• A certain number of 1974 and 1975 XL & XLCH models were produced with modified rocker arms and rocker covers. These were machined to prevent interference between the rocker arm and the inside of the rocker cover due to a thickness variation in the covers. ¹⁾
  • The only suspect rocker arm covers were the ones stamped with the letter “M” in the casting surface (which contains the pushrod entry holes). When replacing a rocker arm with the “M” stamped in the casting, you must grind 0.050“ off the top of the rocker prior to installation to provide enough clearance between it and the cover (when the valve opens).

* . . . Splitting Rocker Boxes

Rocker covers are all the same technically from 1957 to 1985. ²⁾
Different part numbers are a different finish or style.
The should be no concerns about mixing ones from different years respectively per side.

There is, however, a front and a rear.
It has to do with the location of the oil feed lines. ³⁾
The front box has the hole to the rear. The rear box has the hole to the front.

⁸⁾

The 71 up have a thicker rib across the pushrod side and most people feel they're more rigid. ¹⁰⁾
They do look better under a strobe light than the early ones.
¹¹⁾

71-85 rocker boxes (F: 17514-71 and R: 17515-71). 12)

Pushrod tube orientation by Dr Dick ¹⁴⁾
Pushrod tubes seal in 3 places. 2 of those, the upper and lower corks, seal on their flat faces.
Now think about the center cork. What does it seal?
The only joint at center position is the telescope joint. So seal needs to be on outside of the upper tube where it meets the inside of the lower tube.
With the center cork being spring loaded into the flare on the lower tube;
The cork gets squeezed into the flare and 'chinese handcuffs' to the outside of the upper.
Not only does this seal the telescope, it tends to reduce the OD of the center cork as it squeezes around the inner tube.
So what?
Well, if you flip the lower tube so the flat is at center cork, the spring still squeezes the cork.
As the cork gets pushed thinner, it gets smaller on it's ID sealing just as it needs to.
But it also gets bigger on it's OD (sealing to the inside of the spring cup piece.
And trapping rain/wash water inside the cup above the cork (rusting the spring until it snaps).
Meanwhile at the bottom, the cork is getting squeezed into the flare.
Nothing is there for the cork to seal to until the cork gets small enough in diameter.
And fat enough in thickness that the tappet starts to be the thing it wants to seal to.
On a bike with stock or even .436 lift cams, this just creates a leak at bottom cork after enough miles.
But if you happen to have a bike with more lift– say .475 for instance, then the top of tappet may go above the cork seat surface of guide at full lift.
Now that extra tappet-cork kissing contest works the cork into the tube flare real fast.
Maybe fast enough that it constricts oil flow before you get home and notice the leak.
And the ensuing tappet/guide wear from letting the lube escape from the tube each time the tappets hits the cork.
(lifting it off the guide and letting lube out to the wind)
Look at the shape of the rocker box seat. Its reverse flare to squeeze the cork tight to the OD of the bore in the box.
So it don't fall down every time you open the tube.
You late model guys who have the full O-ring setup instead of corks, some of you may suffer from crappy top and bottom O-ring life.
When you take the bad O-rings out, they are smashed flat. To get rid of that, try putting corks in the center position only.
It's the center O-ring getting tighter an tighter that kills the end of the O-ring by not letting the tube to telescope shorter during hot cold cycle.
That's why the center O-rings are always jammed.
Periodic maintenance is one of the hallmarks of a devoted owner.
When possible, while building your ride working to extend the period between these required attentions may keep money in your pocket and your bike on the road.