to shim or not to shim!

It's almost riding time here in north central Ohio snd while I was checking over the bike ( 78 kz1000 ltd) I did the valve lash and found 3 with .04mm or less but the other 5 were .05mm. Should I replace these and take them to .10mm and let them wear back down to .05mm or leave them alone?

You are right at the minimum (or too tight), and according to the guys here, you should shoot for the top end (0.15mm on my 82 1000).

I always set my own bikes (with stock cams) around 4 thou (0.10MM). I never leave clearance at 2 thou or less (0.05MM). If you have to choose between 3 thou (0.075MM) or 5 thou (0.125mm), go wide...

I keep a mix of use and new shims handy to 'fine-tune' my clearances to get them all 4 intake and all 4 exhaust as close to one another in the set of 4 as possibly. It does make a difference...

I always measure each shim with a micrometer and write the actual readings on the shim with a Sharpie before I stash them in my shim box.

Jeff.Saunders wrote:

I always measure each shim with a micrometer and write the actual readings on the shim with a Sharpie before I stash them in my shim box.

Same here.

ditto all the above: one or two thou wide is no problem, tight can mean a burned valve seat and destroyed head.

The spec on my engine is .002 - .004", I keep them .003" - .005". There is no performance difference I can see going a thou or two wider, but I don't have a dyno.

The gospel back when I built engines to race dinosaurs was that the exhaust valves were the most critical for a tight clearance because they got hotter than intake valves and so would expand and contract more due to temp change. Most of the engines I recall always specified a couple of thousandths wider gap for the exhaust, these days the specs seem to usually be the same (?)

The cover is already off. The season isn't really started yet. May as well finish the job while you're this far along.