It should take 25mm diameter shims.
Motion Pro makes a tool that's very easy to use. To save $30.00 just pull the cams. Unless there's been valve work quite often intake and exhaust shims can be swapped and gain proper clearance. I have no dealers left in my area, so if I don't have one the right size I usually end up taking shims to the machine shop and have them surface ground to the correct thickness. Since the CBX and a few of my Yamaha's take the same shims I should probably ante up for a full set, but since I can get 20 shims ground for the price of the kit it just hasn't made sense do buy one yet.
Here's a link to cb750c.com that has a quick guide to adjusting those valves.
http://cb750c.com/modules.php?name=Forums&file=viewtopic&t=12077&highlight=Since the number's quite commonly worn off the shims (from a- grinding 'em like I do, or b- someone put 'em in upside down) a good 0 - 1" micrometer is a must.
The other thing to remember when measuring valve clearance is "go / no-go" For example, if the clearance is given as .003" to .005" so long as your .003" fits and .006"
doesn't fit that one's fine.
My ebay search "cb750 valve shim" turns up shim kits and tools... You may wanna make sure you're searching all sites, including ebay motors
Good luck - I thought I was doing a quick carb clean on a coworkers DOHC CB750C last year, ended up spending 30+ hours chasing a weird revving problem - bike would only rev to 5K then shut off like it hit a rev limiter. I ended up adding a 30 amp relay to power the coils and it was good to go.
