This is a quick guide to take the confusion out of picking the right clutch and chain. You'll also need to be aware of picking the right gear ratio for your application.
If you don't have access to a tape measure, or not comfortable using it, there are some pictures and other common tools to help you.
Measuring your crankshaft: determining bore size
3/4" is the most popular bore. However, you'll still need to double check before ordering a clutch. The easiest way is to measure the crankshaft with a tape measure or you can use a wrench.
You will need to make sure that the wrench is in good condition and fits nicely over the crankshaft. If you have too much play, try a different wrench, measure with calipers, or consider that it may be a metric crankshaft.
Wrenches: 5/8", 16mm, 3/4", 1"
5/8" and 16mm are commonly confused for the same. 79cc and 97cc engines use both, so please measure carefully!
3/4" is the most common - most 3.5HP-8HP engines use this size
1" is found on larger 8HP+ engines. These clutches are typically much more expensive.