Replacing main bearings in a two stroke motor is a big job, entailing a complete strip down of both the top and the bottom end of the motor. In this article I would like to share the experience from my mechanic friend Andy that might save you from some frustration.
The main bearings are two large bearings, either side of the crankshaft, sitting in machined recesses in the crankcase. They are lubricated only by the oil/fuel mix that the motor breathes in through the carb, so their longevity depends on what is, or what isn’t reaching them through the carb.
In the normal run of events, main bearings should last through many top-end replacements, and if they are looked after can last, in some makes, almost indefinitely.
The bike in question was eighteen months old, with about 120 hours on the clock. It had spent most of its life in first and second gear in mountain goat terrain using all the torque that the motor could make. This is a demanding task for a bearing and in many ways much tougher than high speed riding. Higher revs can spread the torque and bearing loadings are therefore reduced.
The owner of the bike heard that he should run a 60:1 fuel-oil mix. Indeed, the bike’s manual says that this is completely possible. However, in this case the bearings soon expressed their displeasure by starting to rustle and hiss quite dramatically.
Not that this lesson was learned on the same bike. This is just common knowledge. But while we are on the subject of destruction, Andy advised me to mention some other sure-fire ways of sending your main bearings to the great scrap metal dealer.
If your air cleaner is not oiled properly, or does not fit properly, or is very dirty, dust will enter the crankcase and the bearings will be compromised.
As the lesson above this is just to mention that water ingress from enthusiastic washing or drowning the bike in a river can result in rust within the bearing which will trash it quickly.
The best story is the guys who dragged on old bike out of the shed. The bike hadn’t run for about three years and the carb was totally blocked by fuel residue. Optimistically, they filled the tank, turned on the fuel and kicked themselves into exhaustion.
Realizing that desperate times required desperate remedies they towed the bike behind a bakkie at high speed, until the oil-starved bearings broke up and locked the back wheel at 60km/h. The ensuing roasties on the rider, (that’s right – no helmet, short pants and slip-slops) were truly magnificent.
So to go back to the bike of lesson no1: Andy established that the mains were trashed, stripped the motor, and installed the first set of aftermarket bearings, along with aftermarket seals. He had used this particular aftermarket make with great success in the past and saw no reason to measure the bearings before installation.
With the bearings installed and the cases bolted together, he tightened up the fly wheel and drive pinion, and the motor promptly locked up solid. So he stripped the motor again and measured the bearings. The right hand side bearing was 1mm thinner than the stock bearing. This meant that the crankshaft was being biased across to the right to the extent that the flywheel was binding on the left side crankcase.
With the motor re-assembled with OE (original equipment) bearings but the aftermarket seals the bike ran beautifully. For 40 minutes. At that point the bike lost power and started pumping white smoke out of the exhaust. Obviously the right hand crankshaft seal had failed. Oil from the clutch side was being drawn into the motor.
On stripping the motor, they found that the lip on the aftermarket seal had detached for no particular reason other than that it was cheaply made junk. So, third strip-down and lesson learned: always use OE crankshaft seals.
For more solid advice on the day-to-day mechanics of a dirt-bike-rider check out the Garage Manual.
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