It did not go bang like that because it was cast Probably drop on the ground a few times before it was fitted {done that a few times
copy and paste
WHY DO TWO STROKE PISTONS FAIL ?
When a two stroke piston fails the rider or mechanic will usually look for a reason which excludes him or her from blame. This is human nature, but unless the real cause is identified, the failure will happen again (less likely second time around when all details and clearances have been checked and a different brand of piston fitted). If the piston is "Genuine" (i.e. thought to be made by the same company as made the bike (wrong)), then the rider will probably look for the cause of the problem. If it is an "aftermarket" piston, particularly an inexpensive one then the piston is most likely to be blamed.
The most common cause is seizure and this is dealt with most extensively by Eric Gorr in his book Motocross & Off-Road Performance Handbook. If you don’t have a copy, go out and buy one. Years ago, such a failure could be blamed on the piston maker. If the piston is made from the wrong alloy, or has the wrong taper or ovality (pistons are not actually round), seizure is inevitable. With modern manufacturing and measuring techniques, this is most unlikely to be the cause of seizure and isn’t even mentioned by Eric Gorr. Wear of the piston pin hole is often attributed to poor quality material, but is actually a lubrication problem. Modern cast pistons are made in batches of 500 - 1000 pieces, so a material quality issue would effect the whole batch and not just the occasional piston.
More rarely, but more significantly from the warranty point of view, structural failure of the piston occurs. The piston has cracked or the ring peg has come out or gone in. This is generally considered to be a manufacturing defect. Very rarely is this in fact the case. The idea that a poor quality alloy has been used just does not make sense either. The cost of the alloy used in a piston is approximately 30p (for a typical 125cc piston), so that using an inferior or inappropriate alloy would not significantly reduce the production cost.
It is important not to consider piston failure as simply a fault in the piston. "Big end" failure is not considered a fault of the con rod or the crankpin. It is a failure of the assembly and replaced as an assembly. In truth, a piston failure is, in most cases, a failure of the piston and cylinder assembly, but usually the piston alone will be blamed and replaced. After all it is the moving part and the one which has stopped moving !
The easiest failure to diagnose is the cracked crown. A crack running from piston pin hole to piston pin hole, across the crown. This is caused by detonation, usually brought about by a cylinder head problem causing water to leak into the cylinder. Obviously a thicker crown piston will withstand the effect for longer, but the fault really does lie elsewhere. If the cylinder has been re-plated of sleeved a number of times, it will be shorter and the compression ratio and squish clearance will need to be corrected to prevent the detonation.
The other type of crack usually occurs across the back of the piston and causes the bottom of the piston to break off. This is almost always caused by excessive clearance causing the piston to rattle in the bore until a crack is started. (TZ350 pistons had a life of 3 hours before cracks developed, less in a worn bore) Don’t forget that nicasil plating is only supported by the soft aluminium of the cylinder and can be packed back rather than worn, so that a bore which looks unworn can still be oversize. There is also the possibility that the piston has been damaged in transit. Top “works teams” X Ray pistons before fitting them ! Once again, not a manufacturing defect ! Forged pistons will withstand this abuse better than cast pistons, but the fall off of performance will be noticed long before failure by all but the most novice riders.
On the subject of forged versus cast pistons, it should be understood that there are pros and cons which are well understood by the Motorcycle manufacturers. Cost is not an issue since the difference at high volumes is negligible. No manufacturer would compromise the performance of a racing bike for a small cost reduction. A forged piston is stronger but cannot not be made from the optimum silicon content alloy and this will require a larger clearance which will always compromise its performance. A further advantage of the high silicon content cast piston is its strength at higher temperatures, particularly at the crown of the piston. This allows a higher compression to be run with a cast piston. Of the four Japanese manufacturers, only Kawasaki has used forged pistons in its two strokes and they abandoned the practice 25 years ago. The highest specific power output achieved by a modern mainstream two stroke is the Honda RS125 G.P. Racer. 50 + h.p. - Yes 400 h.p. per litre (more than an F1 car or a Moto GP bike) and this is using a cast piston (the KTM85 is also close to this performance with a cast piston). Why - because this is the only way to achieve the clearances needed to maximise the power output. A forged piston may perform better when working outside the service limits, but this should not be an attraction for a racer.
Now finally, the most perplexing cause of failure. Ring pegs. On the face of it, if a ring peg moves either in or out, it would be natural to assume that is has not been a tight enough fit. Whilst we have seen batches of pistons where the peg could be pushed in with a finger nail, we have also seen peg failures where the peg has simply been worn away by the piston ring or “worried” until it has loosened. Under these circumstances, we need to look elsewhere for the cause of the ring peg failure. Obviously the peg is being loosened by the sideways forces exerted by the piston ring. These are brought about primarily by the ring moving in and out of the exhaust port (and to a lesser extent the transfer ports) causing it to change shape. If the ports have been enlarged, this will put extra load on the ring peg and may cause failure. If the peg is in the centre of the piston, the loads on the peg will be balanced and less likely to dislodge it. Another cause of ring peg loosening is damage to the exhaust port bridge. If the bridge has not been relieved after re-plating, serious wear and damage will occur and the plating will fail. This puts loads on the ring and peg, causing the peg to loosen and fall out. When the peg is seen to have failed, this is wrongly assumed to be the cause of the failure and not the result.
In conclusion, if a piston fails it is important to establish the cause of failure before simply blaming the piston and fitting another one.
