OzVMX Forum
Clubroom => Tech Talk => Topic started by: evo550 on July 25, 2012, 03:42:35 pm
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Only over exhaust port, 2 brand new cylinders and pistons,both lasted about 1 hr riding.
Standard jetting.
Holes drilled in piston. Getting hot there but why ?
(http://i118.photobucket.com/albums/o82/evo550/046.jpg)
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do ring pins line up with centre of port bridge?
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Havre you relieved the bridge a bit ??? drilling holes in the piston is only half the job.
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do ring pins line up with centre of port bridge?
Pins are at the back of piston.
Havre you relieved the bridge a bit ??? drilling holes in the piston is only half the job.
Not sure what you mean by " relieving the bridge" Bill, both rebores and pistons where done by Serco and I would assume what ever needed doing was done by them.
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do ring pins line up with centre of port bridge?
Pins are at the back of piston.
Havre you relieved the bridge a bit ??? drilling holes in the piston is only half the job.
Not sure what you mean by " relieving the bridge" Bill, both rebores and pistons where done by Serco and I would assume what ever needed doing was done by them.
I wouldn't assume that at all. Infact my last rebore from Serco I recall bevelling the port edges myself. Not hard to do but it must be done.
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Looks like shoddy chamfering you can also V the ring groove in front of the ex port and the main culprit will be ring end gap
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Not sure what you mean by " relieving the bridge" Bill, both rebores and pistons where done by Serco and I would assume what ever needed doing was done by them.
Sand down the bridge a bit - give the piston more clearance over the bridge when its cold. It's the hottest part of the barrel and will expand when hot.
Google "TM300 ring life" or similar - they (ad the 250s) have got a too thin bridge that expands back into the bore. Then the ring wears out over the bridge (ie: The bridge wears a notch into the ring) - if you ignore the loss of compression, eventually the ring snags on the exhaust port, wrecking the barrel as well as the piston and rings.
So... have a close look at the ring wear when you get it apart.
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OK, all the ports have been bevelled at Serco.
As for ring gap....well there isn't any to speak of, both times it was a new rebore with max of about an hour on it.
First time I sent it back they said looks like overheating, which I thought was a possibility as my raditors where clogged with magnesium rot powder.
I have since replaced the rads with nos ones (thanks Hoony), but to no avail.
Is it possible that a non working powervalve (fully closed) could cause this area to overheat at High rpm?
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What Bike is this? We had problems back in the day with RMX250's doing the same thing after resleeving them.I see you have a wiseco piston in there .The problem was that the sleeve thickness was thicker than it should have been and the sleeve wouldn't get hot enough being water cooled ,and so we had to hone it out more than usual to allow for this problem.
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http://www.thumpertalk.com/topic/713203-cr500-exhaust-port-bridge-relief/
I would be cleaning it up an.d checking that exhaust bridge , or going back to your engineer and asking what they recomend
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I reckon it's a case of all of the above, all could be contributing factors but I'm surprised the re-borer didn't advise you.
Or, ride a 4 stroke. ;)
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Is that a 'wiseco' old style piston number [just visible] wiseco's [i was told] require more clearance than the original piston it would have been running due to there different expansion rate. To shoot that theory down though--if it had been running fairly hard for an hour or so then--don't know. Did totally remove a bridged exhaust port [LA or Wiseco sleeve in my GYT DT1 Kit cylinder] about 40 years ago, bit of a brave [stupid] move but still going. Have a fairly thick ring compared to modern thin ones though & never got snagged. [lucky], Made a big exhaust port though. Anyway-back to the problem, mix it 'oily' & richen up the main & needle a bit then put run it on av gas for a try. One other thought, did you say power valves are not working?, wonder if you tie the linkage in the fully open position--put up with a rush of power & see how she goes. [i missed what make the bike is but] Goodluck. ps, hasn't popped a crank seal out from the magneto side?, be running bad & gaspy lean if it had, couldn't see just a simply worn seal leaning things out 'that' much as would always have an oily mix keeping seal trying to do it's job. Sorry, one more thought [you might have said] have the power valves been relieved [if necessary], probably not a lot of clearance between them & piston when shut & think you're on second oversize which [not sure] could be 20 thou or 40 thou depending on how the oversizes go for your model. Goodluck twice. Damn it, more thoughts before i go, had a KX80 when the kids were young [40ish now jeez] & the water pump impeller had become detached from the shaft so just basically therm-siphoned around, did fix it but must've been like it for ages and never gave up.
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Hardex is on to it i think---ignore most of my irrelevant comments. Cheers.
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Thats seems to be a fairly common fault with the guys who did your work, a couple of mates had the same problem recently and the main culprit was not enough bore/piston clearance....a wieco piston is the last brand I would use....go Wossner or ProX first and seizeco last.....also with the cold weather we have been having lately make sure you warm the engine up properly.
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What Bike is this? We had problems back in the day with RMX250's doing the same thing after resleeving them.I see you have a wiseco piston in there .The problem was that the sleeve thickness was thicker than it should have been and the sleeve wouldn't get hot enough being water cooled ,and so we had to hone it out more than usual to allow for this problem.
1987 cr 250, still on original sleeve although out to max oversize.Thats seems to be a fairly common fault with the guys who did your work, a couple of mates had the same problem recently and the main culprit was not enough bore/piston clearance....a wieco piston is the last brand I would use....go Wossner or ProX first and seizeco last.....also with the cold weather we have been having lately make sure you warm the engine up properly.
Wiseco was the only manufacturer that went that big O/S, to save on resleeve....which now is the only option. :(
Ant idea on what the clearance should be..and ring end gap?
Cold weather not an option here 35deg in the shade :P
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Not sure what you mean by " relieving the bridge"
(http://i1260.photobucket.com/albums/ii576/KAB171/w1.jpg)
Dunno if this will help.
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Perfect, I will get a straight edge onto it tomorrow and see if there's any daylight there.
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Talking Wiseco verse original, sometimes manufacturers change the bore & stroke from one model to the next for various reasons & [not sure with yours] you might find one model is ummm 68x70 bore & stroke where next model change the think 70x64 is better & pistons might be identical but a couple of mill bigger. Worth researching maybe.
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This's the other half, Fortunatly a mate gave me this to refer to, so i could check my freshly rebored CR250 motor ,done by a so called expert down here, when i first got into VMX, The bridge wasnt relieved and no holes were drilled in the piston >:(, along with a host of other examples of shitfull workmanship , I done it myself.
Cheers.KB
(http://i1260.photobucket.com/albums/ii576/KAB171/w22.jpg)
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Rule of thumb, Ring end gap should be 0.003"-0.004" for every inch of bore.
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Rule of thumb, Ring end gap should be 0.003"-0.004" for every inch of bore.
What he said, as per Wiseco instructions supplied with new piston.
I recently fitted a Wiseco piston and one ring gap was spot on out of the box and the other needed filing as there was no gap.
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piston to bore clearance too tight ;)
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The only instillation detail that came in the box was position and size of the holes to drill, nothing like KB posted, I'm now wondering if it is a wiseco or a wossner piston ::)
Is a nicosil lining an option for repair of something like this ?
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Does the ring end gap run on the bridge between transfer and boost ports? If it does ring end gap is much more critical than running through the middle of the boost port.
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Wossner have a grey coating on the sides of the piston, but looking at your pickie, you got a seizeco.
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It's obviously getting too hot, if the jetting is standard are you running more compression(has the head or cylinder been machined) or is the ignition timing advanced? There's nothing wrong with wiseco pistons but you may need to relieve the exhaust bridge as stated if not already done...
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Does the ring end gap run on the bridge between transfer and boost ports? If it does ring end gap is much more critical than running through the middle of the boost port.
The ring end gap is behind the inlet port..If the arrow on the top of the piston is 12 o'clock, the top ring gap is at about 20min past and the bottom ring is about 20 min to the hour.It's obviously getting too hot, if the jetting is standard are you running more compression(has the head or cylinder been machined) or is the ignition timing advanced? There's nothing wrong with wiseco pistons but you may need to relieve the exhaust bridge as stated if not already done...
As far as I know everything is standard, I think your on the money with it getting hot, especially looking at the photo to the valve on the left, starting on the outside it looks nice and tan, but it changes colour the closer you get to the exhaust bridge, with the tip looking like it's got white hot.
Why is it getting so hot though?
My question is, Could the powervalve thats not operating and remains closed cause heat build up at higher revs through resticting gas flow?
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pv clearance???? by the photo looks like it might have even scuffed the piston on the left.... not enough clearance n not opening...
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Does the ring end gap run on the bridge between transfer and boost ports? If it does ring end gap is much more critical than running through the middle of the boost port.
The ring end gap is behind the inlet port..If the arrow on the top of the piston is 12 o'clock, the top ring gap is at about 20min past and the bottom ring is about 20 min to the hour.
As far as I know everything is standard, I think your on the money with it getting hot, especially looking at the photo to the valve on the left, starting on the outside it looks nice and tan, but it changes colour the closer you get to the exhaust bridge, with the tip looking like it's got white hot.
Why is it getting so hot though?
My question is, Could the powervalve thats not operating and remains closed cause heat build up at higher revs through resticting gas flow?
The end gaps run on the bridges between ports. There is to much black soot carbon in there for any jetting ignition issues, head gasket would fail if there was water problems.
You would know if the PV was not opening they just struggle to rev and generaly very flat. The exhaust bridge has aluminium smeared all over it.
There's my $0.02 not enough end gap/to little clearnce/no relief is rubbing on the bridge and bulging and heating the piston. Is the ring stuck in the groove at the front of the piston.
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Wiseco pistons are forged, which require more clearance than cast (due to expanding more with heat). Their older pistons were made of a material that had a lot of expansion, even for a forged piston.
Lots of engine shops would hone for a cast piston which was too tight for a Wiseco, causing them to seize.
This is where the Seizco name came from.
I'd be amazed if Serco made this mistake, especially with the newer Wisecos that are much more stable (and by newer, I'm talking "made in the last twenty years"...).
The brand of piston is not the cause of your problem.
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Does the ring end gap run on the bridge between transfer and boost ports? If it does ring end gap is much more critical than running through the middle of the boost port.
The ring end gap is behind the inlet port..If the arrow on the top of the piston is 12 o'clock, the top ring gap is at about 20min past and the bottom ring is about 20 min to the hour.
As far as I know everything is standard, I think your on the money with it getting hot, especially looking at the photo to the valve on the left, starting on the outside it looks nice and tan, but it changes colour the closer you get to the exhaust bridge, with the tip looking like it's got white hot.
Why is it getting so hot though?
My question is, Could the powervalve thats not operating and remains closed cause heat build up at higher revs through resticting gas flow?
The end gaps run on the bridges between ports. There is to much black soot carbon in there for any jetting ignition issues, head gasket would fail if there was water problems.
You would know if the PV was not opening they just struggle to rev and generaly very flat. The exhaust bridge has aluminium smeared all over it.
There's my $0.02 not enough end gap/to little clearnce/no relief is rubbing on the bridge and bulging and heating the piston. Is the ring stuck in the groove at the front of the piston.
The end gaps are behind the inlet port shown in KB17's 2nd scan, illistration "E" and point 4.
I was originally unaware of the meltdown, as I was investigating a hunch that the PV's wern't operating when I stumbled across it. Thread in Honda section "HPP exnaust valves"
Although posssible, I would like to give them the benifit of the doubt that Serco would not make the same mistake twice...so I'm looking for another common denominator, to why it would happen twice to new cylinders/pistons, and the PV's not opening is really the only other possible cause.
Instead of a resleeve can these old iron bores be replaced with nickasil?
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Sand out that aluminium off the bore and whack the lot back to gether. its now found its own clearance
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don't forget just add a new ring or 2 freaky!!!
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Sand out that aluminium off the bore and whack the lot back to gether. its now found its own clearance
don't forget just add a new ring or 2 freaky!!!
Does that mean I can still use the old piston ?? ;)
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You can nicasil over cast iron no problems. You get the aluminium off with hydrochloric acid and a cotton bud. The piston will have more answers. Hondausualy put the end gaps on the bridges.
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You can nicasil over cast iron no problems. You get the aluminium off with hydrochloric acid and a cotton bud. The piston will have more answers. Hondausualy put the end gaps on the bridges.
Pistons a little worse for wear....top ring is seized in,will post pics tonight.
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My 2 cents worth for the evening is, i see lots of talk about ring end gap [gaps] on the bridge. I imagine you mean a bridge on the inlet side or bridge between transfers or somewhere & "not" the exhaust bridge that the main subjects about?. Ony reason i'm saying this is [is the piston in back to front] or are they a "reed holes in the piston" type which couldn't be done wrong, [or a case reed inlet? i.e. piston without reed holes], not familiar with the honda [last one i had was the first elsinore]--long story. Thanks. ps, actually did recently see a reed piston in back to front, was a jack up someone was doing using reed piston in a cylinder with rotary valve, was a good fix as pistons weren't available, went well when he put it back correct way round. Sorry--carry on.
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pic of piston
(http://i118.photobucket.com/albums/o82/evo550/008.jpg)
It was still running in this condition, I only stumbled across it while checking to see if powervalves where working....which they are not...but I'll leave that for another thread. :)
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sorry, me not reading. I see where end gap are.
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pic of piston
(http://i118.photobucket.com/albums/o82/evo550/008.jpg)
It was still running in this condition, I only stumbled across it while checking to see if powervalves where working....which they are not...but I'll leave that for another thread. :)
Surely if the power valve is not working that will meanit's closed or half closed and that in turn would lead to excessive heat build up in the exhaust port and on that exhaust port bridge?
Maybe your problem is related to the power valve.
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The piston looks to have no issue with the bridge or you would see a lot of polishing over the oil holes. Incorrect piston to bore clearnce would show evidence all over the piston.
PV not opening might have contributed with higher dynamic comp than the fuel can handle. Piston gets hot, traps the ring rubs on bore end of.
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Can’t see how the power valve can cause heat build up. All it does is lower and raise the exhaust port.
Trailbike engines don’t get hot because they have low exhaust ports and peaky engines don’t get hot because they have high exhaust ports.
One thing that comes to mind though is that it’s on a large oversize and some power valves can open into the piston area once they are bored oversize.
Have you checked that the power valves don’t protrude into the chamber?
Are there any marks on the tips of the power valves?
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Lozza the piston striations don't match up with the rings ???,especially those 2 "cuts" on the r/h side of piston above top ring(left side of picture),& go back to 1st picture of bore,the hone marks are gone from above the bridge,again the rings don't match to that amount of difference?,is something else going on,is b/end trying to tell us something?,or did Evo not cleanup the previous lot of damage,& thats come back to bite?,just seems a lot more of piston smear damage,& the rings suffered less?, :P
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Can’t see how the power valve can cause heat build up. All it does is lower and raise the exhaust port.
Trailbike engines don’t get hot because they have low exhaust ports and peaky engines don’t get hot because they have high exhaust ports.
One thing that comes to mind though is that it’s on a large oversize and some power valves can open into the piston area once they are bored oversize.
Have you checked that the power valves don’t protrude into the chamber?
Are there any marks on the tips of the power valves?
The valves had to be done at the same time as rebore to ensure clearance, you can sort of an idea in the first pic.
Lozza the piston striations don't match up with the rings ???,especially those 2 "cuts" on the r/h side of piston above top ring(left side of picture),& go back to 1st picture of bore,the hone marks are gone from above the bridge,again the rings don't match to that amount of difference?,is something else going on,is b/end trying to tell us something?,or did Evo not cleanup the previous lot of damage,& thats come back to bite?,just seems a lot more of piston smear damage,& the rings suffered less?, :P
That is some serious detective work. The first pic is of the cylinder after first seizure (I don't have old piston), second pic of piston after second failure, I can get one of the cylinder to match piston pic.
It was given a rebore after first mishap.Pic below.
(http://i118.photobucket.com/albums/o82/evo550/016.jpg)
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errr ok,1st piccie of seizure in bore dosen't belong to the damaged piston shown?,(that all makes sense now ::) ),ok,we now need to see lastest bores seizure damage, :P
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Only over exhaust port, 2 brand new cylinders and pistons,both lasted about 1 hr riding.
Standard jetting.
Holes drilled in piston. Getting hot there but why ?
(http://i118.photobucket.com/albums/o82/evo550/046.jpg)
as above check exhaust port clearance..sorry matcho mick try green herbal tea :)
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Steve,isn't that the 1st seizure,(i think,i'm getting a fugging headache ???), :P
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I've got a shitload of kart pistons on the shelf in my shed with seize marks like that on the exhaust side. From my bucket racing days. It didn't stop 'til I started using a 22mm i.d. tail pipe on the chamber instead of the 18mm I was using before.
What kind of riding are you doing ? Not dirt track by any chance ? If you've got it nailed a fair bit, then any restriction in the exhaust will cause a heat build up-like the power valve for example......Or using the wrong muffler ??.
I'd fix the power valve or just wire it open. Clean up the piston with a smooth file so it doesn't look so horrid. Use some acid on a toothbrush (not your best one !) to get the alloy off the bore-we used to use acid from the swimming pool supply shop I think. You'll need to get all the crud off that's above the exhaust port. Clean up the ring-they are hard as hell and usually can take a pounding. Unless the ring is scored scored then it'll still give you enough compression to run well enough.
It'll probably rattle pretty badly, but Wiseco pistons are tough and can handle a fair bit of clanking around.
Then get it out and cane it. If it seizes again, you'll know it's got nothing to do with the power valve, or the piston-bore clearance.
That's what I reckon anyway.
Nitram
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The cylinder now
(http://i118.photobucket.com/albums/o82/evo550/001-1.jpg)
with matching piston
(http://i118.photobucket.com/albums/o82/evo550/008.jpg)