The engine part 6: rebuild

Two-stroke owners rejoice as you only have to fit a new head gasket and the cylinder head gasket itself. Your four-stroke cousins will have a lot more to do when they decide to get on with the rebuild, too.

With air-cooled twostrokes there isn’t really that much to a head gasket, it’s just a sheet of copper with a series of holes in it. But if your beloved stinkwheels is a water-cooled one then you have a quandary on your hands. Do you use a pattern gasket or do you fork out for a genuine OEM example? Of course we’re making the assumption that the manufacturer still makes the head gaskets… which they may not!

The general consensus is that OEM is the way to go every time unless the pattern gaskets come with a proven record. Little is more irksome than finding that the cooling system is emptying itself into the cylinder(s)!

Of course the same scenarios can be applied to four-strokes, pattern versus OEM, and the same decisions need to be made. However, that said, it is the two-strokes that seem to be most disadvantaged by pattern head gaskets. So back to the strokers; gasket and head in place, add fittings, use a torque wrench, tighten in the specified sequence (if given) and it’s job done or close to it. Should there be no given sequence for pulling the head bolts down, opt for diagonals or as near as possible.

Back to the four-stroke head – the gasket can go on top of the cylinder block taking note of any specific O-rings that may also be required. Workshop or factory manuals are the best sources of knowledge or someone who knows these engines inside out…which means the bloke down the pub should not be consulted. Following the manuals you’ll see that around this point the cam(s) is(are) added and this is where you need to have de facto data readily to hand.

There’s a specific methodology to this particular task and it involves having the crank shaft at a predetermined position which is used as a datum point for the positioning of the camshaft and its attendant sprocket. Essentially what you are doing aligning the camshaft with the crank shaft via pre-set markings while attaching the cam chain to the cam sprocket; of course you did remember to add the cam chain to the crank shaft previously didn’t you? We’ll assume you did and hastily move on.

With the cam(s) at the appropriate position(s) the cam chain is ‘draped’ across one or two cam sprockets according to the engine type. The manuals will give you precise instructions or data regarding how many links/plates/pins/rollers need to be between the uppermost teeth of a double overhead cam engine.

With a single cam you are excused sorting this one out! If the book says x links/plates/pins/rollers then that is the number; x+1 or x-1 will mean potentially instant carnage when you fire the bike up so get this one wrong at your peril! Most manuals advise that the job is done with the cam chain adjuster set to its slackest which makes sense and should be obvious but sometimes isn’t.

One note on cam chain adjusters; the Japanese manufacturers have had a mixed track record on these devices and some of the automatic versions are less than successful. Before refitting the old one or replacement version thereof check out owners groups and forums regarding what’s recommended by owners for your