The engine part 5: rebuild
With the bottom half of the motor assembled and validated in terms of its freeness to turn over and select gears we’re now going to install what is colloquially termed ‘the top end’. If the power unit is a four-stroke chances are you’ll have a cam chain around the crank sprocket; this will need to be pulled up the cam chain tunnel of the cylinders or block and secured in some way to prevent it falling back inside the lower half of the engine. A piece of wire is often used for this and fixed to either a convenient part of the workshop or somehow otherwise retained. The trick here is to remember you’ll need freedom to manoeuvre the top half of the engine onto the pistons and then downwards so don’t paint yourself into a corner. Two-strokes make life easier… but only to a point.
Always fit new gaskets, it’s a basic rule of engine building that will stand you in good stead. And it’s a rule that pays dividends at this particular stage. No one wants a leaking base gasket once the engine is up and running or you’ll be lifting the motor out of your newly restored frame. Buy genuine gaskets from a main dealer if possible; they may be more expensive than pattern ones but are guaranteed to both fit and work properly. If you have to use aftermarket gaskets ensure they’re decent quality.
With a cylinder block running three or more pistons to contend with it makes sense to invest in some form of piston ring compressor. As the name suggests, these devices hold the rings into their grooves (or lands) so that you can fit the cylinder(s) over them. Practicing on a free piston is a good way of grasping how to use the ring compressor but know that you’ll always need loads of clean oil on the piston and rings along with the bore of the cylinders. Four strokes generally run three piston rings and the convention of their positioning is normally with their ends 120 degrees to each other; this ensures the maximum levels of compression with the minimum of blow-by or leakage. However, always check with a manual or accurate source of data in case the manufacturer has a different set up.
We’ll assume it’s a four pot motor we’re working with and move on. With the engine firmly seated on a bench, two oiled pistons are pulled to top dead centre (TDC) and the rings and pistons carefully introduced to the bores. With a little wiggling and cajoling it should be possible to install the piston/rings and then gradually work off the ring compressors. An extras pair of hands makes this job so much easier; one of you can work the pistons home while the other manipulates the block/cylinders.
With a pair of pistons now resident it’s time to turn the crank over bringing the other pair of pistons to TDC or close enough that you can gain sufficient access. Now repeat the process but without evicting the first pair you just installed. This entire process will either work like a dream for the first time you do it or will turn into a nightmare… this is the nature of the beast unfortunately.
Sometimes it’s incredibly difficult to fit the pistons in from the bottom end of the bore; short stroke motors can make this job almost impossible. In these instances the pistons are not fitted to the connecting rods but are pre-ringed. With a circlip installed to the least accessible side of the pistons they are fed into the top of the bore (again using ring compressors and copious quantities of oil) then carefully tapped downwards. Then the gudgeon pin is installed along with the second, new, circlip. If all this sounds like a right faff that’s because it is… once again, no one said it was easy!
Air-cooled two-strokes, singles and twins in particular, are often much easier to sort out. The key point here is to ensure the rings are correctly positioned with their ends either side of the ring pegs. If you don’t two things are almost guaranteed to happen; firstly you’ll get the cylinder jammed on the piston and secondly you’ll break at least one piston ring.
After all this you’ll probably want to enjoy the results of your handiwork and rightly so but just be careful. There’s normally no reason why you shouldn’t turn the engine over but there are a few caveats. Make sure the cam chain won’t get snarled up around the crank and find a viable way of holding down the cylinders… you really don’t want the pistons making a bid for freedom right now! The VJMC – run by motorcyclists for motorcyclists. For membership enquiries only: Tel: 01454 501310 Email: membership_vjmc@yahoo.co.uk Address: PO Box 1949, Yate, Bristol, BS37 0BX Office hours: Monday-Thursday,10am- 4pm (please leave a message)