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Exhaust port bridge missing

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Jul 19, 2007
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#1
I was in the middle of rebuilding the top end and found that there is a small section of the exhaust port missing. I just bought a new piston and ring and went ahead and put that all onto the connecting rod ready for the cylinder and head. My question is...stupid as it may be, how bad would it be to run the bike without part of this bridge? I've been working on this bike ('98 YZ 125) since I bought it 2.5 weeks ago. Now I'm upset and don't know what to do about it. I don't want to spend another 250.00 for a new cylinder. What are my alternatives? Thanks in advance!
 
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May 25, 2007
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#3
I would worry about the rings snagging on the broken parts...People who do hillclimbing have their bridge cut out. I was going to do that myself on my 500 but just relieved the bridge instead. Cut it completely yourself with a dremel or take it to a machine shop and have them do it. If you do it yourself make sure to clean up around the exhaust port (kinda round it off) so the piston rings dont snag, or you'll be going through those like crazy.
 
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#4
I can not imagine a 2 stroke race bike lasting one race with out an exhaust bridge! And if say hill climbers do this on a regular bases?
 
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#6
Some are not designed to have them,it is a little more mathematical. Eliminating it from one that has one should be left to an engine tuner, get 3 opinions.
 
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#7
If your gonna have to buy a new cylinder anyways, why not try it with this one and see. If you send/take a cylinder somewhere with pieces of the bridge missing, their either gonna cut it out and smooth it up or try to sell you a new one. Since a new one is less work...that's what they are going to try to do. All the exhaust bridge is really for is to keep your rings from snaging on the exhaust port...when you have a cylinder bored you HAVE to have the bridge relieved (shaved back a little) or you will go through rings and a piston fast. If you cut it out and smooth out along the top and bottom of the exhaust port (make it like it was never there and so the rings wont snag) then it will be just fine...maybe even a little more power (exhaust getting out faster). Major power gains are from actual porting=raising or lowering the top or bottom exhaust ports 1-2mm depending on what you are looking for. Unless you want to spend 300 bones for a new cylinder or resleave, this is the other option you have. You could also take a dremel to the broken edges and clean them up so the rings wont snag but I think it will continue to break apart.
 

Rich Rohrich

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#8
The chordal width of that exhaust port is way too wide to run it without a bridge. All you geniuses suggesting he run it with no bridge should have paid more attention in geometry class in high school.
 
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#9
Us 500 guys do it all the time...guess all of us are messed up in one way or another :coocoo: ...whats another option then??? Oh wait, that's the only other option there is.
 

Rich Rohrich

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#10
The chordal width of of the 500's exhaust port in relationship to the bore size is much smaller than a 125. So giving this guy advice based on a 500 cylinder, especially a hill climb application that only runs for seconds at a time is a really bad idea.

You're right, there is only one option. FIX IT, or don't ride it.
 
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#11
I have taken the money plunge and ordered the new cylinder from yamaha....ouch!!! I didn't want to risk messing up the new top end I have just put in. Seems that I've just been pouring money into this bike fixing what the yay-hoos have done to put this bike back together in the first place. Why can't people just put things back the way it was or just leave it alone???!!!
 
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May 25, 2007
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#13
That is the better move...are you on the stock bore or are you going to have to bore the new cylinder. Are/were the 125's plated...the wifes 85 is, what a pain! Had to order a cylinder for it too...$400 later...
 

Pete Payne

MX-Tech Suspension Agent
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#15
Rich is right !
MAX chordal width of a 54mm bore with a bridged exhaust port is 51.0mm
MAX chordal width of an unbridged 54 mm bore exhaust port is 38.2mm.
Big difference!