This guide is to familiarize you with valve shims for adjusting camshaft-to-follower ("bucket") clearance. Various overhead-camshaft car and motorcycle engines use select-fit, hardened steel shims between the camshaft and the valve to adjust the valve clearance. These shims are of various diameters and thicknesses, but if care is taken, they can be interchanged between cars and motorcycles, even between and among different brands of vehicles.
Please do not confuse shims meant for adjusting valve-to-camshaft clearance with valve spring shims meant to adjust the installed height of valve springs. Each kind of shim does it's specific job, but both tend to be called "valve shims". Valve spring shims are not covered in this guide.
Shims are "select fit" based on the amount of valve clearance desired vs. the amount of clearance that already exists. If, for example, you want .004 clearance, but you actually have .006 clearance, you'd select a shim that is .002 thicker than what is in place already. Not all the shims in an engine are the same thickness, so you must measure each one individually if the thickness is not printed on the shim. Therefore, an engine may have a dozen different thicknesses of shims already in place in order to provide the same, correct, clearance between camshaft and bucket. This means that if you have an engine needing valve clearance correction, you might get lucky in that the shim you remove from one valve may be exactly what is needed on another.
This is where things get interesting: Let's say you own an older Honda motorcycle, your best friend owns a certain older Toyota car, and your flashy brother-in-law has a Triumph motorcycle. You could arrange to adjust the valve clearance on all three vehicles on the same day, and perhaps use the shims removed from the Toyota to correct the Triumph, and the shims from either to adjust your Honda. Another possibility is to bid on shims listed in an eBay auction for Kawasaki parts in order to adjust your Suzuki. Please note that you MUST BE CAREFUL when doing this, it's easy to get the wrong kind of shim because every manufacturer seems to use several different styles of shims depending on the engine family and the year of production.
Another important dimension is the diameter of the shim. Large-diameter shims are installed on top of the bucket that covers the valve and valve spring assembly. (Shim Over Bucket, or SOB.) The camshaft acts directly on the shim, which transfers the motion to the bucket, and then to the valve stem. It is possible that the action of the camshaft wiping across the shim, in conjunction with excessive RPM, could cause the shim to be "spit out" of the bucket, causing damage to the engine. Since the camshaft wipes across the shim, it is possible (but rare) for the shim to become worn. In normal use, the shims are so hard as to be more-or-less "wear proof" and can be re-used without problem. An advantage to SOB style valve adjustment is that the shims can usually be changed with the camshaft in place using some variety of special tools to slightly compress the valve springs, and then the shims are pried up with a small screwdriver and removed with a magnet. They are replaced in a similar manner. I get upset if I have to pay more than $8 for a new, common SOB shim, but I've heard of them selling for $10--$14 each, brand new at dealerships. It's all about supply and demand...
Small-diameter shims are used between the valve and the bucket. (Shim Under Bucket, or SUB) These shims are lighter weight for greater RPM potential, and they cannot be "spit out" by the action of the camshaft wiping across the top of the bucket. The problem with SUB is that the camshaft(s) and the buckets must be removed in order to access the shims. This is MUCH more labor-intensive, and could lead to improper cam timing and engine damage if the camshaft(s) aren't put back in proper time with the crankshaft. SUB shims tend to be less expensive than SOB shims, to the point where I've seen them sold for as little as 60 cents each, in large bulk assortments. Small groups, or individual shims would be significantly more expensive, of course.
As you have figured out by now, you must match both the diameter of the shim (so it fits your engine family) and the thickness of the shim (so it provides the proper clearance.) You cannot just buy random-sized shims and expect them to be "correct". Shims are nearly always described with metric measurements, I recommend you use a metric micrometer to measure them rather than an "inch" micrometer and then converting to metric on a calculator. If the diameter and the thickness is correct for your application, the engine won't know if the shim was made by Honda or Yamaha or Toyota, and you can use whichever brand is least expensive or most readily available.
Common valve shim diameters, along with examples of applications: Please note that my listing of applications is intended to be a GUIDELINE, not definitive! These examples are "believed" to be correct, but it is up to you to assure you have the right style of shim. Be sure to VERIFY WHAT FITS YOUR APPLICATION before you buy!!!
7.5mm May be called 7.48 diameter. Used in some Honda (For example, CBR1100XX and others), Kawasaki (ZX-6R and others), Suzuki (GSXR600 and others), and Yamaha (YZF 600 and others) engines.
9.5mm May be called 9.48 diameter. Used in some Honda (VTR1000 and others), Kawasaki (ZX-12 and others), Suzuki (Hayabusa, and others), Yamaha (FJR 1300 and others), and Husqvarna TE450 and others) engines
13mm Used in some Kawasaki engines, (ZX1100 and others).
25mm Used in Honda ('79--'83 4-cylinder CB750, 900, 1000, and 1100 series engines, and 1050 CBX 6-cylinder engines, and others), Yamaha (V-Max, and the FJ1100/1200 series), Triumph 3- and 4-cylinder liquid-cooled motorcycle engines, and older Toyota car engines.
28mm Used in some Toyota automotive engines
29mm Used in some Kawasaki (2-valve), Yamaha (2-valve), and BMW K-series motorcycle engines.
29.5mm Used in some Suzuki engines (GS1100 and others)
34mm Used in some Toyota automotive engines
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