As the bike was burning oil I replaced the valve guide oil seals with genuine Honda ones. The pattern gasket set (from Wemoto) is fine in every other respect but Mark White at M&M Motorcycles in Essendine warned me not to risk pattern seals in such an important location. He should know.
A simple gap check showed the rings were beyond their service limits. Dave Silver supplied some genuine new ones, while the excellent Q Prep, a great engineering shop in Oundle run by bike enthusiasts, measured the piston, found it to be OK, and honed the bore for the new rings for £20. I cleaned up the piston and cylinder head with 00 wire wool, water and sieved wood ash.
The 1989 NX650 camshaft had been a risk. Sure enough it was slightly different from the 1990 dead one. But in a good way: the peg that secures the decompressor cam was slightly longer. Thirty seconds with a grinder sorted that. Pulling the decompressor parts off the old cam took a bit of heat and Rupe Farnsworth's three legged puller, but they went onto the new cam just fine.
Phil Joy had warned me to make sure the central journal didn't touch the camshaft, so Farnsworth scraped the journal carefully, removing any potential debris. The grey gloop on the lobes is moly grease, recommended by Honda.
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