Did A Loose Plug Shatter My Piston

[ROOFCOCK]

so i was running 260 mains stock25 pilots no lid fmf fatties ada racing cool head with 22cc domes (i think) reed spacers with boysen power reeds and stock timing

getting beat more than i care to talk about. the 260's were fouling plugs quick, so i jump to 290's 30 pilots 1 1/2 turns out on the airscrew and plus 4 on the timing.

it was running good and on a straight away in 5th gear with a nasty noise it stopped

got it to the truck and noticed a little oil at the base of a plug checked it and it turned right out (not tight enough i guess)

the end was flat the color was chocolate brown and i took it home. when i pulled of the pipe i poured out its contents which was piston fragments and wristpin needles

took off the head and the piston was shattered. finally to the question...did the loose plug lean out the cylinder and destroy the piston or was it something else.

the piston was a namura cast power piston which my bore guy did not like. are forged better than cast, and since its tore down will a port job give me the extra power i need to compete. thanks in advance...roof

[Indubitably]

Next time get forged pistons. Another thing what altitude and what temperature were you at, that may have been lean. The plugs will back out aftermarket heads sometimes get a plug lock from Trinity Racing best 20 i've speant in a long time. A port job will make it go faster. Make sure your compression is right too. do a compression check when you get it back together if it's over 160lbs run race gas.

[ROOFCOCK]

it was about 60 degrees around fredricksburg va between richmond and washington dc. no mountains but dont know altitude... how lean bigger jets? less air?thanks

[supergrzz]

260 to 290 is richer in fuel. That should have not helped the fouling.

As for the loose spark plug. That is an iffy question. I have had a plug

back out and it did not toast the piston. I have heard of other people say

that it has happened to them though. My thoughts..... Its broke now so fix it.

Install forged piston. They are better then cast. I have toasted 2 cast pistons

before.

 

The porting will be a big difference. Make sure you use a good shop for this.

There are different types of porting also. I mean different steps. Play, Drag ect.

 

For your first Jetting after porting I would let a shop get you close. That is a lot of

money to blow on a guess. After porting mine for Play port, I installed 330's.

My carbs were stock and not borded out.

 

 

Good luck.

 

Supergrzz

[DUNEDEMON]

A plug comming loose can cause an air leak, thus producing an extremly lean condition, we all know lean = BAD. I would think that is where your trouble started.

[99screaminshee]

it was prolly the junky pistons i had damm near the same thing happen to my shee piston take a big dump in my crank case get urself sum good forged wiseco pistons and clean out that case real good or u wont have ur new motor for long

the only thing namura pistons r good for is ashtrays they r junk run like hell for a lil while but then they make a mess

[boonman]

Did you replace the upper needle rod bearings when you did the top end last time? I am thinking that the upper rod bearing went, which let the piston crash into the dome, and shattered it. Cast pistons are not as strong as a forged piston. No matter what ANYONE tells you, it is physically impossible. That is an undeniable fact. Now, you were running 22cc domes. What kind of fuel? Was it low octane? Was it old? A problem with detonation will back the plugs out. Not tightening the plugs enough will let the plugs back out. Tightening them too much will cause them to strip the threads in the dome.... There is a fine line. It is about 1/4 turn past hand tight on a fresh plug. Much less on a used plug... Now, the loose plug could have cause it to go lean a little. I don't think it would suck air through the threads as much though.... You would lose compression from it for sure though. Now, you said you were fouling plugs with 260 mains? If that is the case, than why in the hell did you go to 290's? You should be at least a 300 main unless your at 4,000' elevation. Open box and pipes? That is sucking considerably mre air than stock. And 30 pilots!!! That's HUGE!!! Anyways, The end of the plug was flat. This is because it got smashed by the piston. Then, the piston broke apart from repeated hammering against the plug. Now, with all the debris you describe, ad the piston grenading, you need to split the cases and get the crank out of there. Have it checked, trued and welded. NEw bearings if at all feasible. Otherwise, you risk brenading the lower rod bearing and having the same problem all over again. I have had it happen. (the lower bearing after debris in the lower end) Pull the entire motor down. Next time, use Wiseco pistons. They are forged.... And replace the upper rod bearing any time you replace pistons. They are cheap, and how could you not?

[ROOFCOCK]

yes i replaced the bearings fresh 93 octane at 32:1 brand new set of plugs. when i say they were fouling i did not mean gas fouling but more like a sooty black residue. i am pulling the engine out of the frame today and will remove the cylinders to know the extent of the damage. trued and welded...any suggestions on who is best for this. the jetting was from a recomendation from someone on here it seemed to run better at this range and with the 30 it started and idled like never before. what would 20cc domes do for me. i mostly ride woods powerlines and that kind of thing with occasional wide open runs. i am just getting tired of getting beat by my buddies 400ex and running even with my other buddies shee. not to keep running on but would it be better to by a crank already trued and welded or is it cost prohibitive versus having it done to mine thanks for all you input

[99screaminshee]

U have a banshee with mods and UR still getting beat by 400s sumthin aint rite there i dont care if ur fouling plugs left and rite u should still smoke a 400 on a stock banshee thats runnin on 1 cylinder

[ROOFCOCK]

the 400 is hooked up with a big bore kit and other mods and he only beats me in the dirt...need shorter swingarm i guess. on asphalt i can pull him but its still close. all 400's aren't as slow as most banshee owners like to think

[supergrzz]

Throw a long rod kit in there, if you are getting the crank done. I wish I did that

before I had mine done. Now it is going to cost me a lot more. A little stroke would be nice, but sounds like you don't do too much racing mostly playing.

Just my 2cents

[909]

You need to follow the info Boonman said. You will be better off though finding out exactly what happened to the motor in the first place so you can be sure it never happens again. As for the future, I'd split the cases and either re-bearing/true/weld the existing crank or get a wiseco crank. These cranks have come to me out of true from the factory so I quit buying them welded. I would rather deal with the stocker so that I know its true before welding. Yamaha spac is .002, if its ANY beyond that it needs to be fixed before assembly. I have had them come in as far out as .014! Second, if you're running stock porting (which i'd recommend for now) the long rod will get you nothing but you need to run different pistons than stock. Figure it out before buying anything. Next, stick with a 22cc dome and run either premium unleaded or a 50/50 mix of race gas. The mix will keep cost down but make the fuels stable for a consistent mixture. You also didn't mention what oil you were mixing with. Some need to be at 32:1 (Yamalube) and others this is too high (927) From the looks of the plugs as you describe, you are too rich on oil which will also allow too little fuel past the jets causing a lean condition.

 

So here are my suggestions:

Completely tear down the engine, split the cases and rebuild the stock crank.

Fresh bore on the cylinders and new wiseco pistons.

New bearings and seals throughout (only Yamaha parts!)

Check dome condition of head and replace with 22cc if necessary.

Use good fuel and proper oil mixture, rejet according to mods and specs.

Proper break-in procedure, then blow away the 400s!

 

Good Luck!

[MotulMonsta]

Did you say the plug was flat? Like somthing hit it? I say you had an internal failure or swallowed a reed or chuck of metal from somthing that caused the piston to make contact. After that..game over.

[Cotton eyed Joe]

Cast pistons are not as strong as a forged piston. No matter what ANYONE tells you, it is physically impossible

 

Cast machines almost like its "fluffy" and not as tight grained as extruded or milled aluminum. I can blast through it, and its good for somethings, but I use it for jigs and fixtures only. I wouldn't think a cast piston would even be around on something like a banshee where you're turing close to 10,000 every now and again. Forgings are basically a solid billet that is forced into the shape by progressive dies. They also control the amount of heat generated and are more careful about going above and beyond the temperature where aluminum starts to lose its temper. Forgings are all about strength, cast is more about cost, and ease of manufacture.

[boonman]

the 400 is hooked up with a big bore kit and other mods and he only beats me in the dirt...need shorter swingarm i guess. on asphalt i can pull him but its still close. all 400's aren't as slow as most banshee owners like to think

Your correct. Not ALL EX's are as slow as people think. BUT, and there is a big BUT here. They are the exception, rather than the norm. I have been beaten by a couple EX's at Silver Lake dunes. They had no less than 10,000 smacks into their bikes to get them to where they were. My bike is damn near stock. SOme mods here and there, and I was wasting EX's, 440 bored, piped, ignition, cam and what not on them and I had a MotoFast 2 into 1 pipe and a sorry ass excuse for a top end...... it doesn't take much to beat a 400EX. A stock banshee should smoke an EX. No question. And Stan, you hit it on the head with the cast Vs. Forged. it's a cost thing...... No other excuse......