neil is these 2 arrows pointing at the areas you guys are talking about ? the wristpin passing over the aux window and creating a small hole isnt that uncommon. hell even the newest generation ktm are like that. theres no way around it other than a narrower aux window or a plug in the wristpin. narrower windows arent good for power and a plug is a bad idea on a mass produced bike.
on that same note, any machined out areas on the side of the piston for lightening purposes, should be done upwards and behind the wristpin, like in the pic you see a hole above and behind the wristpin. other wise it creates a much larger void, like the esr piston. as for any small hole down in the bottom corner, just drop the front corner of the piston down and the hole is eliminated, but from the pic it appears only about a .020" gap, which i find it hard to believe its causing much problem. but all eddie had to do was look at any modern ktm piston to see what needs to be done. :friendly_wink:
Do you by chance, have an old used piston laying around like the new KTM in the pic ?, if so could you part with it.
Would like to do some measuring on one like that new one.
On the linking deal, in the second pic, where to two color arrow facing the main exhaust port is located. That whole area on the side of the piston in the drawing has been machined out, so to understand what is happening, take a piece of clear plastic & trace over the pattern on the side of the piston, cut it out & then drag it downward just like as if it was the piston on the downward stroke until it reaches the main transfer. Once you have the cut out at the main transfer, if you drag it any lower you can see the link between the aux exhaust port window & the main transfer. When those ports link on the downward stroke, fresh crankcase charge is being pushed out of the main, around the side of the piston & on out through the aux exhaust port in to the pipe.
On the upward stroke of the piston, the same linking is happening, but the returning wave of the pipe is pushing the spent exhaust back through the aux port & down into the transfer.
The biggest problem, is the part of the new fresh charge from the crankcase, being blown out into the exhaust early, which lowers the crankcase pressure when the transfers finely do open.
One other thing might be going on during this whole ordeal, if for some reason the exhaust pressure in the pipe was high, say like maybe the core in the silencer was broken loose & partly blocking the exit tip of the silencer. Sort of like the tater in the tail pipe or cat converter plugged.
If it was blow down creating the problem, then how much more than 32* of blow down would need to be added, 34~36*.
I think the fact, that theres fresh charge being blown out earlier than it should be through the linking process & then the fresh charge being lower pressure when the transfers finely do open, the charge flow up through the transfers, the duration not as long as it normally would be if it were not for the linking, so in turn you have a void to allow the backwards flow of the exhaust down into the transfer ports.
It might be that a pipe with a strong suction wave during that part of the stroke could help some of the problem, but when the pistons on that upward stroke, that returning wave is going to be doing its thing.
Neil
