Honda-SOHC

Exactly that's why I stayed with the original seperate units - Im chicken!

My 500 has eight rubber dampers four are thick on the drive side four are thinner on the overrun.
Looks like the 550 is different.
Definitely another seal on the retainer item 20 in the parts book p/n 90753-283-000 the left side spacer runs inside it plus a large O ring for the sprocket cover I think.

It might be item 27 in that csml link.

No matter what set you need thicker O rings at the headgasket than those in the set

I feel your pain, I've been waiting two weeks for spark plugs that were advertised as in-stock but so far nowhere to be seen!
Was the old sprocket worn out? I was lucky with Ratty because although the drive chain was seized solid the rear sprocket was perfect apart from the PO having it chrome plated! I had that removed and it was refinished in BZP and it has come out like new.

The old sprocket was worn, corroded and was in overall poor condition. I didn't want to use it with a new chain and front sprocket - rightly or (probably) wrongly I slung it. I'll have to wait and see if the other one due next week is ok. And yes the simplest little brackets and fasteners are like hens teeth.

Just took delivery of my rear sprocket and it isn’t the correct one! I got mixed up in my earlier post, the sprocket that’s arrived today is from Wemoto which I ordered after the eBay episode.

(Attachment Link)

It’s out of stock at DS and CMS. I tried two suppliers on eBay, one has no stock despite listing the item. The other reported “stock issues” after I asked for a refund and then a day later promised delivery for next week. The item is now apparently on its way.  I feel a saga coming on….

I feel your pain, I've been waiting two weeks for spark plugs that were advertised as in-stock but so far nowhere to be seen!
Was the old sprocket worn out? I was lucky with Ratty because although the drive chain was seized solid the rear sprocket was perfect apart from the PO having it chrome plated! I had that removed and it was refinished in BZP and it has come out like new.
I think we are both in the same place with our builds. Every time I think I am on the home straight I go to fit another part and I find something else is missing, currently waiting for the two long hex nut studs that hold the headlamp frame to the top yoke. These are due next week.
This is the story when you buy someone else's idea of what they think a bike should look like including pod filters, black painted engine, chrome sprockets and engine plates, fiamm horns, wide wheel rims, cow horn handlebars etc. The result is all the small but expensive and difficult to get parts would be thrown in the bin years ago! It's all thanks to the great people on this forum that Ratty will live again as a 400/4 and not end it's days unloved in a shed.
As for Peter Horton, the man is an absolute guru and a wizard when it comes to restoring our clocks. I can't believe how he brought my clocks back from the dead and converted the rev counter to a 400/4 item from the 250 item I gave him👍.
Good luck with your continuing restoration, it's obviously going to be a great bike when it's finished and looking forward to seeing the finished result 👍.