to develop a film
of lubricating (and sealing) oil, or they don't work.

There's an old (perhaps true) mythos about when Royce/Rolls first
introduced an automatic transmission. They were adamant about making it
to much tighter tolerances than "consumer" trannies, because they
demanded the utmost in 'quality'. Then they discovered that the valve
body and shuttles HAD to be sloppy in order to work at all. G

Lloyd