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porting and polishing?

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ok so i want to port and polish my own head and im pretty mechanically inclined but have never done this befor. i have read a few threads on the topic but they all seem to be about two strokes. do all the same apply to a four stroke as a two? my bike is an 07 crf250r. any tips pointers ect?

also i have picked up a few books along the way which i will study befor jumping in but i wanted some input.

i know its all about the shape of you ports and eliminating turbulance and as for plishing you should not really polish the intake side so much as clean it up. polishing it will not allow for propper mixture of fuel and air. the exhaust however should be polished for volocity.

volocity is actually where the misconception about back pressure began.

obiously no exhaust at all would let the most air out. right? NO! thats wrong. the reason for a smaller exhaust is not to keep a back pressure its to create vaccume in an engine to pull the fumes out.you know when you drive your car down the road and it sucks the dust up like a vaccume behind the car. the pipe does this same thing using air.

the waves of air in a tube will make for a vaccume behind them so not only is the air being pushed out by the engine but its also being pulled out by the air befor it. the smaller the pipe the faster the air moves making for more vaccume. the trick to an exhaust is to find a balance to air volocity in the exhaust to create the most vaccume while leaving it open enough to allow the air to escape.

and thats why we polish the exhaust side. to create a little more volocity. i found all this info in some other threads and thats the gist of what i know. any suggestions?

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Without access to a flow bench, there are really only two things that you can do on a 4 stroke head without ruining a head.

The first is to just do a general clean up of the port area to remove any casting irregularities and smooth the transition from the port to the seat. Just don't get too carried away.

The second area that can be worked is the combustion chamber. The area around the valve seats should blend into the surrounding surface of the combustion chamber. This is usually an area that can use a lot of work. From the factory where the top angle of the seat is cut there is usually a step that can be safely eliminated. By blending the seats to the combustion chamber (also called "unshrouding the valves"), you will allow airflow to increase when the valve is at very low lift, effectively 'tricking' your engine into thinking it has more cam duration.

Improvments in performance are going to depend on how bad these problem areas were before doing the work.

Don't get carried away with cutting up the guides. Shortening the guides will only reduce valve life and can cause oil consumption problems.

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when I get my head out I will take pic. I think I have a good grasp on what you are saying but I will take pics and post them befor getting into it to be sure. What do you suggest using to do this I have a dremel tool and hand files also how about polishing the exhaust side

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A dremmel will work fine for what you need to do. Leave the files in the tool box.

Polishing the cc and exhaust ports doesn't really help anything but it does keep carbon from sticking and not building up as thick. Polishing is something that has gone in and out of favor with porters over the years.

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what should I use to polish?

Use cratex wheels on your dremel for polishing.

Should I size exhaust and intake boots?

Do you mean port matching the carb boot to the head?

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yes. i mean the carb boot to the head. right now there is a slight lip where it tapers down into the head.

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yes. i mean the carb boot to the head. right now there is a slight lip where it tapers down into the head.

Yes. Port matching is okay. You only need to be concerned with anything that obstructs flow. If the mismatch is from the boot being smaller than the port, don't try to cut the boot to match.

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Also pay attention to the piston, eliminating/blending any sharp edges. It is good that you have already learned that if a lot is good and more is better than too much is just right. In ports bigger is not better. (in most cases).

Edited by ben williams

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Don't match the exhaust port to the pipe, you want to have a step from a small port out to a bigger pipe. You can match the intake boot to eh intake port if the port is smaller than the boot. Don't make the ports any bigger, just clean up any sharp edges and the step between the port and the seat. Most modern MX bikes come with very good ports these days, so very little is to be gained without a lot of porting experience.

With older engines with very poorly cast ports it was possible to get as much as a 10% gain with just a piece of sand paper on your finger by cleaning up the ports, these days are long gone now though.

Edited by KJ790

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In the case of YZF's, one of the cheapest and easiest "port jobs" you can do is to discard the OEM exhaust gasket and substitute a piece of 8 gauge bare copper ground ground wire cut so it fits end to end it the flange pocket of the head. The OEM gasket overhangs into the gas stream.

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In the case of YZF's, one of the cheapest and easiest "port jobs" you can do is to discard the OEM exhaust gasket and substitute a piece of 8 gauge bare copper ground ground wire cut so it fits end to end it the flange pocket of the head. The OEM gasket overhangs into the gas stream.

when you say ground i assume you mean solid bare wire probably 10ga would work better.

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Yes I do mean bare solid wire, and no, 10ga won't work as well, because 8ga fills the gap more precisely than 10ga does. On a different engine it might be that a different size wire is a better choice.

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Well, it would, but is that beneficial, and if so, how?

Here's the point of it. Picture a tubular passageway with a square ridge in it, like a 4x4 laying across your driveway. That's the end of the stock YZF exhaust port in totally stock form. Now picture the same thing but this time we have a 4x4 trench cut in the tube, like a trench across the driveway. That's the same port with too small a gasket in it. If it was your driveway and you were trying to flow water down it, which one would be worse?

The gasket needs to fill the gap without hanging out of it. In the case of the YZF, that means 8ga bare copper is better than 10. You could also lap the joint together so there was no gasket, but then the header would need to be tightened squarely, tightened first, and the alignment of the rest of the system would be much more important.

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I never thought about the "trench" it would leave. Goodpoint.

Why should I not shave out the boot from carb to head to match the port? Is that pot meant to be smaller for volocity?

Should I atleast round off the sharp edge of the rubber to cut down on turbulance?

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I never thought about the "trench" it would leave. Goodpoint.

Why should I not shave out the boot from carb to head to match the port? Is that pot meant to be smaller for volocity?

Should I atleast round off the sharp edge of the rubber to cut down on turbulance?

The boot should closely match the diameter of the carb bore as it's manufactured from the factory. I've never seen one that didn't. If there is a mismatch between the boot and the carb, it might be the wrong part.

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i have three crf250rs 2-06 and 1-07. they all have a boot that is slightly smaller than the port.if i had to guess i would say they are off by approx. 1/4 diameter. meaning the boot over hangs inside of the carb about 1/8th of an inch restricting the air flow into the port and likely causing a lot of turbulance. any suggestions? im not sure if i should leave this lip alone or what.

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In all reality any turbulence created that far up the port runner typically has little effect on performance. About 90% of your performance is dictated in the end of the port from the valve guide on, the shape of the valve seat and valve face itself, and the combustion chamber shape around the port.

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