The engine part 3: rebuild
On the premise that your engine has been gone through with the proverbial fine-tooth comb, you’ll be wanting to begin rebuilding but before you start there’s a few things that need to be addressed.
The outer engine cases will be either bare alloy or painted and in either instance chances are the finish will need to be redone. The apparently bare alloy used on many bikes was, in fact, covered in a transparent lacquer. There are similar modern materials but they need to be applied with a great deal of preparation and care.
The alloy needs to totally free of any polishing residue, fingerprints, oil etc. Aim to use a self-etching clear lacquer and spray the cases when the weather or your spraying area are warm. The results should be okay but it’s worth knowing that the stone chips etc. will allow moisture under the clear finish and, potentially, ruin it. For this reason many VJMC stalwarts choose to leave the alloy bare and protect it with a decent wax polish.
If the outer cases and possibly the main engine case themselves are painted you’ll need to find a paint that is both heat resistant and petrol tolerant. Little looks worse than a newly refinished classic sporting engine paint that’s discoloured or peeling off. If you’ve had the main engine cases blast cleaned you’ll need to clean out every single threaded hole and dowel location. The best method is to blow each hole with an airline (wearing safety glasses), wash the holes with brake fluid in an aerosol can and run a tap down each thread then blow clean once more. Cleaned alloy cases oxidise rapidly so give them a good coat of something like ACF-50 to protect them.
As we are looking at engines on a generic basis it’s not viable to cover every aspect of an engine rebuild but there are some general principles that should be followed regardless of type, make, size etc. Replace every seal as a matter of course; one reused seal that fails may require a fundamental rebuild. The same goes for gaskets and talking of those apparently low tech pieces of thick paper, never use a cheap one unless the originals are totally unavailable.
The OEM gasket is always the one to have. Pattern gaskets can be okay but some are badly made while others simply do not fit. Before fitting components or sub-assemblies back together, offer up the gasket and double check it actually fits. Most Japanese bike engine cases separate on the horizontal axis meaning many bearings are positively located via pegs or roll pins.
When installing cranks into bare cases it’s vital that the bearing locates accurately onto the pegs or pins. Similarly some bearings are located via semi-circular retainers; make sure they are all present and located securely. Mating surfaces need to be totally clean. If a stud has pulled up slightly proud of the surface this will most likely have come about by previous over tightening. The stud should be removed and the case very carefully redressed back flat.
Run a tap down the hole and ensure the thread is viable; if not it will need to be recut via a Helicoil or similar. The mating surfaces of the crankcases are normally sealed via purpose designed sealer otherwise known as a gasketing agent. Unless your manual tells you otherwise never use a silicone-based sealer between crankcase halves. Excess silicone can break off and block vital oilways or the oil pump pickup, wrecking the engine in seconds. The correct material is often referred to as Yamabond or Hondabond.
When the various shafts have been installed the cases should go almost together via hand pressure and a little wiggling; if they don’t something is out of place – don’t force it. Once sorted begin to add the bolts that hold the engine together and just pull them down gently. Your manual should explain the order in which the bolts are fitted. Work systematically tightening the bolts as per the specs. When the engine is together ensure the crankshaft is free to turn; if it’s not give it a firm tap with a copper mallet to settle it in the engine cases. Assuming the crank now revolves freely you’re good to proceed.