How do I measure my sprockets, clutches, and chains?

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.  Common sizes are 3/4", 5/8", 1", and 16mm.
 


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.
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Measuring sprockets for chain - "what is my chain size"?

When we talk about chain size, we are not talking about length.  We are talking about the pitch of the chain so it rests properly on your sprocket(s).  The most common sizes are #35 and #41.  However, don't rule out #219 or #530.

#35 Chain - 3/8" Pitch

That's 3/8" from tooth to tooth on your sprocket.


 
#40/41/420 Chain
1/2" Pitch

That's 1/2" from tooth to tooth on your sprocket.

(Pro note:  if you have a #41 sprocket, you can put #41, #40, and #420 chain on it as well.  However, if you know you have a #40 sprocket, you must use a #40 or #420 chain.  The #41 chain will be too thin to rest on the #40 sprocket.)

(Pro note:  for motorized bike conversions, many of you have #415 chain and sprockets.  These sprockets are very thin.  When you are purchasing a #41 clutch to match, your #415 chain will NOT fit over the sprocket as it is too thin.  You will need to buy a #41 chain and may need to replace your rear sprocket as well as there may be too much play on your existing, thinner sprocket).



Other Sizes:

#219 - 1/4" Pitch - 1/4" from tooth to tooth.

#530 - 5/8" Pitch - 5/8" from tooth to tooth.