The DR900 - DR650 Big Bore Stroker buildup

The plan being to build from the best pieces from these three motors I laid out all three transmissions for inspection.

No single gear cluster was obviously better than the others so I pulled them all apart and compared all three of each piece.


I'm surprised at how poor all the gears looked. The only exception - get this - is the third gear set in each transmission. All three third gear pairs looked great! All three 4th drive gears (mainshaft) looked too far gone to put back in so I ordered up a new 4th drive gear. The mainshaft is on the bottom. 4th drive is the second gear from the left. 3rd drive is the third gear from the left.


Here's the bits that go in between the case halves (minus the transmission, crankshaft and balancer. Not much to it really. Some DIY guys dread the thought of splitting the cases but it's not that complicated just unfamiliar if you haven't been inside a motor before.


There's a few things that can be done while I wait for the 4th drive gear to arrive. The oil pump idler gear was very wobbly on the shaft. It's been running at an angle with a side load. Check out the wear on both sides of the teeth. This came out of the lowest mileage motor but it's the worst of the three gears I have. I'm kind of surprised we don't see more failures related to this idler gear. I ordered a new idler gear. I think this part of the motor needs all the help I can give it.

The gear rides on a short pin that's inserted through the case from the inside. The pin is loose in the case and the gear is loose on the pin. This makes the gear run cocked at an angle between the drive gear on the back of the clutch basket and the driven gear on the oil pump. I took the best of the three pins I have and epoxied it into the case. This will hold it firmly in place and will prevent it from falling inside if I ever have to remove the outer snap ring that holds it in place.

I also put some shims between the snap ring and the case on the outside. This will also help keep the pin firmly in place. They're in there nice and tight.

I installed the shift drum, oil pickup screen, crankshaft and balancer.

With the longer stroke there's just barely enough clearance between the big end of the rod and the balance shaft. In stock form there's about 6mm of clearance here. I'll grind a little off the center if the balancer just to be safe.

Plenty of room now. Maybe almost 3 millimeters.


I disassembled the top end from the low mileage donor. All the top end stuff stays together to keep things simple and organized.
This motor will get a 190 cam and a different cylinder to accommodate the big bore piston.


With the 4th drive gear installed on the mainshaft the gear clusters can go into the case.
You can see the new dark colored 4th drive gear on the left and the very shiny pair of billet 3rd gears from Nova in the middle.


On most Japanese motorcycle transmissions it isn't possible to put the shift forks in the wrong place.
Either they won't all line up with the shift drum or one won't slot onto the wrong gear or something.
Not so with the DR650. In the pic below the fork on the left could be swapped with the top fork on the right and everything would fit together just fine.
If those forks are swapped it won't shift into 5th gear. The pointed tang on the fork on the left is relieved for clearance. It needs that clearance to slide the gear all the way into 5th.


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