exhaust port issues

[hontrx265r]

So I pulled my cylinder off to see how things are going inside. It has a good few hrs on it and has actually run pretty well. I noticed exhaust gas showing signs all over the lower end and transfers on the underside. Started to talk to a few people and did some rough measuring. What I have come up with is I believe the piston cutaway on the bottom of the piston is rising above the aux. Exhaust port at t d.c. and allowing exhaust gas to re enter the crankcase. Thoughts and opinions? Im going to contact eddie tomorrow if possible and see what he says but I see this as an issue. I did a rough illustration of what I believe is happening. I cant get it to load right side up the boot shape is the piston pin area and the line tagged problem is the piston cutaway.

[Hawaiiysr]

I believe this is called port linking. Neil has spoke of this many times. Its an issue with wiesco pistons. They claim there is no issue but as you found there clearly is.

Neil and I'm sure many others will have a word to say about this. In a nut shell, you are correct with your findings.

[hontrx265r]

I think it may be a little different the. What your thinking of though. This is the new style piston it is not linking between the ports. The aux port is being opened when the piston is at t.d.c. allowing the gas to enter beneath the piston.

[udontknowme]

if i understand you correct ,the side of the piston cutaway exposes the bottom corner of the aux exh window

since the cylinder is off just put the piston in the bore at the normal tdc location and look underneath and you can get a better idea how big the hole is. no need to draw it on paper. if its a small hole like your picture suggests then i wouldnt even worry about it.  if it was a large hole then you would probly want to fix it but that would require stripping the plating and weld then replate. ideally you do want the piston to keep the exh sealed off but if its a pretty small hole then it likely will be more grief to fix than its worth

theres a number of reasons why the hole would be there. if thats a special made piston then maybe they didnt take enough time to design it correctly. if its a piston that was originally used in a engine with a bridged exh, the side skirting is often times cut higher which can lead to shortcircuit holes when theyre used in engines with aux windows

[hontrx265r]

It was all straight from eddie its a 310. The hole would be very small just as illustrated, however it coated my crankcase and underside of cylinder pretty heavily. Leading me to believe its a problem.

[udontknowme]

coated it with what ?   that small of hole will have little negative affect. unless the hole is much larger than your pic suggests but from what i can see it looks very small.  if your getting black residue down through the transfers into the case and even into the reed cage area then most likely its exh blowing back down the transfers and boost port.

[hontrx265r]

I am getting the exhaust residue and why would it do that?

[udontknowme]

you might take a few pics so we can get a better idea where this residue is and what it looks like.  on engines that arent setup well its not uncommon to see blackened transfer tunells and even black down in the case and reed cage area

[hontrx265r]

It is in exactly that in the transfers on the rod, under the piston. Etc. Black exhaust residue. The motor actually runs well, but it looked so wrong to me as I have not seen that with any of my other motors.

[udontknowme]

if its black in the transfers you can be sure the exh is blowing back in but the hole under the piston isnt causing that. black on the underside of the piston is usually from the heat generated on the crown and the oil chars onto the underside. i dont think the hole would have much to do with that either. ive had piston undersides turn completely black and there was no shortcircuit hole.

[hontrx265r]

Its not black from heat though its exhast it wipes right off. What would cause so much exhaust blowing back though

[fearlessfred]

Is this a spacer plate setup and could the spacing be incorrect

[udontknowme]

what color is your premix ?  mine is very dark in color. red oil mixed with blue gas makes a dark purple color. possibly thats what your seeing. heres a piston with black underside. if yours looks similar then its not from them holes. told you already its because the crown is getting hot enough to char the oil. other pic you can see in the red circle is black because exh is pushing back down there and yes it wipes off easy, again it has nothing to do with holes. from the pic you showed, them holes are small and certainly not turning the whole crankcase black. now if your drawing is wrong and the holes are much larger than what your picture shows then maybe its possible the holes could be part of your problem. but still, at tdc it seems highly unlikely any exh is entering through them holes and going into the transfers. that usually happens on the down stroke as the piston uncovers the trans windows.

[hontrx265r]

It is like the first pic. Like I said its exhaust not heat. I know what a hot piston looks like. My drawing while not accurate is damn close. the holes are no bigger then my drawing. So if we are concluding that its not from my assumption. Again i ask what causes so much exhaust to blow back in. Too much back pressure? No spacer plate, short rod, stock stroke 310

[udontknowme]

the reason for the black is because when the transfers open, the cylinder still at a higher pressure than the crankcase and cylinder is still full of hot exh to make matters worse. so when you have a high pressure area and a low pressure area what happens. the high pressure tries to go to the low pressure area. and thus you get exh going down in the crankcase. in a nut shell, your blowdown (amount of exh area above the transfers) is likely not sufficient