Dynoray That's what I said and think. It hand a crack first, than a hole. That's why you see oil stains on the bottom of the piston. If it blows thru it also suxs at the same time, no Ifs or buts about it. I still think this is a jetting issue. I looked at the head again and I think it was jetted lean on the mid.

later

 

Hey Doctorturbo, I don't think this Rap is turbo charged, therfore there is negative pressure in the cylinder during most of the intake stroke.
CCing is a very simply task.
Easiest way, if you don't have plates already made up to CC head and cyl separately, is to assemble the engine.
Tilt engine so plug hole is the highest point.
Bring piston to TDC
Fill cylinder to bottom of plug hole with oil from a burret (if not available get a 100cc syringe)
It should be in the 68cc range
Take the Rad of bore 102/2=51
51x51x3.14x84=686.04 (swept vol, but you knew that)
686.04(Swept vol)+68(Comp vol)=754.04 (total vol)
754.04(total vol)/ 68 (comp vol)=11.088:1 (Compression ratio)
That easy.

 

That sounds easy enough. What type oil? How do you get it out after you're done? Will it seep past the rings or what? TIA

 

Just use what ever oil you use in the bike.
Put a piece of fuel line on the end of the syringe and suck the oil out.
Make sure you spin the engine over a few time (with a rag over the head) before you put the plug back in.
Hope this helps
Ray

 

Thanks.

This has to be a world record or somthing. Over 100 posts to a single topic and not one bash or argument, just helpful opinions and IMO some pretty darn good info from many posters of this forum. I do believe this is what Dbeck had in mind once upon a time. [img]i/expressions/face-icon-small-smile.gif[/img]

 

Dynoray, the combustion chamber is under a positive pressure 50% of the time. The only time it's not, is when you have either in intake or exhaust valve open; and even then it's close to neutral, depending on the throttle setting.

If you have any hole bigger than the hole in the piston, it will draw from the least resistance.
If you are at 1/2 throtle and once the intake valve opens up and the piston drops away, the hole in the carb will have less resistance than the hole in the piston. It will suck from the carb, not the crankcase; or should I say, not very much from the crankcase.

If you are wide open, you have no vacuum and there will be no draw on the tiny hole in the piston.
Once you get to compression the pressure wants to go south to the crankcase.
Bang, the piston lights off and again, the pressure wants to go to the crankcase.
Then the exhaust opens up and again the pressure is almost neutral, so very little contamination again.
Then the whole thing starts over again.

The worst time will be at idle, and even then not much will make it's way into the combustion chamber, depending on the hole size in the piston.

I've seen pistons with no and very few rings on the lands in running engines. They don't smoke all that much, not like one would think anyway. But man do they smoke and puke out the breather!!!!!!

wshrdskin my ISP is not answering my email so I will just PM you the info you wanted.

 

Can we argue now?[img]i/expressions/face-icon-small-smile.gif[/img]

 

Your forgetting piston displacment in the crank chamber . as far as close to neutral ? check for vacuum .
I Believe the piston is flexing under combustion downward> = leaking and then there is momentary pressure balance then it seals up on exhaust stroke .
Works much like a reed valve but then work hardens until you get oil exchange .
Depends on when you catch it . two of the pistons Ive looked at the 1st had a small amount of oil returning into the chamber the 2nd was a lot more but the second was riden after the smoke was noticed for quite a while until it got to smoking pretty bad .

Just think how boring this would be if you would have bought a ROSS piston

 

AMEN to that!!!!!!!!
It's a lot easier to come to the forums when you can get reliable information and not a bunch of bashing. Good job guys!!!!!!!!
[img]i/expressions/face-icon-small-cool.gif[/img]
I'd like to see you put another JE in and fix the timing issue and see if timing is the real problem or if it truley is the piston.

 

""""""""My Builder is putting a 12:1 JE is my 686. Should i tell him to go 11:1 before it is too late? I'm getting scared now. I ride everything from Dunes to trails and, i just wanted more power to keep up w/ my buddies Bansheees. I'm nothing close to a mechanic and, these problems explained above would prolly cost me 1k per time i needed the piston replaced by a Local Stealer.

So what is the true recomended Octane to run in a 11:1 and, for 12:1 pistons? IF it is 93 for 11:1 will 100 octane hurt anything? Or, Vise versa.

While on the subject of higher compression = how are starters holding up for you guys? Is 12:1 gonna make my starter crap out? I really can't afford nor, repair these types of problems. I'm starting to wish i'd have stayed piped! OUCH!

Please advise, i got "Butterfly's" now!"""""""""""

Any ideas for me? I need to decide by today weather to go 11:1 or, 12:1 JE (they are putting it together today). Biggest ?'s are will i go throught starters w/ the higher comp and, can i run the gas i can get at a Regular Gas Station 100-110 octane and, be ok?

We have nothing like the California, AZ, ect Guys in the way of performance shops or, special places to get gas and high performace work. So this Decision is Crucial while it is thousands of miles away!