Suggestion on valve shims

 Hi All,

  OK it's valve adjustment time for my 1980 CB750.  So before pulling the
cover I
went by my local Honda dealer parts dept and asked about shims since I'm
sure I'll probably
need 1 or 2.  To my disappointment they had none.  (plenty of the small size
shims
for newer bikes, nothing for mine though)  They can order them - if I want
to wait
a week, and the shims are $8  -each-.

  So now what?  Do I go to the trouble of taking everything apart just to
get some
shim measurements then putting it all back together, waiting a week, then
taking it
all apart again?  Do I get an expensive shim kit that I'll never use 99% of
the
shims in the kit?  Do I call around to other bike repair shops with the bike
apart
and ask if they have a shim in the measurement I need that they will sell or
swap?
Do I scavenge bike wrecking yards for hashed-up bike engines that I can get
for a song?

  Any ideas of where I could quickly get a few shims that I need for a few
bucks would be most appreciated.  What do other people do?  (besides
take it to some mechanic and let it be his problem?)

Ted

 Ted Mittelstaedt wrote:

Apparently (just saying this based on a couple of links I found), Toyota
twin cam engines use 25 mm diameter shims, so do some Triumphs and quite
a few Yamahas - perhaps Toyota shims will be easier to find locally.


By the way, I've never adjusted any kind of shimmed valves, but suspect
I may have to soon on a friend's 1982 CB750C - am a little puzzled that
clearance is supposed to be something like .05 mm whereas the shims
people seem to be selling online for cb750's all seem to vary in
thickness by exactly .05 mm.  I'm assuming that if the valve springs and
tappets/cam surfaces aren't too worn then usually valve wear would cause
clearance to become tighter over time (I'm guessing because of valve
shoulder wear), so one would likely find after measuring that clearance
needs to be increased.  Say for argument's sake that one finds it is
necessary to remove .03 mm of shim thickness, wouldn't that mean
removing, say, a 2.35 mm shim and replacing it with a 2.32 mm shim?  Or
do these bikes come stock with several shims whose thickness isn't a
multiple of .05 mm or is my understanding all balled up?

 paul c wrote:
...

Looks like I was a little balled-up, just found some manual pages at
http://www.cb1100f.net/Other/CB750FManual/Honda1979thru83CB750ServiceManualChapter03InspectionandAdjustment.pdf
and it seems that clearance is .08 mm with tolerances of plus .05 and
minus .02 mm.  There's also a chart to follow for replacements.  So I
guess the key is that plus tolerance.  It also sort of makes sense to me
that the plus tolerance would be greater than the minus, as I imagine
it's better for valves to be slightly 'loose' than slightly 'tight', at
least as far as engine wear is concerned.

 

I once tried to extend the valve adjustment interval on my GS1100 by
setting the
valve clearances to the minimum, wrongly assuming that the cam lobes
would wear and that the valve clearances would loosen up...

I set all the intake valves at less than 0.003 inches. They burned
after about 30K miles, but I was also having fuel starvation problems
and worn valve guide oil seals leaking oil onto the valve heads, and
keeping them from seating properly.

Normal valve seat erosion under normal operation will cause the valves
to "sink" into the seats by a few thousandths of an inch, taking up
valve clearance at the stem end.

 

.08 mm is 0.003152 inches. An American feeler gauge would work just
fine.

.13 mm is 0.005122 inches. Again, an American guage would work just
fine.

.06 mm is 0.002364 inches. That's a very tight valve clearance...

You can futz and putz around with metric tolerances and waste a lot of
time if you
don't realize just how small the variations are. I use American
guages, which are close enough for a reasonable individual to use.