Sunday, September 20, 2009

WR250R Sunstar 12T Front Sprocket Install

To combat the increased size of the D606s and to get a cheap feel for 13/46 gearing, I changed the stock gearing from 13/43 to 12/43 with a new Sunstar 12T sprocket (520) part number: 90-1512 or Mfgr# 38812

The first step was to loosen the rear wheel and slide the axle part way out so the adjusters were free. That created enough chain slack to pull the chain off the rear sprocket

I used a 10mm socket to remove the front sprocket cover's three bolts.

Then I grabbed a huge breaker bar and a 22mm socket. I stood on the rear brake and loosened the nut on the front sprocket. An impact wrench and 22mm impact socket would have been easier.

The old and new sprocket side by side. The stock is on the left and has a lot of rubber on it which reduces chain noise. I haven't noticed a lot of new chain noise with the new sprocket.

Install the new sprocket, put everything back together, and tighten it down. The last step was to re-adjust the chain.

The new sprocket definitely compensates for the gearing change by installing the D606s. Removing a tooth from the front is the same as adding three teeth to the rear. By adding the D606s I added about 1-1.5 teeth in the rear, so this change isn't as much as I had originally hoped. I do notice a difference, but it isn't as much as I expected; in all gears there is slightly more zip.

I am likely going to change the gearing to 13/47 when I can afford a new chain and sprocket kit. I will also keep the 12T around to swap to get really low gearing for trail riding.

The speedo is way off now with both the recent changes, so the next step is to borrow a GPS and get my SpeedoDRD installed.

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  1. Great articles Chris, but dont you mean "Removing a tooth from the front is the same as ADDING three teeth from the rear." instead of "Removing a tooth from the front is the same as removing three teeth from the rear."

    Correct me if I am wrong but adding a smaller front sprocket is the same as adding a larger rear sprocket - no? I switched to a 13-47 and loved it on the dirt for the low end torque but it really limited what the bike could do on the highway so I switched to a 13-45 setup recently which I really like so far but havent taken it on the highway yet. Low end feels great though. Less torque than the 47 but still better than a 43 off the line.

    1. you are correct! thanks for catching that typo. I didn't do the 13/47 yet. I'm leaning towards 13/45 this winter and probably a fresh set of D606s.

  2. Front sprocket needs a size 27 socket (not 22), same as the rear axle nut.