A timing wheel is a disc with graduated degrees all over it. I used my 11" Moroso wheel and a couple of adapters I made up on my lathe to fit the Vertex crankshaft.

Here's what it looks like:


You use it along with a pointer so that you can figure out all your timing needs. You set the pointer with a depth gauge so you can ensure the pointer reads the same value on either side of zero when the piston is a specific depth below TDC. i.e., my gauge read .105" at TDC, so I'd turn the crank so that the piston was at .120" both before and after TDC and made sure the pointer read the same value on the timing wheel at both times. Then I verified that at .105", the degree wheel read zero. The reason for this method is that the piston moves VERY slowly at TDC and it's difficult to know exactly when it's bang on TDC. You can be off by a couple degrees, and when you're doing precision work, it matters.

Of course, you're doing your best guess when determining when the port is actually closing, so you've got to run it 'round a few times to be sure.