Mike- the diameter is only important with respect to where you mount it (clearance to other parts) and the needed resolution between teeth that your ECU needs. Plus where/how you mount the sensor. A 3" wheel with no place to put the sensor makes no sense and a 12" wheel similarly. 3" or 4" minimum I have seen, most are 6" to 7". And anywhere up to 60-2 teeth (most I have seen and used, may be more in some applications). And a crank trigger doesn't care what engine it is on. I use a modified SBC wheel with 2 teeth on my 4 cyl. You program the ECU to do what you need firing order wise. The wheels are pretty much universal but of course the mounting bolts you use to put it on your damper or pulley or whatever may need customizing. Or am I totally off base and you are just using a crank trigger for a distributor and some ignition box, no EFI etc. But still, pretty universal and exact diameter dimensions not critical, just the tooth size and count.