uwenr Posted December 29, 2008 Report Share Posted December 29, 2008 (edited) This is the second time I've broke the bottom off the right piston. Both times it broke at the bottom corner of the windows then the pieces punch through the cases. Its the 7 mil in the signature the only thing not listed is it blaster pistons. Any ideas would help Edited December 29, 2008 by uwenr Quote Link to comment Share on other sites More sharing options...
alcoholbanshee Posted December 29, 2008 Report Share Posted December 29, 2008 This is the second time I've broke the bottom off the right piston. Both times it broke at the bottom corner of the windows then the pieces punch through the cases. Its the 7 mil in the signature the only thing not listed is it blaster pistons. Any ideas would help Are you opening up the windows on the piston............I have seen a lot of times where people port match them and this happens. It weakens the piston greatly, not worth the smalll power gain IMO. Another cause would be the piston rocking in the bore because of the bore being too large. And last but not least, do you warm your engine up or just start it and give it hell? You should always warm the engine or it will cause this problem. Quote Link to comment Share on other sites More sharing options...
uwenr Posted December 29, 2008 Author Report Share Posted December 29, 2008 Are you opening up the windows on the piston............I have seen a lot of times where people port match them and this happens. It weakens the piston greatly, not worth the smalll power gain IMO. Another cause would be the piston rocking in the bore because of the bore being too large. And last but not least, do you warm your engine up or just start it and give it hell? You should always warm the engine or it will cause this problem. The windows are opened up but not where they are breaking, the bottom is the stock window for a blaster. This time i was warming it up in the garage to do a oil change. I havent check the sleeve yet. Quote Link to comment Share on other sites More sharing options...
dajogejr Posted December 29, 2008 Report Share Posted December 29, 2008 The windows are opened up but not where they are breaking, the bottom is the stock window for a blaster. This time i was warming it up in the garage to do a oil change. I havent check the sleeve yet. Check the piston to wall clearance, and leave the window alone. I too have seen the skirts crack from opening the window. I understand it's not opened at the location on the piston that is breaking, but by weakening the skirt as a whole....it could very well be the problem. Quote Link to comment Share on other sites More sharing options...
desertdeweller Posted December 29, 2008 Report Share Posted December 29, 2008 I wonder if the cylinder port work could be causing it? Quote Link to comment Share on other sites More sharing options...
deckheight Posted December 30, 2008 Report Share Posted December 30, 2008 There is not a hell of a lot of material left there! Besides whats already been mentioned, if you have any type of detonation going on in that particular cylinder (even just a little) it's going to push the piston around pretty good and the vibrations from the deto itself and the piston banging around are not going to help your cause. You notice anything that might lead you to believe that the problem pistons combustion chamber is running hotter than the other slug? Piston wash, carbon color, texture, etc. Short of that, stop massaging the windows, make sure your ports are well chamfered and maybe try rounding off any sharp edges on the window itself to help relieve any metal stress??? Gotta be a reason just the one side is doing it... Maybe??? Quote Link to comment Share on other sites More sharing options...
dajogejr Posted December 30, 2008 Report Share Posted December 30, 2008 Normally deto will show on the crown first. How does the crown of the piston look? Did it back out any spark plugs? Quote Link to comment Share on other sites More sharing options...
uwenr Posted December 30, 2008 Author Report Share Posted December 30, 2008 Normally deto will show on the crown first. How does the crown of the piston look? Did it back out any spark plugs? Top of the piston looks fine I will get a pic when I get home. Never backed out any plugs, mic'ed the sleeve and its loose like .006 the new piston rocks in it. Going to get it bored today. I know im pushing the limits of this motor, but I like the fact it does good against the cubs. I guess I need to check more often to see if the skirts are colapsing. Are you guys not cutting windows in your pistons at all? Here is a pic of the new piston Quote Link to comment Share on other sites More sharing options...
dajogejr Posted December 30, 2008 Report Share Posted December 30, 2008 Nope...drop them in. I've never seen proof that windowing a piston does anything notable on the track or performance otherwise. I have seen a few broken skirts. I would drop them in. Don't touch a thing. A loose bore in addition to a weakened skirt could very well be an issue... Process of elimination. Quote Link to comment Share on other sites More sharing options...
FASTOYS Posted December 30, 2008 Report Share Posted December 30, 2008 IMO, it has all been addressed; Detonation, too loose of cyl. bore, cutting/weakining the windows, etc.. IF you have .006 cyl/to piston clearance i would think we would see ALOT more blowby past the rings then we see in the pic.. ALso it would sound like a rod knocking when you ran it (im exagerrating but it would be noticebly louder). Check your piston to head squish and make sure it is cut big enough for your BIG bore pistons. Example: 68 mm piston needs head cut out too 68.15 or so, so the piston has no chance of hitting the edge of chamber. I would not reccomend cutting your pistons Especially if it has already happened twice and breaks your cases. I dont think beating a cub is worth that $$$$$$$$$. Plus im sure if your beating a cub now you will still beat it without cutting the windows as i can promise you that is not what is putting you in the front but it sure is slowing you up when you have to buy new cases and tear it all down! :sad: Quote Link to comment Share on other sites More sharing options...
uwenr Posted December 30, 2008 Author Report Share Posted December 30, 2008 The squish is good Im running a stock head without any spacers, the piston goes up into the head. Havent heard anything but with the drag pipes with stingers you can't hear much. Havent had to buy cases yet just welded the holes and trenched again. As far as I know every builder here cuts windows. I agree the windows dont help the fact im breaking the piston but I dont think its the culprit, my builder has built 4 of the stock cylinder 7 mils and not 1 has done this. Not trying to argue with anyone just looking for ideas. What about over revving? The last time this bike ran I was at the dunes running the hill and couldnt keep the back tires on the groud through the whoops. Quote Link to comment Share on other sites More sharing options...
shanYE west Posted December 30, 2008 Report Share Posted December 30, 2008 I'm going to add another vote to window mods on the piston.. I've used a set of moded pistons one time a while back.. guess what happend to them? Quote Link to comment Share on other sites More sharing options...
lunatic Posted December 30, 2008 Report Share Posted December 30, 2008 I have never ran modded pistons but i tore one of my pistons(513 series) out that had about .011" clearance(skirt collasped) and it never broke a skirt. Im pretty sure if it was(modded) it wouldnt be pretty.lol I guess if you replace them at about .0045" they might live but thats a lot of maintenance to worry about. Quote Link to comment Share on other sites More sharing options...
uwenr Posted December 30, 2008 Author Report Share Posted December 30, 2008 I'm going to add another vote to window mods on the piston.. I've used a set of moded pistons one time a while back.. guess what happend to them? It ran good? Quote Link to comment Share on other sites More sharing options...
dajogejr Posted December 30, 2008 Report Share Posted December 30, 2008 It ran good? He has a custom ash tray... LOL. Quote Link to comment Share on other sites More sharing options...
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