The transmission gears are engaged by way of gears with dogs (male engagement) and slots (female engagement).
Gear dogs may either slide into their mating gear slots or mesh to their mating gear's dogs depending on which gears are being mated.
The dogs are undercut starting from the outer corner toward their root. This undercut is designed to keep the dogs from slipping out of gear under heavy load.
With a 90 degree angle instead, the sliding gear dogs could simply spin straight out of mesh (pop out) with high centrifugal force.
With the undercut, they can't simply pull out. The sliding gear would have to first separate from dog mesh and then pull out.
The outside edge of the dogs also have a back cut (or chamfer) to help guard against chipping when sliding in or out of mesh against it's mating gear.

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A Sportster is not a 1974 Chevy pickup with a six cylinder motor and manual shift and should not be shifted like one.
Below are a few common things that can cause gear dog wear/damage while shifting.

* It's best to shift soon after pulling the clutch:

A “fast shift” doesn't mean slamming the shifter pedal. It means to be sure and shift soon after pulling the clutch.
This retains current speed of both the mainshaft (MS) and countershaft (CS) gears.
If you wait too long after pulling the clutch lever before making a shift, it could cause grinding noises and gear dog damage.
The MS and CS should both be at different speeds but traveling at the same respective speeds both before and after pulling the clutch lever in.

The dogs should ideally not hit side to side during a gear slide while changing gears.
While the mainshaft and countershaft will not be at the same speeds, they are both in a respective speed with the sliding gear engaged.
When the clutch lever is pulled, the mainshaft is not being “powered” by the motor. The countershaft however is being “powered” by the rear tire.
So the mainshaft has the ability to slow down shortly after disengaging the clutch.
The sliding gears on the mainshaft are splined to it so if the mainshaft is turning, the splined gears on it are also turning.
And with all the gear's teeth always in constant mesh, the corresponding gears on the countershaft are also turning (these are sleeved over bearings).
Works the same with the sliding gear on the countershaft with corresponding sleeved mainshaft gears.
The sleeved CS gears also slow down even though the countershaft is being powered by the rear wheel as they are not spline to the countershaft.
They also provide friction on their corresponding mainshaft gears which helps to slow them down, which slows the mainshaft.

If a shift doesn't happen until after the mainshaft starts slowing down, the dogs/slots may not match up and hit side to side.
This slows the sliding gear even more with the fork still trying to force the sliding gear in and the edge of the dogs end up slapping their mating dogs sides.
And the dog corners take the brunt of the blows.
This can end up rounding and mangling the dog corners and cause a gear(s) to pop out of dog mesh with their mating gear.

* It's best to shift with a little attitude instead of a lazy shift (easing the shift pedal with your foot).

That doesn't mean slamming the shifter pedal but rather with a enough force to ensure constant travel “to home” of the sliding gear(s) affected by the shift.
A lazy shift allows the (gear to be engaged) to spin normally but slows down and repositions the sliding gear).
So the sliding gear hits the sides of it's mating gear (which is now traveling faster than the sliding gear) and sort of bounces back against shift fork motion.
The shift fork is trying to mate the 2 gears but both gears try to repel each other traveling at different speeds.
Finally, a “window” opens where the sliding gear can enter the mating gear's “dog land”.
So you get the noise that sounds like grinding gears until that “window” opens.
The brunt of abuse once again is on the dog corners.
The gears themselves are pretty tough but constant abuse on the dogs can round the corners and cause the gear(s) to pop out of dog mesh with it's mating gear.

Below is basically a list of possible wear conditions that could cause shifting problems.
Click Here for more detailed information with pics of the associated wear discussed below.

The shift forks are a brass/bronze material. Excess wear from the forks may show up as brass “flakes” in the primary oil.
The job of the shift fork is to deliver the gear to the right spot, not to hold it there. ²⁾

There is designed slop in the shift forks just as there is with most of the shifting components.
The ID of the round end of the forks slides along the shift drum but with a wide clearance.
So there is some slop in that connection alone but that connection also allows uninhibited side movement of the forks.

Worn fork pin holes:
Each fork ID has a pin hole with a solid pin through that sits in a groove in the shift drum.
The pins just sit there with a cotter pin on top of them (not thru them) to keep them from coming out of their bores.
The pins have a habit of wallowing their bore holes during operation which could eventually lead to a gear not fully seating in it's mating gear's dog land.
This could lengthen the time the gear actually hits home to mate or serve as an accomplice to a gear popping out of home place.

The gear dogs can wear on the sides. ³⁾ They will sometimes become rounded off instead of being flat on the edges. This can cause the gears to not sync 100% and pop out of gear.

Worn dogs 4)