Exhaust bridge

[UtahTrailRider]

Can someone explain to me what the deal is with drilling pistons on the exhaust side. I've heard the CR125 needs the piston drilled but I sure didn't in my last rebuild. Now I'm all paranoid I'm gonna shred something on the hot side.

Save the two stroke.

[frdbtr]

Can someone explain to me what the deal is with drilling pistons on the exhaust side. I've heard the CR125 needs the piston drilled but I sure didn't in my last rebuild. Now I'm all paranoid I'm gonna shred something on the hot side.

Save the two stroke.

 

The only pistons that you need to drill are forged pistons like Wiseco or Wossner.  You don't drill a Cast OEM piston (or prox which is really a OEM piston with a different brand on it).  In my experience, the Wiseco piston comes pre drilled for the 125's so you are probably fine no matter what piston you are using.

[UtahTrailRider]

The only pistons that you need to drill are forged pistons like Wiseco or Wossner. You don't drill a Cast OEM piston (or prox which is really a OEM piston with a different brand on it). In my experience, the Wiseco piston comes pre drilled for the 125's so you are probably fine no matter what piston you are using.

it was rebuilt with a Wiseco. I guess we'll see how important of a step it is by the end of my next race haha.

Save the two stroke.

[frdbtr]

it was rebuilt with a Wiseco. I guess we'll see how important of a step it is by the end of my next race haha.

Save the two stroke.

 

All of the wiseco 125 pistons I have seen, standard and over bore, were pre drilled.  I wouldn't stress over it. 

Edited March 8, 2014 by frdbtr

[callen8365]

Sorry to thread jump, but I to installed a wiseco piston into my 2000 cr250. I didn't drill any holes into it, nor where there any holes predrilled. I don't recall reading anywhere (other then on here) that you had to drill any holes. Does this mean I need to take apart and drill some holes? I also have 5 hours on the piston.

Thanks,

Chris

[Foleyracing]

from wisco web site

http://wiseco.com/PDFs/Manuals/MSeries.pdf

[bcislandrider]

Sorry to thread jump, but I to installed a wiseco piston into my 2000 cr250. I didn't drill any holes into it, nor where there any holes predrilled. I don't recall reading anywhere (other then on here) that you had to drill any holes. Does this mean I need to take apart and drill some holes? I also have 5 hours on the piston.

Thanks,

Chris

if its a wiseco it should need drilling for the 250.

The instructions on how to should be on the green paper that is wrapped around the piston in the box. And it is very important.

[ChrisLPD]

Can someone explain to me what the deal is with drilling pistons on the exhaust side. I've heard the CR125 needs the piston drilled but I sure didn't in my last rebuild. Now I'm all paranoid I'm gonna shred something on the hot side.

Save the two stroke.

 

OK, this is an old-school 2-stroke engine builder trick Utah,...but that one that works VERY Well! The idea is that the exhaust port bridge in the 2-stroke is a necessary evil. It is the one fault of the efficiency of a tuned 2-stroke engine. Its necessary because it acts as a land for the rings to run against as the slipper piston passes the exhaust port on its stroke to keep the ring compressed. It’s the evil because it’s the absolute hottest part of the motor and the absolute MOST suseptable to cylinder failure or nikasil plating damage for 3 reasons…

1. It is the thinnest section in the combustion chamber and can not dissipate heat. (Imagine taking a torch and heating the entire surface of a sewing needle. If you heat the needle’s eye, the shaft, and the point evenly, which part glows red first? The needle point does because it is the thinnest surface area that can not dissipate the heat).
2. All of the expanding burning hot gasses must pass by it. (Physics of Convection Dynamics)... (Imagine taking that same blow torch and blowing the flame evenly thru the center of your cylinder. It heats up in 360 degree radiation of the heat source. Now focus the flame onto one part of the cylinder. In addition to the heat from thermal radiance, the surface contact point heats up dramatically hotter because of the convection (movement) of the hot expanding gas molecules are bombarding and moving around it. Same thermal convection effect as the hot expanding combustion chamber gasses bombarding and having to move around the exhaust bridge.)
3. In lieu of both of the above critical faults, the exhaust bridge is NOT encompassed by the water jacket to get cooled

 

Most cylinder failures in a properly jetted and sealed 2-stroke with good fuel and lubrication are on the exhaust bridge because it gets so damned friekin hot. This is especially true of plated cylinders where you first start to see the plating flake off around the exhaust bridge. Drilling the the piston is MASSIVE insurance against this failure from the bridge warping, cracking, or flaking its plated coating. This was done by numerous factory teams and engine builders in the good'ole days before MX had cams, valves, and timing chains. This was when the manufacturers wanted to sale thumpers and pressured the AMA to establish the rules that a thumper was allowed twice the displacement to be "Fair" against the oil burners.

 

This is how its done...When I set up a 2-stroke, I use set-up dye. An easily removable (wipe-off) acrylic-like paint that you can spray on a part to visually inspect the wear and contact areas during motor building. I spray the dye on the front of the piston (exhaust side) and allow it to dry. I then install the piston on the wrist pin (without the rings) and lightly seat the cylinder down on it.

 

Once I have the cylinder lined up and seated, I look thru the exhaust port (open exhaust port with no pipe), I cycle the crank (via kick starter) until the bottom ring land passes approx ½ inch above the top of the exhaust port into the compression/combustion chamber. Using an etching tool I lightly trace both the outer left and right sides of the exhaust bridge into the set-up dye on the piston, and then disassemble.

 

Now with the piston removed I can see where the exhaust port bridge is in the etchings of the dye and I find the center vertical axis between these two etchings. This is the center of the exhaust port bridge. Starting ¾ of an inch below the ring land, I drill two .050 holes vertically (one on top of the other) about ½ inch apart down the center axis between the two vertical left and right marks.

 

The idea concept is to allow the positive pressure in the crank case (on the pistons downward stroke) to mist just a fine little amount of the cool, raw fuel/air mix onto the exhaust bridge thru the piston holes each time the piston passes the bridge. The cool, raw fuel mix acts as a cooling agent and will surprise you by how much this little trick will drop the temp of the thin little exhaust port. At 6000 RPM’s the critical heat temp on that thin little exhaust bridge is getting misted and cooled with fuel and intake air 100x a second!

 

Some will say this looses horsepower, I don’t believe so as long as you line the holes correctly and mist the center axis of the bridge and not vent your positive crankcase pressure out of the open exhaust port. Even if does, the loss is very, very, very minimal…but IT WILL SAVE YOUR $400 plated cylinder.

 

As frdbtr pointed out… This is a commonality of Wiseco recomendation in SOME of their piston kits that most people don’t read. (Some applications do not use a single port exhaust bridge). I am not familiar with the other Wossner brand he named, but wiseco recommends it because IT DOES WORK and their pistons are FORGED instead of cast and do not lose integrity by drilling the holes in the piston wall.

 

A Note about OEM and cast pistons… The same cooling effect would be benefited from an OEM or any other piston. It’s not often recommended for cast pistons because the “sand-cast” piston will weaken at the metallurgical level when you drill holes in it unless you stress relieve and re-harden it.

 

Please, please, consider all of the above My Opinion Only,... but as an educated opinion I have been building cast-iron and nikasil 2-stroke motors for 25 years and I have yet to have a cylinder failure of the exhaust bridge since I learned this from World Champion Melvin Cooper (Cooper Company Machine and Racing) building 2-stroke Hydroplane racing engines back in my twenties.

Edited March 8, 2014 by ChrisLPD

[ChrisLPD]

Wrong person sorry, was trying to respond to someone else and wouldnt let me delete.

Edited March 8, 2014 by ChrisLPD

[ChrisLPD]

Sorry to thread jump, but I to installed a wiseco piston into my 2000 cr250. I didn't drill any holes into it, nor where there any holes predrilled. I don't recall reading anywhere (other then on here) that you had to drill any holes. Does this mean I need to take apart and drill some holes? I also have 5 hours on the piston.

Thanks,

Chris

 

See Response Above #8

Edited March 8, 2014 by ChrisLPD

[ChrisLPD]

The only pistons that you need to drill are forged pistons like Wiseco or Wossner.  You don't drill a Cast OEM piston (or prox which is really a OEM piston with a different brand on it).  In my experience, the Wiseco piston comes pre drilled for the 125's so you are probably fine no matter what piston you are using.

 

This is absolutely 100% Correct for Wiseco. Not familiar with Wossner pistons, but im assuming they are forged if they recommend this. Read my response above if anyone wants to know why. DO NOT drill an OEM or cast piston unless you stress relieve it and re-harden it after drilling!!!

[Kenneth Sawyer]

I am going to follow manufacturers recommendations and some of your opinions and do some drilling. .050 if I understand correctly. I have a 1985 Kawasaki kdx80 with a fresh new nikalsil plating. My son I have a bike project.