144 Exhaust Port Bridge

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Aug 21, 2000
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#1
There is a similar question asked in the Forward Motion forum, but not a definite answer. (BTW, do not in any way take this as a negaive for the 144 kit. The F.M. 144 kits have been awesome in all three bikes.)

Situation involves 3 different big bore (144) bikes. All three bikes run great (RM/YZ,) but a groove is forming in the piston and ring that appears to be from the exhaust bridge. In the F.M. forum, one possible theory is that the bridge area is expanding more (from heat) and needs to be relieved. Has anybody that has worked with the 144 experienced this? If so, how much should be removed, and can it be removed without destroying the plating.

BTW, I would normally just replace the ring/piston more often, but (especially) in the case of the last bike, the piston and ring show almost no wear except in this one area.
 

bclapham

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#2
i think it is a common problem when you goto that big a piston size, if you read Eric book you will see that he notes this issue and i believe he relieves the bridge to help matters, but i suspect there is only so much you can do. it certainly occurs on mine, i just change out rings and pistons frequently since it keeps me busy and gives peace of mind!:)

i suppose you could always buy a ktm since i dont think it has a bridge, or you could get an 04rm125 since they have copied KTM- the RM's are real good at 144 and looking at how good a mildly modded 04 is, i can only imagine how good an 04 rm144 would be.
 
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#3
Thanks BCL. I should have read the section in Eric's book on big bore's again before posting (RTFM!) Anyway, I noticed in another post that you have an 02 RM144. I am considering trading topends for a 144 (originally built by Eric) for an 02 RM. I know how well the RM144's perform. My only concern is how often that I will have to replace rings/pistons in the 02. (In my previous experience [98 RM,] the interval was about the same as with the stock bore.)
 

bclapham

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#4
i am actually running a 97 or a 98 cylinder on my 02 and i still see the bridge wear on the piston but nothing that makes me loose sleep at night- indeed the cylinders seem pretty much the same but just have different PV setups. like i said, mine wears a bit but its not that big a deal to swap the piston or rings out sooner rather than later. i have nothing to compare with, the last 125 i had was a 1981 kx125- i bought this bike without a cylinder so i have nothing to compare with. i ride for about 1-1.5 hours every week- i try to do a ring every 6-8 weeks and a piston every other 6-8- i suppose thats around 10 hours on a ring and 20 on a piston- more than enough for me. i must call eric for a new one coming to think of it!


i hope steve sees this since i would like to know how long he goes on a top end since it sounds like he rides a bit harder than me.
 

elf

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#5
I think how the piston wears is directly related to how hard you ride the bike. On my ktm144 I can go 20 hours of play riding and the piston looks almost new. I like sand tracks so I try to race at alot of those and after about 10-12 hours its ready for a new piston.
 
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#6
I have an '02 kx144 done by MaxPower. I keep getting flat spots from the exhaust bridge on the exit side of the piston. My piston (when new) was gold colored on top and the sides are coated with some black stuff, which wears off upon installation just about. They also put the same gold colored coating on the head. It is flaking off from the inside of the combustion chamber and now allows combustion gasses to work their way up through the head bolts, which causes a sticky ugly mess. I just cleaned it up as good as I could and replaced the head gasket, hopefully it will stop leaking. Yet to ride (its -25F today). Have not had the bike ported. The bike does have more low mid power, and is great for my wife to ride (its her race bike). I will keep bikes stock in the future, that seems to be the most reliable route as a whole, and reliability is more important to me at this point. Always a trade out I guess. Hoping the new KXF will be reliable.
 

elf

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#7
AARONA Dont know what the gold stuff is but I think the black stuff just helps the piston break in quicker. I dont think the gold stuff has anything to do with gunk coming out of your head bolts. That would be a gasket problem. I had Eric Gorr do my motor and I have not had any problems at all. Except jetting issues which were no fault of Erics. If and when I buy a new 125 I am going to have the same thing done to my new bike I will buy a four stroke when they make them as light as a two stroke.
 
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#8
AARONA, the flat wear spot is caused by the exhaust bridge bulging into the cylinder when hot. This is generally corrected by removing a small amount of material (relief) (0.002 0.003 inches may be a good place to start) from the bridge. It seems that your cylinder probably does not have the bridge relief everyone here is talking about.

Chris
 

Eric Gorr

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#9
On cylinders like the YZ, the piston will always wear near the bridge because the bridge is too narrow. But welding the bridge wider hurts performance. The key is to use good oil and change the rings every 4 hours especially if you're a revver. I give people the choice of wide or narrow bridges. Normally when a guy wants a 144 for an enduro application I weld the bridge because I know those guys don't like to do much top end maintenance.