TRX250R.ORG
Workshop => Engine and Bottom End => Topic started by: hontrx265r on December 07, 2014, 08:33:42 PM
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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. (http://forums.trx250r.org/vbulletin_imports/486_3902_2017-04-18_2771.jpg)
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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.
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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.
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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
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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.
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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.
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I am getting the exhaust residue and why would it do that?
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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
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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.
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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.
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Its not black from heat though its exhast it wipes right off. What would cause so much exhaust blowing back though
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Is this a spacer plate setup and could the spacing be incorrect
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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.
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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
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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
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Makes sense. I'll see what I can do about that tomorrow.
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No matter how you look at it, the aux exhaust port should not be exposed to the lower end .I have never had residew in the lower end in 27 years of riding the seems to me someone went a little too far with grinder.the angle those ports are at ,it wouldn't be hard to do.
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I agree with that fred. I will be contacting eddie to see what we can do about it. He resleeved this cylinder right after I got it because it had rotated.
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It seems to me that Eddie always does his best to make things right.please post on how it turns out.
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No matter how you look at it, the aux exhaust port should not be exposed to the lower end .I have never had residew in the lower end in 27 years of riding the seems to me someone went a little too far with grinder.the angle those ports are at ,it wouldn't be hard to do.
ideally you do want the exh sealed off. but if the hole is quit small then you dont have much to be concearned with. to fix it correctly will probly be far more grief than its even worth, unless esr will strip the plating, weld up the hole then replate all at eddies expense, but im thinking eddie will tell him the same thing i just did, it aint that big of a deal. either way, hes not losing a bunch of power nor is any kind of damage going to occur. i guess it all boils down to who is paying the bill to fix it. any black residue that may be in the tunells or lower end is probly from what i pointed out early, a lack of adeqaute blowdown area, not the hole under the piston. its a common problem on engine combinations that arent setup well. even on a well designed engine you may see blackened tunells if the engine is continiously operated below its intended rpm range
you can ask 1000 different people and get 1000 different opinions on the matter. i happen to know that a small hole makes little difference if its there or not. hell the pic i posted above has the same kind of hole from the factory. theres no sense in diving into the reasons why because there really is no good reason but i can say those particular engines were as reliable as anything else out there. i went to the trouble to weld the hole on one of my engines but the plating was already off and i was doing other work on it also. i certainly wouldnt go to all the trouble of tearing down a perfectly good engine just to weld that small hole. now if half the window was exposed that would be a different matter but that doesnt appear to be the situation here
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if you look at this piston it intentionally drops down in the front to prevent the aux exh window from being uncovered at tdc. so ya its best to keep them covered and i dont think anybody with 2t experience would argue otherwise. but on the other hand i dont think a small hole is worth the trouble of trying to fix.
and heres something else to think about. ya youll be keeping the window covered if you weld the corner but youll also be making the window smaller. so anything you might gain by closing the hole will probly be offset from the window being smaller. so theres a good chance youll go around in a circle and have gained nothing
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It is a sleeves cylinder so there is no plating or rewelding. Being that Eddie did the sleeve work, weather it be ok or not ok or reliable or whatever. I still want to contact him and address the issue. I do appreciate all the help though.
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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. (http://forums.trx250r.org/vbulletin_imports/486_3902_2017-04-18_2143.jpg)
You are correct in what you are thinking. I tried for quite a few years to get LA Sleeve & CT to change the piston design, so to stop the port linking, but it fell on deaf ears.
When I spoke to Eddie about the port linking issues, say a year after he came out with his own cylinder castings, he changed pistons & got his on design you see being used today.
The issue when your using the old design Pro-x piston, from the piston being machined to much on the sides to lighten them, lighter piston, more hp output. If it was just a slight opening or just opening up into the wrist pin hole, then the effect wouldnt be as bad, but the length of the machining on the pistons sides & the aux exhaust port window, the duration of the open area is to great during the cycle of the piston at that time in the stroke.
Do know though, the linking issues not as bad as it was a few years ago, because of the design of piston Eddie is offering today.
I'm thinking it was back in 98~99, maybe 2000, did a test on a 310 Pro-x cylinder. Welded up the aux exhaust port of the sleeve & then going back & opening up a smaller window so that there would be no way for the port to link.
That little 310 at a national TT event, was sitting in third place behind Tim Farr & Shane Hitt until the small o ring on the head dome melted & blew out the coolant.
It was part my fault, because I took time & watched the rider running the R which had a 450R as well & was spending all his attention to the 450. If the 250R had been attended to like normal, the radiator would have been checked & would have been found the coolant to be low. By it not, when it went out for the main & the coolant low in just a few laps it over heated & melted the o ring ending that little 310s life.
He ran it till it wouldnt run no more, it pure turned the crank black it go so hot, but it was gittin it done for a little while.
That test, proved to me how big of an issue that port linking ordeal was on these cylinders & something needed to be changed, but deaf ears wouldnt do anything about it until I went over it with Eddie. In less that 6 months time after our talk, ESR started offering the piston design you get today when you order a cylinder kit, so we got to give Eddie credit for doing that.
What would really be nice, if the machining on the piston sides were just like a CR250R piston, then there would be no linking at all.
Today, when I'm working on the aux exhaust port windows, I do not touch the rear part of the window, not widen it any at all so to prevent as much as I can any port linking. Its all I can do other than welding up the window in the sleeve & then re-cutting the window.
When you order a new replacement sleeve for these cylinders, should you have one sleeved, the new sleeve even after the CNC machining is complete, the window opening is cut to far around because of the angle of the cutter.
Heres something odd to think about, say your building a 270cc OEM TRX cylinder using the YZ250 piston, specially on a cylinder that is all ready ported. If you check that build when the piston is at TDC, you'll notice the skirt of the piston is uncovering the floor of the exhaust port maybe like a mm or two, but it has no affect on output & if anything its helping.
I dont know how it could be tested other than on a dyno & two different cylinders to check. My guess is, that at that part of the stroke the returning wave in the pipe is pushing some of the fresh intake charge spent out into the pipe back into the crankcase & acting like a turbo effect. I have nothing to prove this though, but I do know every one I've built that ended up with that little piston skirt gap will tote the wheels out of a corner & make for one happy rider using it.
Neil
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neil is these 2 arrows pointing at the areas you guys are talking about ? the wristpin passing over the aux window and creating a small hole isnt that uncommon. hell even the newest generation ktm are like that. theres no way around it other than a narrower aux window or a plug in the wristpin. narrower windows arent good for power and a plug is a bad idea on a mass produced bike.
on that same note, any machined out areas on the side of the piston for lightening purposes, should be done upwards and behind the wristpin, like in the pic you see a hole above and behind the wristpin. other wise it creates a much larger void, like the esr piston. as for any small hole down in the bottom corner, just drop the front corner of the piston down and the hole is eliminated, but from the pic it appears only about a .020" gap, which i find it hard to believe its causing much problem. but all eddie had to do was look at any modern ktm piston to see what needs to be done. :friendly_wink:
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this is another way to go about it , which reduces linking holes to a bare minimum. you see the front corner of the piston is dropped down to eliminate the aux window from popping open at tdc. then the side of the piston is lightened above and behind the wristpin which eliminates any huge voids as the wristpin runs over the windows. yes there may still be a small hole as the wristpin passes over the back edge of the aux window but the linking between exh and tansfer port/crankcase is kept to a bare minimum
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Those pistons with the cutaway having that little dip would prob work perfect. Udontknowme although I do believe this is going on and I do think its minimal. I 100% agee this could be more of a setup issue with not enough blow down. Arlan and some other reputable builders have said this is not hurting anything, but I do believe its not working at its peak.. and well that is what we are, here for. Cleigh I also appreciate your post and have read alot about linking from information you have posted. It has shed alot of light on how things are working. As for the yz piston maybe the tiny gap isnt bad but I feel like the amount of exhaust entering my crankcase is not just from this aux port as udontknowme originally stated in some of our first posts, and is so much I cant see it working efficiently. If its filling with old mostly burnt gas it doesnt have room for new good fuel charge. I need that greater pressure to be from intake charge. Im working things out slowly and will get this pump working good or ill start over with another..
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not much you can do besides find another piston with less of the side machined out. but still you cant completely eliminate the link between exh and crankcase because the wristpin still passes over the back edge of the aux window. the best you can do is reduce the empty void in the side of the piston by using something like the pics i showed
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Yes I completely understand the linking and the fact it is what it is. I was referring to some better cylinder porting and building a more efficient setup. Lets skip subjects and address another issue I found. I was not running a reed spacer. Im running v force three. They were new less then 15 hrs on them. The upper reed pedal seemed very close to the intake roof after pulling the reeds it was clear they were hitting the roof. They have begun to chip ever so slightly. So is the fix the reed spacer to back it up a little and let it have more room? Or reshape and raise the intake ceiling?
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I was not running a reed spacer. Im running v force three. They were new less then 15 hrs on them. The upper reed pedal seemed very close to the intake roof after pulling the reeds it was clear they were hitting the roof. They have begun to chip ever so slightly. So is the fix the reed spacer to back it up a little and let it have more room?
I used reed spacers on all the 350cc and below motors. Otherwise the reeds only lasted a couple of seasons. Other builders take material away where they hit. The v-force are new to me. But I have heard you can bend the tabs further with a heat gun.
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Its not black from heat though its exhaust it wipes right off. What would cause so much exhaust blowing back though
I have not been on the site for a while and just noticed this thread.
If the inside of all of your transfer port tunnels are black (and usually the reeds will also be black) it is from exhaust entering the transfer ports through the transfer port window. When this occurs the port will usually be darkest at the port window and the color will get lighter as the distance from the port window increases.
If exhaust was entering the hole your have pictured, the inside of the transfer ports would not be black due to the small amount flow that is occurring through this hole.
Like Udontknowme said, the blackening is due to insufficient blown-down for the RPM where you are spending a lot of time. Increasing the blown-down will help this problem if you have the right pipe and a carb large enough to properly fill the crankcase at the RPM where the backflow is occurring.
Severely over-revving the engine past its power peak can also cause this problem even with the correct porting, carb and pipe.
Port linking (any open path from the crankcase to the export) is sometimes beneficial when the piston is near TDC on some two-strokes. Port linking when the piston is mid-stroke or lower is seldom beneficial to power.
On the piston up stroke, whatever is in the exhaust port close to the port link will be sucked into the crankcase from the exhaust port. Depending on the RPM, some fresh charge may still be lingering in the exhaust port when the port linking occurrs. On the piston down stoke, crankcase mixture will be lost to the exhaust port.
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neil is these 2 arrows pointing at the areas you guys are talking about ? the wristpin passing over the aux window and creating a small hole isnt that uncommon. hell even the newest generation ktm are like that. theres no way around it other than a narrower aux window or a plug in the wristpin. narrower windows arent good for power and a plug is a bad idea on a mass produced bike.
on that same note, any machined out areas on the side of the piston for lightening purposes, should be done upwards and behind the wristpin, like in the pic you see a hole above and behind the wristpin. other wise it creates a much larger void, like the esr piston. as for any small hole down in the bottom corner, just drop the front corner of the piston down and the hole is eliminated, but from the pic it appears only about a .020" gap, which i find it hard to believe its causing much problem. but all eddie had to do was look at any modern ktm piston to see what needs to be done. :friendly_wink:
Do you by chance, have an old used piston laying around like the new KTM in the pic ?, if so could you part with it.
Would like to do some measuring on one like that new one.
On the linking deal, in the second pic, where to two color arrow facing the main exhaust port is located. That whole area on the side of the piston in the drawing has been machined out, so to understand what is happening, take a piece of clear plastic & trace over the pattern on the side of the piston, cut it out & then drag it downward just like as if it was the piston on the downward stroke until it reaches the main transfer. Once you have the cut out at the main transfer, if you drag it any lower you can see the link between the aux exhaust port window & the main transfer. When those ports link on the downward stroke, fresh crankcase charge is being pushed out of the main, around the side of the piston & on out through the aux exhaust port in to the pipe.
On the upward stroke of the piston, the same linking is happening, but the returning wave of the pipe is pushing the spent exhaust back through the aux port & down into the transfer.
The biggest problem, is the part of the new fresh charge from the crankcase, being blown out into the exhaust early, which lowers the crankcase pressure when the transfers finely do open.
One other thing might be going on during this whole ordeal, if for some reason the exhaust pressure in the pipe was high, say like maybe the core in the silencer was broken loose & partly blocking the exit tip of the silencer. Sort of like the tater in the tail pipe or cat converter plugged.
If it was blow down creating the problem, then how much more than 32* of blow down would need to be added, 34~36*.
I think the fact, that theres fresh charge being blown out earlier than it should be through the linking process & then the fresh charge being lower pressure when the transfers finely do open, the charge flow up through the transfers, the duration not as long as it normally would be if it were not for the linking, so in turn you have a void to allow the backwards flow of the exhaust down into the transfer ports.
It might be that a pipe with a strong suction wave during that part of the stroke could help some of the problem, but when the pistons on that upward stroke, that returning wave is going to be doing its thing.
Neil
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Does anyone have a pic, of the new ESR piston & then an old Pro-x to show the machining on the sides around the wrist pin hole. If so, we can see the difference between the two & kinda understand how much better the new ESR pistons are.
If you have a pic of a piston for the CR250R/TRX250R 85/86 year, that would be nice as well.
Neil
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There are some posts somewhere in the forum with those pics.
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Yes, but good Lord finding them.
Neil
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Thanks guys! This is the information, a successful forum creates! :applouse:
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Do you by chance, have an old used piston laying around like the new KTM in the pic ?, if so could you part with it.
Would like to do some measuring on one like that new one.
Neil
what size piston you looking for ?
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Like 72mm & up would be nice, but the KTM not that big. It dont have to be a good or reusable one, just need to measure the machining, location of everything. If seized, that wont make any difference.
Thanks, Neil
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i have one laying around like the pic in #25 post . its vertex brand at 72mm. the pic above it is the wiesco version, i do have one of these also but its installed in the engine. the wiseco version is similar to the vertex but with some minor differences. obviously the cutout area above the wristpin is different and the dipdown area in the side skirt may be alittle different as well but its hard to say without looking at both pistons side by side. they are flatop dome and have a sqaurish shaped cutout on the intake side. like a cr500 but more sqaure. something else to keep in mind, because the ktm cylinders are plated and were never intended to be bored larger, the piston is only offered in 72mm. so you wont have a bunch of extra oversizes to choose from. i can let you borrow this vertex i have but i will need it returned. the reason is because im using it for a project of my own, although its not honda related. if you want a used piston to keep then i can talk to one of my aqaintances that does ktm rebuilds and have him send me a used one, then i can send it to you, chances are it would be vertex brand however. it would be free other than a couple dollars shipping but i dont know how soon he could get me one. might be a week or maybe a month. its hard to say. so you can borrow this vertex if you want. myself i always use wiseco but unfortunatly i dont have a spare wiseco to send you and i have no clue where you would get a cheap used one. possibly ebay maybe. also i dont have a spare wristpin to send but it uses just a standard 18mm
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neil i sent a email to my piston guy to see what used stuff he has sitting around. let me see what he says first. if hes got one then ill be able to get you one of your own to keep so mine wont need to be shipped back and forth