Most riders like to get their hands dirty to some extent, spannering away on their bike to the best of their ability and only calling on their dealer’s service department when the job at hand is outside their realm of capability - a realm which (these days) invariably involves four-stroke engines.
Why? Well let’s not shy away from the fact that if you make a mistake assembling a four-stroke dirt bike motor, the consequences can be expensive. Stuff up the power valve on a smoker and it will run like a two-legged dog, but do no real damage. But mess up the cam timing on your four-stoke, and you will unleash merry hell inside your engine.
But remember… your bike’s cam timing is not rocket science. Check and double check that the rotating assembly is at Top Dead Centre (TDC) and that the timing marks are correctly positioned before you put the lid back on it and you can’t really have any problems to do with your cam timing. You can’t even mix up the inlet and exhaust cams - each won’t fit in the other’s cradle. Getting the maths wrong when shimming your valves can be frustrating, but similarly to the cam timing, as long as you double check that it’s correct before starting the bike, you can’t go wrong.
Most other engine assembly practices such as installing the piston, preparing the gasket faces and correctly torquing down the head are no different to rebuilding a two-stroke, so if you’re comfortable doing that, then you should be able to do this.
So to guide us through the process of freshening up your bike’s top-end we enlisted the help of ‘Doctor’ Ross McWatters at the Aussie KTM Race Team workshop to help us get to the bottom of things
Should I be scared?
‘I don’t think anyone should be scared of doing maintenance work on their four-stroke, but they should definitely have a respect for it,’ said Ross. But it’s that cam timing business that everyone’s afraid of. There’s a perception out there that it’s some kind of black art, and for the fear of bent valves in the event of a minor oversight, most owners keep the tappet cover firmly fixed on their four-bangers at all times.'
Take a look at the photo (at the beginning of this article) of the cams installed in the head. See those timing marks on the front of the cam gears? Whenever you’re working on an engine, as long as you keep it at top dead centre (on a KTM engine you can physically lock it there), all you need to do is ensure that these timing marks are aligned correctly on reassembly (as pictured) and you quite simply can’t go wrong with cam timing. If you do this, you are ensuring that the position of the cams relative to the crank (and hence the valves relative to the piston) is correct, and that’s all there is to it. Myth busted.
DIY cost vs dealer cost?
It’s worth bearing in mind that there’s some considerable cost savings to be made by doing the work yourself. Costs would of course vary from dealer to dealer, but a main dealer would probably charge you 3-4hrs labour to do a piston and rings in a four-stroke, or about £250, plus parts. To check your valve clearance, a shop will take around half an hour (and charge you about £30), and if they need shimming you’re up for about an hour and a half’s labour (roughly £120) plus shims.
Obviously the cost of the parts remains the same whether you do the work yourself or get the bike shop to do it, so these approximate costings represent money in your skyrocket every time you crack out the spanners yourself. In terms of specialist tools you’ll need a ring compressor (these are available relatively cheaply from any good automotive toolshop), a telescopic magnet for safely retrieving valve shims and other small parts, and some metric feeler gauges for checking and shimming your valves.
How often should you do it?
Of course this depends upon the bike and what type of rider you are. But if you’re a hard rider, racing in a championship then a top-end rebuild should be carried out about every 25hrs, or roughly halfway through a season, assuming you use the same bike for practising on.
Regular racers should be able to go all season long on the same piston and rings, but it’s worth considering a top-end rebuild during the off-season – especially if you’re planning on using the same bike for racing on the following year. If you’re just trail riding, then you can expect to get a good couple of years out of a piston and rings, by which time it would be worth checking the camchain and valves for wear and tear. Valve clearances however should be done much more frequently - every few months or so, depending on use.