New to forum not Honda though! Picked up a 77 SOHC 750 which i was told has an 836 kit on teardown i found the top of piston missing a small piece showing the ring also which is strange the camshaft is marked made in USA !! I was thinking of powdercoating the cases ,,jugs and head any opinions on that ?? Pics later today ,,thanks for having me!!!
Sounds like it was preigniting or running very lean and melted the piston top. That piston will have to be replaced, depending on what kit it is you may have to replace the whole set. Made in usa on the cam means it is a performance cam of some sort, you will need to look for numbers stamped on it to determine what you have. Normally cam markings are on the end of the camshaft, sometimes they are under the washer end but most of the time they are on the opposite end. Some times it can be on the shaft itself. You will need to determine what you are going to do as far as rebuild before you get to deep into powder coating, it is more durable but requires more prep work, I find good engine paint last a very long time if you clean the surfaces well, costs less and is easy to do it yourself at home with no special tools.
The pistons are old design, you more than likely wont find a replacement. The cam is a Kenny Harmon D which was there small cam. works good for street use.
Wiseco is the best, they are forged and are proven to be good pistons. You may want to consider using the Cycle X 850cc kit, it is a slight overbore for 836 kits, they are cast pistons and shouldn't be used for racing or a lot of high rpm use but they are good for street motors.
