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XFR/Firecat/Sabercat Regular 128" XF / Firecat/Sabercat chassis

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Old 04-01-2011, 10:12 PM   #1
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blown piston w/ pics what could cause this?

My sled was running fine then all the sudden it seemed to drop to one cylinder and would not go over 6000rpm and started to run like crap. I figured I lost a piston...and I was right. Compression is 135/115 on clutch side. I pulled the y pipe off and this is what i found. The bottom of the piston looks like it has a piece broke off and the side looks scuffed and it doesnt look like the good piston. What could have caused this? I am thinking maybe the piston was hitting the powervalve.....There was also a lot of oil in the airbox boot right before the throttle bodies. What do you think I am looking at to get it running again? Just new rings and pistons? The motor turns over fine with no problems and starts fine and doesnt have any unusual noise but i am worried where that broken piece went and hope the crank and rods are fine. Any input would be appreciated.
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blown piston w/ pics what could cause this?-p4010616.jpg   blown piston w/ pics what could cause this?-p4010615.jpg  
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Old 04-01-2011, 10:17 PM   #2
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When the good piston is all the way at the top of the cylinder I cannot see the bottom of the piston or the connecting rod but on the bad piston i can feel it with my finger that it is scuffed on the top of the piston close to the manifold opening and scuffed a bit on the side as well..........looks like a chunk broke off the bottom of the piston and you can see the connecting rods.....just to verify what we are looking at here....4000 miles on sled.....always run 91 oct. thanks
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Old 04-01-2011, 10:36 PM   #3
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the cylinder is probably scuffed up as bad as the piston. You'll probably have to replace them both. The pieces of that piston are probably rattling around in the bottom end, putting all the business in there in danger of getting eaten by aluminum chunks. If it were me doing the work, I'd tear it down all the way and repair what was needed as I put it together. Chances are you are looking at just a piston and cylinder, but further teardown would tell you what you need to know.
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Old 04-02-2011, 10:16 AM   #4
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That engine needs to be torn down. You will probably find some damage inside the crank case halfs as well. No doubt it going to be messy. If you pull the cylinders, I think you will see a lot of pieces in the case, & probably some good marks in there too. The biggest concern is the bearings. If small pieces get into the bearings your looking at over $500 just in the lower half. I would pull it apart, clean out everything & see where your at. If you just rebuild the top & don't get the bottom cleaned out 100%, you will be doing it again
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Old 04-02-2011, 10:30 AM   #5
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This picture is the bottom half of a case after loosing the top land of one piston. This engine only ran for about 6-7 seconds after it broke. Gotta keep in mind that the bottom case is also the intake system. Anything in here that doesn't get into the bearings, is headed back to the cylinders at some point. Bad news in any direction.....
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Old 04-02-2011, 11:03 AM   #6
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Worst case scenario it knocked a hole in the bottom of the crankcase. Best it just needs to be honed. I have seen both with broken skirts. I have been lucky and have not seen it eat a crank. If it did knock a hole in the case there is a good chance that a tig welder will fix that. If it looks real bad in the bottom end you may want to check the squish and see if it bent a rod. A lot of times it will grind up the aluminum piston and just spit it out the exhaust. We had one bust a skirt this year and all we did was hone it and put it back together again. That was over 500 miles ago and it has been running fine ever since.
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Old 04-02-2011, 01:24 PM   #7
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Same thing happened to me this winter. 2003 F7 EFI at 3200miles. Blew a piston skirt and tore up everything. Check out my pic album to see the damage. New cylinder used or repaired 200-300$, complete top end kit is another $300. The debrise tore apart my reeds so that was another $250 because I got v force. I sat there in a parts washer cleaning my lower end and every bearing theroughly. I did not find any significant problems in the crank but if you do not clean it out perfectly it will cause serious damage again. Mine is all back together and prrrring perfectly again. Good luck!
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Old 04-02-2011, 06:51 PM   #8
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I would also determine why it went down. It looks like it got hot. With your listed mods, you are probably lean in the middle if you don't have a fuel adder installed. X2 on the teardown.
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Old 04-02-2011, 08:04 PM   #9
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i wonder if it would be cheaper just to drop in a used engine. I have done all the mods myself but have never separated a crank case and reassembled. I have no problem with redoing the top end but am unsure about the bottom end.
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Old 04-03-2011, 02:08 AM   #10
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the bottom is not that hard if you only need to clean it out and inspect. If you have bad bearings or want to replace them that is where the job becomes harder and maybe not left to a first timer. taking apart, cleaning and getting it sealed up again is not to hard so long as you pay attention to where things go as you pull it apart and if you understand how the coil side comes apart. Might just be me but it seems like people want way to much money for used, non rebuilt motors these days... IMO.
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