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How to Port and Polish

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Hi,

I just snagged a junk two stroke cylinder and thought that I might try my had at porting and polishing it instead of practicing on my good cylinders that I need for my bikes. From what I see, there isnt too much to it, but how would you, for example, raise the exhaust port or the boost ports? Is there a specific way to do it, or sould I not be concerned?

Thanks

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I've heard of people raising the ports by using an extra base gasket or two. Not to sure what the benefit of changing the port timing is, but if you search I'm sure you will learn more.

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Porting a two stroke has a lot to do with the specific engine and some respond to certain modifications better or worse than others. Porting a four stroke is not simple, but IMO, there are a lot more factors involved with porting a smoker and doing it well.

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I paid the guy that taught me to port cylinders and how to figure out time area and to be able to figure out what works and what does not work $100.00/hour and this was back in 1992. About 80 hours later it was all clear --- i could even see through my wallet, But seriously it was well worth the investment.

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Ok, so what are some of the factors involved, example increasing the overall feel of the powerband? From what I've read, porting is similar to timing, so would you heighten the exhaust port or something like that? Also, what would be the benefits of raising the boost ports and polishing them? Improved throttle response?

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8000$ to learn how to port? Aren't there engine building tech classes you can take? I remember i tried to get job at a machine shop to learn all about porting but the guy wasn't interested because I didn't take machine shop at the tech center during high school... The way they have highschool set up now is you either can learn tech skills like welding or auto shop, or you can go to college. cant do both. Thats why college students now a days don't which of the wrench to pick up.

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Yes, it's all about port timing. Raising the exhaust port makes it open sooner. This moves the general power band UP in the rpms, so you lose some low end. Same thing happens when you lower the intake. You can widen ports a SMALL amount before the rings want to go OUT the port instead of staying in the groove. But these changes are engine specific. Certain motors need more help in one area than another and if you just go nuts with a grinder, it will be a disaster. A safe approach (which you may not be interested in) is to smooth out casting bumps and improve the inherent flow this way. As for "polishing", I was told that you can polish the exhaust port as much as you want. The smoother, the better. Intake ports should NOT have a shiny finish. The slight texture helps the mixture atomization.

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First you have to analyze what you have now.

Measure the Deck height, port timing edge heights, port widths, overall port durations and blow down area.

Port roof angles, squish clearance , squish band width, trapped volume and the head volume.

This will give you an overall picture of what you have and you can determin what changes to make .

Lots of engines have adequate tranfer/intake area and the timing is good of the transfers is good , just the exhaust may be timed a little off- or vise versa . So you need to know what you have so you can determin what to adjust.

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Any porting spec for 2005 CR 250 Hon ? I wish to improve overall but low to mid and torque is best. Thanks.

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Yes, it's all about port timing. Raising the exhaust port makes it open sooner. This moves the general power band UP in the rpms, so you lose some low end. Same thing happens when you lower the intake. You can widen ports a SMALL amount before the rings want to go OUT the port instead of staying in the groove. But these changes are engine specific. Certain motors need more help in one area than another and if you just go nuts with a grinder, it will be a disaster. A safe approach (which you may not be interested in) is to smooth out casting bumps and improve the inherent flow this way. As for "polishing", I was told that you can polish the exhaust port as much as you want. The smoother, the better. Intake ports should NOT have a shiny finish. The slight texture helps the mixture atomization.

Did not know that, I would have polished them for sure given the chance before I read your post.

I would think that if you use a powerwing or what not to help with atomization, polishing out the intake to reduce drag would then be more beneficial yea?

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Any porting spec for 2005 CR 250 Hon ? I wish to improve overall but low to mid and torque is best. Thanks.

Better yet, does anyone have any model/year specific porting templates?

I remember back in my RX7 days people would port all the time, they had some porting templates that you could buy that would allow you to make the proper cut.

If there was a sleeve that you could insert, even half of one for a cylinder that would align with the dowel pins in which you could color in the area for porting with blue marker. Would be pretty cool.

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Did not know that, I would have polished them for sure given the chance before I read your post.

I would think that if you use a powerwing or what not to help with atomization, polishing out the intake to reduce drag would then be more beneficial yea?

About the only time you can really get away with polishing intake ports is with a gaseous or "dry" fuel like propane or CNG...which rules out the average two cycle application. The texture makes a thin boundary layer on the port walls that helps keep fuel from collecting on them in larger droplets and runs. That's why it's generally fine to mirror polish the exhaust side; there is no "wet" flow and the finer finish doesn't lend itself as much to carbon buildup.

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Yes, it's all about port timing. Raising the exhaust port makes it open sooner. This moves the general power band UP in the rpms, so you lose some low end. Same thing happens when you lower the intake. You can widen ports a SMALL amount before the rings want to go OUT the port instead of staying in the groove. But these changes are engine specific. Certain motors need more help in one area than another and if you just go nuts with a grinder, it will be a disaster. A safe approach (which you may not be interested in) is to smooth out casting bumps and improve the inherent flow this way. As for "polishing", I was told that you can polish the exhaust port as much as you want. The smoother, the better. Intake ports should NOT have a shiny finish. The slight texture helps the mixture atomization.

:banana: Correct statement. I ported for years (late 70s, 80s firt part of the 90s) and back in the days cyl casting was bad, realy bad casting flaws. Just cleaning the casting faws made a bike run cleaner and crisp. But in todays 2 stroke bike the ports are realy very good. If you want to do anything it would be just to mirror polish the exh. Todays bikes the port timing is so well rounded for bottom, mid and topend and I don't feel that porting is that important unless you can ride the wheels off your bike and need more top end.

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8000$ to learn how to port? Aren't there engine building tech classes you can take? I remember i tried to get job at a machine shop to learn all about porting but the guy wasn't interested because I didn't take machine shop at the tech center during high school... The way they have highschool set up now is you either can learn tech skills like welding or auto shop, or you can go to college. cant do both. Thats why college students now a days don't which of the wrench to pick up.

If the teacher knew what he was doing, $8000 is a bargain.

Your statement about college students not knowing which end of a wrench to pick up is false. I turn wrenches for a living and in 1 year I'll be a ME. My classmates work in machine shops. I will agree that there are a lot of students that are all books and have no clue how to actually work on something, but the few that have the knowledge and the hands on experience are dangerous:banana:

To the OP, there are a ton of things that go into the porting of a two stroke engine. You should check out A. Graham Bells book if you really want to learn something. This is a good site too http://www.macdizzy.com/

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Any porting spec for 2005 CR 250 Hon ? I wish to improve overall but low to mid and torque is best. Thanks.

any specs out there have been developed with somebodies time and money and they wont be given away,generally raising the transfer ports will help low and mid range,its easy to mess up a transfer port roof so the best way to accomplish this is to just raise the whole cylinder with a .060" spacer or some extra gaskets and cut the head the same amount to retain squish clearance & compression ratio.you must know how to measure your squish clearance if you plan on doing this.

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I agree with you. I would rather to pay someone to port my cylinder but fortunately or unfortunately, I can't have that done in Afghanistan where I'm at right now. So, With some know how and detailes, I've got to do it myself or perhaps take the specs to an machine shop here and have it done. It's the only fun I've got after duty hours. Thank you all for help. Oh, Aaron9696024, Thanks but those links are broken.

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