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Lapping synchros

Lapping Synchros

Rob: You made reference to 'lapping synchros' when giving some Moss box
troubleshooting hints.  This means what?  My Moss box and overdrive have
been apart for a while but are due for reassembly this winter, so perhaps
you could expand upon the synchro topic for myself and others on the
mailing list. - Dave Quirt, 58 XK150S 3.4 OTS

This process is mentioned in the manual on page F26 paragraphs 5 & 6, but
the author used the word "grind" rather than "lap". Perhaps lap is American
machine shop slang, I don't know. Anyway, the idea is to smear some valve
grinding compound on the synchro cones and then rub them together. Clean
the gear wheel and sleeve, then rub them together dry to gauge by feel in
your hands the friction in the cones. Now smear in the paste and start
lapping. Keep doing this until you are convinced you have full contact of
the gear cone on the inside of the sleeve. Clean them off and see how they
feel, should be noticeably more friction than when you started. This is
what the
manual means by "a good bite". However, there are two things to inspect for
before you start lapping, particularly on the second gear synchro, because
it seems to get the most wear. 1. The gear wheel cones have longitudinal
grooves, which may be filled up with metal rubbed off from the inside of
the sleeve cones. Take a tiny chisel and chip out those grooves. 2. With
the cones held together there should be a gap between the flat face of the
sleeve and the flat face of the gear wheel, like about 1/16 inch (1.5
mm).If this gap is down to zero, this is the definition of "worn out
synchro", and lapping won't do any good. Don't panic, your parts are not
junk yet. Take them to a good machinist, where he can put the sleeve in a
lathe and shave off the face enough so you have a good gap. I took off
.025" on mine, probably could have taken off a bit more. Now you can lap
them. I did all this on mine and it shifts like new, well ok, near as I can
guess what "new" was like. - Rob Reilly

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