The engine part 4: rebuild

The grunt work (largely) is complete and now it’s time to get some finesse. Pistons and their rings. Measure, prep and don’t skimp!

So hopefully we now have a basic bottom half of an engine that has a revolving crankshaft and a gearbox with all the relevant cogs. Before getting too excited it’s a fine idea to check that the former can transmit its potential motive force to the latter’s output cog. If it doesn’t the engine will need to come apart again; it’s best to suss this out now rather than when the engine is in the frame! Temporarily attach the gear pedal to the gearshift and with the aid of a mate turn the engine over.

You’ll need to protect the con rods with some cardboard or similar but with a little care they’ll be fine. With the engine revolved in its normal direction of rotation carefully select each gear ratio and ensure that the output shaft revolves. Don’t expect the same number of turns from crank and sprocket as the engine turns. The gearbox alters the two relative speeds… that’s its job! Finally ensure that the neutral position is easily and repeatably selectable.

Well done you’ve just rebuilt the bottom half of your engine! Almost without exception Japanese bike engines are installed in the chassis as totally built and complete units. This is fine if you’re working on a production line with all the appropriate lifting gear but not necessarily ideal if you’re working off the floor of a draughty shed.

If you chose to drop the bottom half of your engine into the frame now to reduce the mass of the final lift you’ll need to be certain that it’s possible to fit the cylinder head in situ and access all of the requisite bolts. If it’s not feasible to work like this then the engine will have to be installed as a finished lump and lifted in with one or more mates to help.

Next up it’s time to fit the piston rings to the pistons which can be a little fraught first time around. The piston rings are precision made circles of springy ferrous alloys – hard wearing yet fragile at the same time. They are generally location specific and should always be installed as per the manufacturer’s instructions. Twostrokes normally run upper and lower rings and furthermore these have to be located such that their open ends sit either side of a ring peg in pistons grooves. Four-stroke rings generally come as a trio and are again location specific but don’t sit aside ring pegs.

In all instances the rings and pistons need to be well lubricated in engine oil before installation. Old hands (sic) seem to almost magically slide rings in place with their thumb nails. Novices and the less adept will be using piston ring expanders! If the rings refuse to sit in their grooves don’t force them in; you’ve probably got them mixed up. Four-stroke rings are normally fitted with their end gaps 120 degrees to the ring above and below.

If the bike is a four-stroke with a cam chain let’s hope you installed the chain on the crank previously and move on from there. The pistons now need to be installed onto the con rods with new bearings where appropriate. This can be something of a fiddly process but needs to be done carefully and methodically.

Noting which side of the engine you’ll be working from, select a piston and install a new circlip on the opposite making sure it’s properly installed in its register. From the ‘working’ side, slide in the gudgeon pin through the piston part way then guide it through the top of the con rod the bearing if fitted and through the other side of the piston until it makes contact with the previously installed circlip. This process is made infinitely easier if messier by lubricating everything first in clean engine oil.

The piston should now just rock when the piston is tilted. Now install the second circlip into its groove. The same process is repeated on the other piston(s). Although a relatively simple process it’s not without issues and concerns. Firstly never, ever, reuse a piston circlip; they may not appear worn or compromised but don’t take chances. They cost pennies but if they fall out can totally and utterly wreck an engine. Secondly, when installing circlips protect the motor by pushing clean, lint free rag into the mouth(s) of the crankcase. One circlip dropped into the crank can waste hours of time trying to remove it.

Now go and have a brew because next time we’ll be adding the cylinders which is an art form in its own right!

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