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.
The slotted gears are also undercut to match the dog machining.
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.

¹⁾

The gear dogs can wear just about anywhere around them for several different reasons.
The most common thing that happens is when the corners get rounded off instead of being flat on the edges. ²⁾
A gear may hop out of dog mesh.
It feels like the transmission jumped into neutral for a second and then jumped back in gear.
The dog(s) 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.

This is the common cause of hop-out, although checking the pawl adjustment with a .116“ drill bit per the service manual should certainly be done first. ³⁾
But you can check that on your way to pulling the transmission.

Worn dogs 4)

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 or contribute to 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.
Any hinderance to the sliding gears for whatever reasons could contribute to gear dog wear.
However, worn parts are generally described as inherent with use and the prime contributing factor to dog wear is generally associated with shifting habits.

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. ⁵⁾

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.

Scars/grooves in the bore IDs:
The fork to drum clearance is pretty wide but the fork IDs are relatively smooth still when new.
The round end of the forks can get scarred and/or grooved by the sharp edges of the shift drum channels when a gear backs out of dog mesh.
The gear forces the fork to reposition itself against the drum and the fork ID gets rammed into the sharp edges of the drum channel(s).
The grooves do not necessarily present a huge problem unless they are jagged or scrubbed up enough to hinder movement of the fork along the drum.
It's possible to lightly sand the fork ID and reuse the fork unless the damage is excessive.
But until addressed, the damage can allow for erratic movement of the gear towards it's intended home position.

Bent shafts:
Bent shafts will hinder movement of the sliding gears as well as create more pressure against each shaft which creates more heat in the system.

Removing thrust washers can distort them and re-using them can end up with pointed edges to catch or hinder movement of the sliding gears.
The sliding gears will slide over the area where the thrust washers sit and the washers should be tight against the respective shafts.
The washers keep the stationary gears from moving side to side.
A loose or worn thrust washer can allow the stationary gear to reposition itself and/or hinder the sliding gear from going home.

Worn/damaged shift drum channels:
The shift drum has 3 channels cut into it and the fork pins travel thru these channels.
As the drum is rolled by the shifter mechanism, the channel directions change to whatever direction need for the next gear change.
These channels run front to back until a gear needs moving and the respective channel turns so the pin can carry the fork/gear toward/away from the adjacent gear.
The channels are wider than the pins = more slop in that connection also.
It is conceivable that the slop mention so far is designed to allow some flex of the gears when they don't line up for a proper shift.
If the dogs hit side to side, the slop allows a momentary stop or slowing of delivery so the dogs can mate when the window of opportunity arrives to do so.

The channel edges are cut sharp when new but not razor sharp. They get sharper as the pins are forced against them during shifting.
If a gear pops of dog mesh, it pushes first thru the clearance mentioned and then against the shift drum channels.
A prolonged condition can widen or scar the channel(s) which can result in erratic operation when shifting and/or gears not mating solidly.

If a gear does pop out of mesh, it can reposition the forks to where the sharp edges of the drum channels can cut grooves into the fork IDs, mentioned above.

The shifter shaft assembly ends with a shift pawl that raises or lowers a pin on the end of the drum that makes the drum turn.
The detent plate has valleys where the drum stops at the correct respective gear positions.
These valleys are direct relationships to the gear when it's in “home” position (or in dog mesh). If the drum is allowed to rock in and out of the root of the valley, damage to the dogs may occur while achieving dog mesh.

Detent arm spring:
The detent arm under the plate has a roller on the end that the plate rolls against with the other end of the arm being spring loaded.
A stretched or especially missing spring could keep the detent plate from stopping in the correct place, allowing the plate to “float” around the valley.
This could allow for movement of the dog a “homed” gear to pop back out of dog mesh.

Detent plate positioning:
The detent plate is notorious for getting tilted due to the shift drum pins walking out.
Click Here and view the sub pages on the top of page for detent plate issues.
The detent plate should sit upright 90º to the drum. If it sits tilted, the angles may skew and hinder gear slide timing and/or positioning of the gear.
Result could be erratic movement of the gear towards it's intended home position and/or gear dog damage.