I tried searching this website for this question but couldn't find any.
So what exactly is piston slap? is it harmful for the engine? can it be avoided?
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Piston slap is the rocking of the piston in the cylinder (as opposed the desired movement of the piston up and down the cylinder). It happens when there is "too much" room for the piston in the cylinder and the skirt of the piston hits the cylinder wall. Like this:
It can be a design flaw, and it is also exacerbated by wear over time. Aside from rebuilding, there isn't much you can do about it.
Piston slap is a noise created by the piston to cylinder wall clearance being excessive, or the cylinder bore being out of round, so the piston skirt slaps in the bore as it travels up and down.. Normally this occurs on idle and overrun. Many two stroke single cylinder race engines suffer from this and will run just fine for ages with slap, but these are normally serviced & rebuilt regularly enough that failure is rare.
If pistons and bores are setup/machined to correct specs slap shouldn't occur. Cylinder noise like this should always be checked out as ignoring it could lead to piston skirt wear/failure over time.
So in answer to your question.. If piston slap is just ignored and left to slap away it can be very harmful to an engine, and yes it can be prevented/avoided. Or at the very least minimised.