LOL! :lol:
Yes, the famous M7 nuts... never leave da house without a spare in your pocket... (don't ask, in '99 I chugged across half of Europe with one missing... ;-) )
They can come off if frequent checking their seating is neglected and/or they haven't been properly torqued to begin with...
The issue is, that it now would be wise to check all, gaining access however means removal of the heat shields and by the looks those bolts will snap off...
And since one downpipe already came loose, replacing all exhaust manifold gaskets (at least the two at the shown LHS head) seems recommendable as well...
Anyway, the M7 manifold flange nuts are used on 1100 and 1300 (possible other models as well), likely that a lister can offer to drop a spare one into a bubble-mailer and send it to you...
(I lurked on eBay till a full set from a wrecked 1300 came on for peanuts... but the postage for one sent from Europe to your place would cost more then I'd paid for the 8 pcs set...)
What would you need if going for the full job (item numbers for doing both sides):
- 8x 90304-MM5-000, nut, flange, 7mm (that's the one you're missing)
- 4x 18291-MN4-920, gasket ex pipe (copper gaskets manifold to head)
- 8x 95701-0601000, bolt, flange, 6x10 (bolts fixing the heat-shields on the manifolds)
- 2x 94103-06400, washer, plain, 6mm (washers for forward bottom fixing heat-shields)
- 8x 18292-415-000, packing protector (secret tip/improvement from pre '94 models! Place them between the heat-shields and the exhaust bracket to eliminate rattle/ping...)
The bolts fixing the heat shields will likely shear off... don't bother, you drill out the stub: a) punch a centre dent, b) place a big flat screwdriver between pipe and bracket (avoids puncturing the downpipe!), then pre-drill with a 3 or 3.5mm bit, c) carefully widen with a 4.8mm bit but keep centred (possible the remaining metal of the stub will already fall out), d) re-cut the threads with an M6 bit for blind-holes (their shorter)
Once the heat-shields are off, you can deal with the manifold gaskets, remove the nuts holding the flanges, gently pulling the downpipes down, small screwdriver to pry out the old gaskets, then place in the new copper rings (I always "glue" them in place with some copper slip or thick marine grease so they won't fall out the exhaust ports while working), seat the downpipes into the ports again, apply a slight smear of copper slip on the M7 studs, place the chromed nuts back on, set the flange plates evenly!, torque nuts with not more then 12Nm/8.85ft lb...
Now the heat-shields with the "new" packing washers:
Insert the new M6 bolt from outside and slip a packing over the threaded part from the inside, they'll grip there.
Apply some copper slip onto every bracket we just re-cut the threads, we want to avoid them rusting shut again...
Gentle place the heat shield towards the downpipes and "feel" the M6 bolts into the brackets and set them 1~2 turns by hand, then the next bolt, etc...
Don't forget to put the large washer on the low forward facing bolt...
Once all are have found their threads, tighten them evenly (don't overdo it though, re-cut threads and that...)
Button her back up and go ride silently again ;-)