How to set a Balanced GPR Engine with a Senda Performance Crank

toratora · Post by **toratora** » Sat Jun 16, 2018 1:39 pm

It's a strange feeling when you search for help on the web, and your posts on how to do things are the closest you find.

With the help of my friend Ed, we cracked an important Euro2 Derbi engine secret. There are two versions of of these engines. One that has a balancer, and one that doe not. The balanced engine has markings on the gears to indicate how to set things so that the balancer is properly aligned with the crank. All of the performance cranks I've ever seen for the Derbi engine are designed for the non-balanced engine.

Now let me quickly state that when I say balanced in this context I'm not talking about machining the crank so that it is itself balanced. You could, and would probably benefit from doing that with either engine.

Here's a stock GPR balanced crank from the clutch side.

Here's the Airsal crank.

And a stock Senda non-balanced engine crank

By comparing these you'll notice that the balanced crank features a stepped design, while the non-balanced cranks feature a taper design. It may be a little difficult to see with these phone photos. My apologies.

Here are the two different primary drive gears for these cranks. The one on the left is the stepped design (I believe it to be Derbi part #00H02610381), and features a notch on the top of the gear, and a woodruff key slot. Notice that the taper gear on the right (I believe it to be Derbi part #00H02601381) features neither of these items. Yes you will need to source one of the taper gears if you upgrade your crank on one of the balanced engines (which is why I included the part numbers).

If you have a Senda with a non-balanced engine, and you upgrade your crank you'll not run into this issue. But if you have a GPR which features a balanced engine you will run into an issue when you go to set the alignment for the balancer, because there is no mark on the gear.

Ordinarily what you would do is set the balancer to align with the case mark, the mark on the balancer gear like this:

Then you would turn the crank so that the notch on the gear aligned with this mark, but now you cannot because there is no mark on the gear. What do you do?

Well as it turns out it's pretty easy. Aligning to that mark would actually just set the crank to TDC (top dead center). So all you have to do is set the balancer to its mark, set the crank to TDC, and then set the clutch basket into place. Mystery solved.