As part of the rebuild on this 420 I am going through the transmission. The person that I bought the donor engine from mentioned that it slipped out of gear occasionally. When I inspected the gears I found a few dogs that had the corners wore down.
When you grind the "gears" in a transmission you are not really grinding gears. The gears are always meshed together, the grinding comes from the shift dogs. The red arrow in the above photo points to the rounded off corner of one of these dogs. As you shift the gears slides on the shaft and the dogs engage on matching bumps on the second gear. You cannot see them, but the yellow arrow points too them. Rough use can cause the corners to wear down and eventually they will not stay together.
There are a lot of pieces to keep track of inside the transmission. I put these stands together to help me keep all the gears, spacers, and what not in order as I took them off the shafts. You can see in this photo that some of the gears have a light coating of rust on them. Those are the gears that I took from another machine to replace the ones with worn shift dogs.
Like most ATVs and motorcycles the engine and transmission are all one unit. This photo shows the case just before I put it back together.
To the left of the green line is the engine stuff, crankshaft and balancer/camshaft. To the right is the transmission, forward gears, reverse gear, and shift drum. It all uses the same oil for lubrication and cooling.
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