relieving exhaust bridge for 125cc

[georgieboy]

Hi all, checked the archive but every topic refering to 250cc's.
Does relieving the exhaust bridge counts aswell for 125cc's?
And if so, what wld be the measurements? 0.05mm?

 

[Jman271]

I would not suggest touching the bridge on a 125, at all. Unless you get it tig welded and then profile it differently, I would leave it alone. This area can be a big weak spot,,,,some YZ 125's come to mind as it is very thin to begin with.

 

[cujet]

I believe what he meant is that the surface of the exhaust bridge that supports the rings is the area of relief. Yes, 125's get a slight relief also, especially if there is a problem with the bridge bowing into the cylinder. Because 125's often have such large exhaust ports, they are succeptable to bridge distortion.

Chris

 

[georgieboy]

thx guys for the replies. Yes i am talking about relieving it to compensate for the heat expansion. 125's get away with smaller tolerances piston/cilinder, but how about for the bridges. They do get hot but i can't see any relieve by looking at it. Piston has no scratches in that area so i was just wondering if they get away with no relieving at all?

 

[marcusgunby]

They do have a relief-at least the ones ive seen have.

 

[bclapham]

read Erics book page 122- he states that the bridge getts hotter, expands more and needs extra relief in the region of 0.001-0.003 inches, whatever that means :silly: :confused:

 

[georgieboy]

Bruce, lucky i ve got the book. Will read that page, thx

 

[EricGorr]

You can safely polish the exhaust bridge if it gets scoured from a minor seizure, using a "red stone". That is a small rectangular stone block of fine grit and soft material, they are commonly sold at industrial supply stores. They're used to clean and sharpen lathe tool bits and sold in different grit/colors. Red and green are the finest and softest. Use the stone to buff up and down the length of the bridge until its smooth.

All OEM cylinders have relieved bridges that are machined with CNC grinders, but aftermarket plated cylinders are hit or miss. I use a three stage process on my cylinders, starting with a soft stone, then a fine sanding roll, and finishing with a Cratex 120 grit rubberized abrasive.

Some 125 cylinders like the YZ, TM, and CR have such narrow bridges that they tend to wear a notch in the piston and ring. Relieving the bridge more just makes it worse. The solution is to either replace the ring often (4-5hrs.) or have the bridge welded wider during the cylinder replating process. My company and RPM offer this service and fitting of the exhaust valves is required to accomodate the weldment.

 

 

[O'MALLEY_808]

Hi,

Im not knocking Eric's advice as he more than likely knows what he is talking about but the place that does my engines (BONDI Engines) gets me to drill two small holes in the piston line with the exhaust bridge so that this area becomes lubricated. He then takes this area where the exhaust bridges contacts the piston and lightly sands this small area on the piston so that oil can be trapped here and also allows for some minor clearance.

Thats just what I do to my pistons and I havn't had a single problem.

Some wiseco pistons already offer the two holes in the piston in line with the exhaust bridge.

 

[Rich Rohrich]

Originally posted by O'MALLEY_808
Hi,

Im not knocking Eric's advice as he more than likely knows what he is talking about

Yeah more than likely he does. :silly: :laugh: :silly:

 

[steve125]

Originally posted by O'MALLEY_808
Hi, 

He then takes this area where the exhaust bridges contacts the piston and lightly sands this small area on the piston so that oil can be trapped here and also allows for some minor clearance. 

 The proper way is to relieve the exhaust bridge, not sand the piston.

 

[MikeS]

Originally posted by EricGorr
Some 125 cylinders like the YZ, TM, and CR have such narrow bridges that they tend to wear a notch in the piston and ring. Relieving the bridge more just makes it worse. 
 

Sanding the piston would make it worse. In our TM we use the OEM piston (great quality) and the ring gets grooved after a few hours. The psiton does have the oil holes in it.

Eric has done the 144 kit for us and the wiseco he uses is not better. Actually the 144 kitted TM seems better on the ring. I guess the bridge gets wider as it gets bored.

 

 

[EricGorr]

Actually some thermally challenged engines need as much help as possible. Oil holes in the piston are a good idea if deburred properly. Wiseco shapes the piston to add a bit more clearance at the bridge so the oil holes aren't too neccessary. Another thing that I do when I prep a cylinder before plating is to relieve the bridge and all around the exhaust port with a fine sanding roll. That way the plating thickness will be extra thick so when the bridge is relieved and the port chamfered, the plating doesn't wear out prematurely. None of the aftermarket plating companies do that because of the time factor. That was also the underlining thehe to an SAE tech paper that I wrote in 1999. We built a special liner jig with the ability to mechanically distort the liner to simulate thermal conditions of a running liner, so we could hone the cylinder before and after plating, then install it in an outboard engine block and have it straight and round with equal plating thickness. Mercury picked up on the idea and built a better jig with more sections and hydraulic operation.

Theres a lot to making a 2-stroke top end more reliable for the hard useage and dirty conditions imposed by off-road riding.