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Mike's wire-gauge idea is a good one, especially for static measurements, since
wire can flex more easily than a blade and conform better to any curvature that
may be present on the mating surfaces. Wire is rather difficult to use when the
engine is running, although perhaps Mike's forked arrangement can be set up so
that it provides a stop that doesn't allow the wire to be slid inward beyond the
midpoint of the valve stem.
It's true that if one surface is convex and the corresponding mating surface is
concave, a flat gauge will tend to tighten up within an effectively larger gap,
creating the impression that the gap is smaller than it actually is. Generally,
however, it has been my experience that on a flathead engine, the hardened
adjusting screw in the tappet and the end of the valve stem tend to wear more or
less equally in terms of flatness. I just had a look at a set from my disassembled
T-245 and both surfaces appear reasonably flat.
A larger problem arises when you have an engine where the end of the tappet
adjusting screw is a larger diameter than the valve stem, in which case a worn
area (even if flat) in the adjusting screw creates a depression that can really
throw off your measurements. The same phenomenon can take place if the valve
diameter is a larger diameter than the end of the adjusting screw (unless the
adjusting screw is softer than the valve and tends not to create a depression in
the end of the valve stem). Note that if there is a difference in the diameters of
the adjusting screw and the valve stem, even a wire may not be able to conform
totally, due to the steep break at the edge of the worn depression, which can be
bridged even by a piece of wire. This should not be a problem with the T-245,
however.
Another approach that is almost foolproof is to measure the actual gap with a dial
indicator (measurement taken directly off the tappet) by measuring the distance
the tappet must move upward to break the valve-to-seat contact, then back the
crank off again and see what size feeler gauge fits snugly in the gap, and
subtract the difference to see how much thinner the feeler gauge must be to
provide the proper gap. The dial indicator is a wonderful invention...
Alan
"Norona, Mike" wrote:
> One minor note. If the lifters have not been crowned (machined flat where
> they contact valve stem), they will have some degree of dish-shaped wear. A
> flat feeler gauge may bridge across this cavity, and not give a true reading
> of the actual gap between the curved surfaces of valve stem and lifter.
> (The valve stems may also be rounded, but this won't affect a flat feeler
> measurement.)
> One way to overcome this is by using a wire feeler (like spark plug gauges).
> I ended up making my own, using a forked arrangement to hold a span of music
> wire (single-strand, not wound/wrapped), which can be had in various
> diameters at any guitar/music shop, etc. (or you can get 100-ft lengths at
> the hardware store).
This archive was generated by hypermail 2b29 : Mon May 01 2000 - 05:30:05 PDT