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Discussion Starter #1
'88 indy 400, 8000 miles. I was running 7800 rpm across a lake, engine died. Towed it home, found a hole in the clutch side piston. Mag side piston still black, and mag side plug is rich brown.

So - 2 sets of questions. As far as fixing it, what needs to be done? Can I just replace the one piston and rings? Do I need to go up a size and rebore, same size and hone, or just throw in a new one? Do both pistons at once?

Next set of questions. I need to fix the cause. What is most likely? Cracked carb boot? Crank seal? Carb setting? I haven't changed the main jets, the needle has the clip in the middle setting, and both carbs have identical settings for the air screw.

Thanks for the help, again.

Tim
 

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I would say the cause is wear and tear. If the motor has 8000 original miles, it doesn't suprise me that you blew a hole in your piston. Maybe someone else can tell you more information with pictures about why it happened.
 

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you answered your question. Personally, 8000 miles, I would do a tear down, replace both pistons, hone the cylinders if they are within spec, and check the bearings while you are in there. Thats my opinion, since every time I do something to "get by" it never fails, should have done it all from the beginning. get a Clymer book, take a few hours, and go thru it. Sounds like a leaking pto seal, and wonder where those little pieces went? down in the crank.
 

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well seeeings how 2 years ago i blew one piston and i bored it becuase the cylnder walls were F*ed so i replaced all 3 pistons and then a year later the crank comes unglued id replace evreything and with 8000 miles id say it was time for it to blow a piston.. mine had 5000 but it was all to the handle bar but thats what id do
 

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you need to investigate the damage, is the cylinder scored, or is it ok? that could be the difference between boring out or just leaving it. The cylinder head could have gotten messed up, along with the crank if there was a lot of metal bouncing around.
depending on the damage, at the least you should do BOTH pistons with rings. If the damage is really severe, id say part out the sled and get something new.
 

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most likely it was a crank seal that was leaking or some kind of an air leak to burn a hole in the center of the piston but to be sure like the other guys said I would go thru the whole motor just to be safe
 

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Just buy a used 400/440/500 fugi...they're everywhere, are cheap, and use the same bottom end. Some of that metal that use to be a piston is now in your crank bearings. You'll need a few new bearings, 2 seals while you're at it, and at least one piston and gaskets. That will get expensive quick.
 

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Originally posted by XCRacer440
[br]I would say the cause is wear and tear. If the motor has 8000 original miles, it doesn't suprise me that you blew a hole in your piston. Maybe someone else can tell you more information with pictures about why it happened.
Wear and tear? Wear and tear doesn't explain a hole in the piston ever. You can run a motor 8,000 miles, and yeah, maybe the skirt gets clipped off or the rings develop low compression, but that never explains a hole in the piston! I would investigate this further, looking through the carbs and the inspecting the seals as you remove them.
 

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one big bummer is now theres no way to diagnos the problem. if you plan on keeping the sled awhile its best to do it right and replace alot of stuff. yes it gets espensive. anoth alternative is finding another motor but thats a risk too!
 

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http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ih=013&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=230077777271&rd=1,1

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ih=020&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=300070414517&rd=1,1
 

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I have a used 500 motor from a 1997 Indy...125 lbs. comp in both....ready to deal and wheel on this one. Pm me if interested.
 

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Originally posted by XCRacer440
[br]I would say the cause is wear and tear. If the motor has 8000 original miles, it doesn't suprise me that you blew a hole in your piston. Maybe someone else can tell you more information with pictures about why it happened.
I would definitely agree with Triple650Indy. I would consider low compression, a broken piston skirt, a dried out seal or something normal wear and tear. Breaking a crank, burning down a piston, or throwing a rod would all go under the category of catastrophic failure in my book. Check the carb, check the seal, the carb boot, fuel line, etc. While you are at it, check the parts on the other side as well. It is possible that the other side was ready to go as well. Good luck.
 

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Last time the carbs were cleaned? Look at your main jet,almost certian it will be plugged a bit. If they are not look for other air inlet points.
WOT , engine leans with plugged main , started to detonate, = hole in pisiton.

http://www.theultralightplace.com/pistons.htm

Taken from http://www.misterfixit.com/deton.htm

Check for a lean fuel mixture. Rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, carburetor gaskets or the induction plumbing downstream of a fuel injection throttle can all admit extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by dirty fuel injectors, carburetor jets clogged with fuel deposits or dirt, a restricted fuel filter or a weak fuel pump.
 

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i dont think it would be w4ear and tear. i know guys that run 10000 miles no prob on one motor. i would guess carbs/or gaskets
 

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Discussion Starter #15
Thanks for all the info so far.

Carbs are spotless, I've been through them several times already this winter. Carb boots were fine when taken off.

However, the one oddity on tear down was that the water pump belt was loose - loose enough to lift off of the pulley. Also, the piston near the pump (mag side) was nearly clear of carbon, whereas before it was fairly heavily blackened.

Also, the rubber boot from the airbox to the carb was loose on the pto side - not a good seal. I'm thinking that these two factors contributed to the failure. Water pump pumping only enough to partially cool the mag side cylinder, PTO side not getting enough, heat builds... cold night, dense air, lean mixture - detonation.

Plausible?

Tim
 

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plausible, i would say yes. i stuck a piston in my sled last year in a similar situation. was wot across the lake, very little snow(little cooling), and jetted at the limit. the piston was starting to dish out, so had it kept going, a hole would have developed. so in your case, i would say you are correct with this assumption, given all else is in proper working order and is jetted correctly. if the cylinder cleans up well enough, you could just hone it and put a piston in. only having to replace the base gasket as a minimum and the piston/rings. if you need a step by step walkthru of rebuilding the engine, let me know and i will type something up. please ask if you need any more info. -Polaris Doc
 
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