Making a bridged exhaust port?

[neutral4x4]

I have a question for you guys. It doesn't specifically have to do with saws in my case, but it could apply to any 2 stroke I guess.

Is it possible to add an exhaust bridge to a cast iron cylinder with a single port? I've never done it.

Is it possible to weld anything in there with any accuracy?

Would it warp the cylinder?

It probably would need to be re-honed at the very least, but would it need to be bored out?

I know it would need more piston clearance than the rest of the bore so it doesn't stick when it heats up.

Anyone done it? Or even better, done it and have pictures and results?

Should I try it or am I just :bang:?

Thanks guys!

 

[srcarr52]

Welding cast iron?

Welding cast iron is always a crap shoot. When the weld cools it will causes cracking in the cast material in the heat affected zone of the weld. The best way to do it is to peen the weld while it's still glowing to remove the tensile stresses in the weld and take it little by little. But welding a bridge in an exhaust port will probably cause too much stress on the cylinder and end up cracking radially in the bore.

If you are going to try it heat the whole cylinder up as hot as you can get it such that the temperature gradient is minimized. But I wouldn't try it on something that couldn't easily be replace.

 

[trappermike]

I doubt you will ever successfully weld a bridge in an exhaust port.
In the 60's and 70's bridged exhaust ports were popular in racing 2-strokes,it allowed a wider port without the piston and rings actually "Falling out"into the port and being destroyed. There were drawbacks of course,the wide exhaust port exposed the exhaust side of the piston to a lot of exhaust flame,resulting in seizures,also the bridge could expand out into the cyl. scuffing the piston.

Now a bridged intake port is another story,I wished sawmakers would adopt this older idea,it has many benefits.

 

[ncfarmboy]

I doubt you will ever successfully weld a bridge in an exhaust port.
In the 60's and 70's bridged exhaust ports were popular in racing 2-strokes,it allowed a wider port without the piston and rings actually "Falling out"into the port and being destroyed. There were drawbacks of course,the wide exhaust port exposed the exhaust side of the piston to a lot of exhaust flame,resulting in seizures,also the bridge could expand out into the cyl. scuffing the piston.

Now a bridged intake port is another story,I wished sawmakers would adopt this older idea,it has many benefits.

Now you have my curiosity up. Would you explain this further? Always want to learn and there is always something.
Shep

 

[trappermike]

Same thing with a bridged intake,you can make it quite wide without the piston skirt wanting to "Hook" it(Then the piston literally bursts!) or the edge of the piston skirt wearing badly.
Also the bridged intake does a lot to cure "piston rock",you see the intake port and exhaust port are open at the same time,you have the front of the piston wanting to lean and hang out the exhaust port,and at the same time the rear of the piston can tilt out into the intake port,causing the piston to rock a lot and causing unusual or severe wear to it with large ports,sometimes the "Mysterious" and complete destruction of the piston when it hooks a port edge too hard and shatters into many little pieces.A bridged intake allows a very wide port but still supports the piston skirt reducing piston rock and accompanying problems.
A wide intake port allows more area for more flow and power,but does not increase intake duration,blowback and loss of low speed power.

 

[Rusty-880]

You dont peen it with a hammer...
To weld cast iron properly you have to preheat the cylinder and bridge piece to specific temp, weld, then post heat and allow to cool slowly. My brother is a boilermaker.

On a side note from what I've read on bridged exhaust ports they can give a tendency for localised seizing due to the temp differences between the bridge and cylinder. I wonder if you radius on the bridge so the rings go out of the cyl a little but still supported by the bridge if this would work better.

 

[trappermike]

I remember on one brand of motorcycle engine(Motocrosser) with a bridged exhaust port,the piston had 2 very small holes(Approx. 1mm) drilled through the skirt in line with the bridge to allow the cool crankcase fuel/oil mix to squirt through and cool and lubricate it.
I never really saw any big problems with bridged ex. ports,maybe today it's just cheaper to not use them....