Yeah, it's pretty amazing to find out there is precious litle related to "pasting" a center cylinder in betwen the jugs of a 500.
Differences top to bottom:
1. Triple rockerboxes not even a vague resemblance; rockers, valves, cams, etc. of course additional qty.
2. 3-pot head not even close to 500 +1 pot (cylinders close enough, not totally different)
3. alternator on right, in timing chest
4. 3-piece crankcases
5. one-piece crank
6. dual crank main bearings blocks
7. COMPLETELY different clutch assembly with multiple containment sections in wierd configuration
8. Oil filter setup internal to engine
9. Oil sump setup with strainer like the old pre-unit engines
10. Timing idler gear on a huge shaft with retainer circlip (nice touch)
Tranny is virtually identical to Bonneville, both of which do NOT resemble the 500 very much at all.
The engine is NOT 50% heavier than a 500, it's about 100% heavier than a 650!