Finished

It took a few days to sort out the odds and ends that inevitably arise: frayed carb cable, sticking brake pistons, carb setup and that kind of thing. So far the bike's not gone over 4500rpm out of consideration for the new rings. But it feels absolutely lovely, and handles a treat. So far the cracked head hasn't made its presence felt. Long may it continue. I used Castrol Power 1 after interviewing Matt Symonds, head of Castrol's bike development programme last year at Pangbourne. I am convinced they make some of the best oil available.

Things that have worked:

The rebuilt oe shock (Falcon Engineering)
Carb refurb (JRS of Swansea)
The pattern gasket set (no oil leaks)
The 1989 camshaft and new rockers/followers/sub followers
Five Wheel Steel paint on forks, pillion hangers and swing arm etc
Simoniz tough black paint on footrests and sidestand

Things that haven't:
Hycote high temperature black exhaust paint
The pattern Honda grips (they smell of creosote)

I'll report back in 1000 miles to say whether the engine is still OK.

NX650 top end rebuild

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.

NX650 bottom end rebuild

The only reason the cases had to come apart was the worn-out splines on the gearshift shaft. The Dominator is actually pretty simple to strip and reassemble if you have the factory manual. You don't need any special tools beyond a clutch centre holder, a very long 5mm Allen key which you can hook up to a torque wrench (for the engine casings), and a universal flywheel puller. A handy substitute for the Honda cam chain tensioner holder tool is a small punch, or even a ground-down nail would do.

NX650 cylinder head

The only way out of the morass of worn parts is to spend money.

Repair options first: Phil Joy at Joy Engineering, Ware, no longer repairs cylinder heads as he did for many years. He suggested one person who might do the job, but then pointed out the scored central journal is not critical. That reminded me of a stripdown Honda UK did of a 1989 Dominator used by Performance Bikes. Nine months from new the central journal was scored, but it went as well as ever and there was no other engine damage.

This gave me hope, but we still had a cylinder head crack to deal with. I soon gave up looking for a second hand head. They aren't available, for obvious reasons, and used engines on eBay or from breakers sell around the £650 mark. Not a risk worth taking. They could have cracked heads too. New heads are £800-£1000 as far as I can tell.

The Cylinder Head Shop told me they could pop out the valve seats, grind the crack out, fill it with weld, remachine and then refit the seats for about £350. As a bonus, they would thicken the plug area to take a long reach plug. I sent it off. Six weeks and many calls later they hadn't started the job, so I got it sent back. They apologised and offered to get it done pronto, but I had lost all faith. The highly respected SRM Engineering reckoned a repair was not worth doing. Can you see daylight through the crack? No. Was it running OK when you got it? Yes. Well, you could always try putting it together and running it as is, they suggested.

I got a third opinion from Ducati bevel expert Tony Brancato, who I went to see on a job for Bike. Tony said he could repair it, but it would never be as good as a new. That settled it. I decided to replace the worn cam parts and re-use the old head. The cracks happen, apparently, due to overheating. I will make sure this engine never runs low on oil again.

Dave Silver Spares quoted £203 for a new camshaft. I found a cam billed on eBay (usa-motorcycles-inc) as an XR650/89 Dominator fitment in the USA for about £75 and went for that. Dave Silver supplied two followers and two sub-rockers for another £70 or so.