Yamaha TRX850 Enthusiasts Forum

have just checked my shims left reads inlet, 15 15 15 spot on. Exhaust reads 23 23 2mm out. Right reads inlet reads 12 13 15 and exhaust 23 23 all very close according to the book. My question is is this close enough or will they have to be changed to get them all spot on. Thanks Killer.

2 mm out? Yes you have a BIG problem!
Watch those decimal points.

Inlet: 0.15 to 0.20

Exhaust: 0.25 to 0.30

The exhausts at 0.23 are too tight and even those inlets at 0.15 that`s the MINIMUM clearance remember, I would prefer to see 0.18 personally.
Always aim for middle tolerance / clearance, err towards the upper limits, too tight is a big problem, a tad loose is not as bad although performance can still be lost.

May as well change them as you're already in there!

Thanks Mincehead for your reply, have removed shims and here are the results.
Left Cyl. Reading left to right Inlert. 15 15 15.
Shim size 176 182 179.
Shim required?

Exhaust reading left to right 23 23.
Shim size 78 78.
Shim required?

Right Cyl. Reading right to left. 12 13 15.
Shim size 168 181 99
Shim required?

Exhaust reading left to right 23 23.
Shim size 78 78.
Shim required?

I cannot make nor tail from the book, anyone out there that can tell me what size shims I now need. 70 years of age and still learning
Thanks Killer.

Try think like this:
Your measurement is the gap between the valve and the shim.
If you want to decrease the gap you'll need thicker shim.
If you want to increase the gap, you'll need thinner shim.
Marks on the shim is the corresponding thickness in 100's of a mm. (i.e. 180 is 0.04 thicker than 176)

So far the math, but here's the awkward thing. Replacement shims come in gaps of 0.05 mm, i.e. 170, 175, 180 and so on. You'll have to compromise...

Example:

Left Cyl. Reading left to right Inlert. 15 15 15.
Shim size 176 182 179.

15 is a bit tight (although within limits, but you can expect to see the gap decreasing over time). To be "safe", you'll aim for the upper limit, i.e. the biggest gap within tolerances, right? So, to increase the gap, you'll need thinner shim. Next thinner from 176 (remember, 0.05 increments) would be 175. Your new gap will be the difference between the old shim and the new, i.e. 176-175 => 0.01. New gap should be 0.16, 0.01 bigger than before. Next thinner would be 170, but then the gap would increase with 0.06 (176-170), which would give you a gap of 0.15+0.06=0.21. (This is a tad too big, but it's on the safe side, see Mince's comments)

Hope this helps!

Excellent post Hans, that`s it in the simplest terms.
Again, a `tapping` tappet / valve clearance is far better than a tight one and a valve head or seat being burnt / eroded from blow-by of burnt gasses.
IF anyone is completely anal about such stuff a surface grinder with a magnetic bed could be utilised to finely adjust the stock Yamaha shims to achieve the perfect clearance. If I had a surface grinder I could actually do this myself, used to work one years ago making gearbox slip rings for Albion in Glasgow.

Thanks Blokes for your information, I now understand how it all works. I was looking to have all valves perfect but now see that without surface grinding it is almost impossible. I have had air cooled B.M.W.s all my life and am used to the old lock nut tappet set up so these shims are all new to me. Once again many thanks for your advise and direction and to the forum moderators thanks for a great site. Regards. Killer.

killer wrote:Thanks Mincehead for your reply, have removed shims and here are the results.
Left Cyl. Reading left to right Inlert. 15 15 15.
Shim size 176 182 179.
Shim required?

Exhaust reading left to right 23 23.
Shim size 78 78.
Shim required?

Right Cyl. Reading right to left. 12 13 15.
Shim size 168 181 99
Shim required?

Exhaust reading left to right 23 23.
Shim size 78 78.
Shim required?

I cannot make nor tail from the book, anyone out there that can tell me what size shims I now need. 70 years of age and still learning
Thanks Killer.

Hey Killer, if you haven't already, download the service manual form the download section on here, it has a chart of shim sizes that tell you the required shim.

Takes the guesswork out of it, especially if you've had a few beers

G,Day all, Shims done and new cam chain installed. Went to local Yamaha shop for shims and you can just about get any size you want so where this .5mm steps came from is beyond me. I have only one thing to say after a life time of air cooled B.M.W.s what a pain in the Ar++ of a job. Never again for this little black duck. Regards Killer.