PlayGround

Head to head straddle mount (or center mount or head steady):
There is only the one upper motor mount assembly from 57-76 consisting of a support brace between the heads and a cross mount from there to the frame.
The support brace (straddle mount) is engineered to deal with any production tolerances and most owner created misalignments and is the first thing to assemble on top. ^{1) 2)}

The straddle bracket (16250-57) mounts from head to head and a cross brace bolts to the straddle and ties back to the frame.
The straddle bracket changed in L1971 but the part number stayed the same.

Service Manuals:
There are no instructions or mention of how to attach these mounts in the FSMs other basic statements to the effect of “install the bolts”.
Why does it matter if the washers are on top or below the straddle mount?
According to Dr Dick, the washers being on top under the bolt head is responsible (at least in part) for cracked heads at the threaded mounting holes for this mount.

Looking at illustrations in the FSMs:
In the 1964 FSM, there a 2 illustrations in the book showing the left side of the bike and a very good detail of straddle mounted.
On page 1B-7 picture (1B-2), in the front of the book, shows the washers on the bottom of the mount.
On page 3A-17 picture (3A-8), in the engine section, shows the washers on top of the mount under the bolt head.

The 1967, 1969, 1976 and 1978 FSMs all show the washers between the head and the straddle.
These old books were printed using dot printers and scanning a picture to get a better view is not that easy as the dots in the ink get bigger as well.
But the washer doesn't show up on top in these books.

So even if one were to try and research from the manuals, it's still a crap shoot from there deciphering washer orientation depending on what manual you have.

Parts Catalogs:
The parts catalogs, in general, show exploded views of how the parts go together (which is generally a good visual for installation).
However, 1957-1978 parts catalogs all show the washer under the bolt head (on top of the straddle mount).

Interestingly, the 1979-1985 parts catalogs all show the washer under the straddle mount between the heads.
Starting in 1979, the head straddle (head strap) is no longer tied back to the frame. It just supports between the heads.

See Factory Top Mount Installation per Dr Dick below for top motor mount instructions.

Washers on top of the straddle mount. (as shown in the parts books and most FSMs) 4)	Machined area on head for the straddle mount. (notice the binding that occurred in the left pic) 5)

There are three straddle mounts from 1957-1985;

• 1957-E1971 (factory fitted with the -57 head castings).
  These were straight along the edge that faces the rocker boxes.
  You shouldn't use this mount on any other head casting with out checking for clearance.
• L1971-1976 factory fitment.
  These were shaped like “Ben Franklin's eye glasses”.
  This part can be used on all 57-76 no matter what heads you are running.
• 1977-1985. All 3 holes are slotted.
  This should not be used on 76< frames.

1957-E1971 straddle mount 6)	L1971-1976 straddle mount 7)

⁸⁾

1. Assemble the straddle to the heads:
   • A 1/8” thick 1“ dia. washer goes BETWEEN each head and the bottom straddle mount (1 on each end of the straddle mount).
     Then the bolts (7/16”-20 X 7/8“ bolts 5/8” hex size) get put in with no washers on the bolts (between the bolt heads and straddle).
     The head is machined where this washer sits. The washer raises the strap above any unmachined nubs around the washer seat. ⁹⁾
     It keeps interference at bay. If you look at the different strap profiles, you see the 1000 mount is also relived for nub clearance.
     The 900 strap is not relived and when you see heads that are broke at the tapped hole, you will see most are 900.
     Busting the mount lugs is because the washers are put on top smashing the strap into the nubs and splitting the head.
   • No washer between the bolt and the top of straddle mount. Both the versions of straddle mounts get assembled the same.
2. The cross mount to straddle;
   • Gets placed under the straddle mount (all years)
   • Gets bolted with no washers (58-64).
   • 57 gets the choke mount placed under the cross mount.
   • 65-76 uses the stud in the horn mount instead of the bolt.
   • 66 gets the choke lever assembly added to top side of straddle.
   • Install Flexloc or Nyloc nut.
3. The cross mount to frame;
   • You will notice there is a giant space between the frame and the cross mount.
   • The cross mount and the frame surfaces will be parallel. If they aren't then something is amiss.
   • Select the correct pair of washers to fill 'your' space between the cross mount and the frame. This will take two of the three washers.
   • Take the remaining washer and slip it over the bolt. Both get slid in from the bottom, thru the cross mount, washer stack, and frame bracket with the nut on top.
   • At this point you got:
     • Three 1-1/8“ OD washers (two thick, .160”) and (one thin, .090“/.110” depending on what year).
     • A Flexloc or Nyloc nut
     • A 7/16“x20 bolt, either 13/16” hex head 57-E72 (this bolt may be crossed drilled for a cotter pin/safety wire for R model use), or 5/8“ hex L72-76.
       This bolt eventually goes in from bottom with either a castle nut or nyloc depending on year.
       • The bolt with the 13/16” thin hex got 1 of the 2 thicker washers slipped on then these 2 items went from the bottom thru the cross mount.
         Then the other thick wash, then thin washer (these are slid between the top of cross mount and the bottom of the frame mount), then frame mount.
       • 73-up used a regular 7/16“x20 bolt, not 13/16” hex, instead.
       • Now the nut on top.