When you say "TIGHT" do you mean there is NO clearance? If so, are any of the shims for any of the other valves thinner? If so you could swap them around just to get a measurement. I just went down to my local Harley Davidson dealer (some harley's use the same 10mm shims) and traded shims for what I needed at no charge. I only needed 1 new shim to get all 4 valves in spec. If they are at .000" I would just start with a shim that is at least .006 thinner and go from there.