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There are a few ways you can go about selecting the new shim. The quick way is to add or subtract half the adjustment range to the shim you've removed. This assumes the valve was marginally out of spec. A more accurate but time consuming way is to measure the shim you've removed, and use this along with the present clearance and new clearance target to calculate the new shim size. I would not recommend that you sand down a shim to change it's size. This would probably only work if you can guarantee that the shim was not just surface-hardened.
Replacement shims are available from Aprilia in 0.025 mm increments, ranging from 2.300 mm to 3.000 mm. This is a 10 mm diameter button shim, and is probably available for other manufacturers too. Shims are almost always marked with the size in mm on their surface (sometimes the decimal is dropped). The shim that I removed from my bike had no markings, so I had to measure it with micrometers. A dial caliper doesn't really have enough resolution, but you can use it in a pinch.
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Measure the shim with a micrometer if it isn't marked. |
An Intake Valve example. My smallest micrometers measure in inches, as do my finest clearance feelers, so I'll do the math in inches here. If you are using measurements or shim markings in mm, divide the measurement in mm by 25.4 to get to inches, or multiply inches by 25.4 to get mm.
| [1] Clearance measured: | 0.0045 | in |
| [2] Add size of shim you removed: | + 0.1026 | in |
| [3] Result: Clearance with no shim | = 0.1071 | in |
| [4] Subtract desired final clearance (center of range = 0.0060) | - 0.0055 | in* |
| [5] Result: New shim size required | = 0.1016 | in |
| Multiply by 25.4 to get mm: | x 25.40 | mm/in |
| Result: New shim size in mm: | = 2.5806 | mm |
| Round to nearest 0.0025 mm: | = 2.5800 | mm |
Note! When I did this, I thought the intake clearance spec was 0.005 - 0.006 in, but I've since been told that its 0.005 - 0.007 in. So the target I should have used to center the clearance would have been 0.0060 in, not 0.0055 in. What I did is OK, but its more likely my valves will be out of spec again at the next interval because I set them closer to the limit than I had to.
For an exhaust valve, the desired clearance (line 3) is 0.0100 in.
Be prepared to settle for a different shim size if your dealer doesn't have a large selection in stock. There's a ten shim size range that will probably set up your clearances correctly. You may want to redo the shim selection example plugging in the extremes of the range for line 3 to see which shims are acceptable. When you get your new shim, you may want to measure it just to be sure it's marked correctly. The new shim I installed in this example measured 2.578 mm.
Some dealerships are cool and if you go to the service department they'll just swap you shim for shim, no charge. My Aprilia dealer and Yamaha dealer were like that. Other dealers will insist you buy the shim, which I've done. After a few services, you have enough shims to swap around yourself. The worst are the dealerships that won't sell out of service stock, and want you to order the shims and wait 5 days for them. There is a chain of dealerships like this in Boston, and I take great pleasure in bring all the bikes I've bought at other dealers to them for inspection every year so I can show them the business they are losing.
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