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  #1  
Old 13-01-2013
oops oops is offline
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YZ250 rebuild/port matching

Hi,

I am in the process of doing a full rebuild on my 2009 yz250 engine. I will be installing a new crank and piston. The gearbox bearings were done not to long ago. I figured whilst I have this apart I might as well tidy the ports up a little, match them to the case halves, measure the squish and hopefully set that. I don't want to make radical changes to the motor, just set it up the way the factory Intended. I have attached the photos with what I was planning on doing, any advice would be appreciated since I haven't done this before. Is it worth polishing up the exhaust port a little?

Also is the pattern on the piston normal?

Cheers
Adrian.
Attached Images
File Type: jpg Piston in cyclinder.jpg (58.6 KB, 58 views)
File Type: jpg Piston.jpg (42.0 KB, 58 views)
File Type: jpg Cylinder base.jpg (43.7 KB, 68 views)
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  #2  
Old 13-01-2013
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choppiektm choppiektm is offline
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How many hours on that piston?
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  #3  
Old 13-01-2013
oops oops is offline
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Not to sure a couple I'm guessing. I do have a shiny new wiseco one and an hour meter to go in though
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Old 13-01-2013
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Gasman Gasman is offline
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don't round the transfer port bridge............just leave it.

don't add epoxy resin either, no need to and to risky.

also don't smooth things out too much or remove too much material, it upsets the crankcase pressure.

the trick is to match the ports up so the fuel going up doesn't have any resistance........you need to have the cases split and the crank out to do it properly.

just clean out any big lumps in the casing, leave the rest as is.
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Old 15-01-2013
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Could it be that the die's used at the yamaha factory are getting old, when comparing the finish of the port walls it is a lot lumpier than an RMZ head I have sitting on my bench?

Thanks for the reply gasman.
When you talk about matching the crankcase with the cylinder do you just grind the inner edge off? just below the gasket surface the crankcase walls are pretty thin, so that kinda limits how much to take off, is this correct? I have attached a pic of what I mean.

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Adrian
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File Type: jpg crankcase.jpg (61.6 KB, 42 views)
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  #6  
Old 15-01-2013
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Don't let gasman scare you off. Build her up with some Devcon. I have never seen it come off.

Also, don't be afraid of smoothing things. There is that much turbulence the intake charge will go through after the case line of the the transfer ports, a smooth-ish run in wont upset things.
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Old 16-01-2013
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I posted up mine last year

http://www.dirtbikeworld.net/forum/s...hlight=porting

my understanding of porting is just take the dags off, as the fuel/air mixes better ..aslo a golf ball goes further with dimpling than smooth.

and don't take too much out or it stuffs up the crankcase pressure/volume


plus subsonic air goes quicker over smooth edges and supersonic air goes quicker over sharp edges.(crankcase air is subsonic)

cheers
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Old 16-01-2013
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Id build it up with epoxy as first preference before i ground the cases back. Make sure the area under the epoxy is clean and roughed up nicely and it wont come off. Ive never seen epoxy come off thats been applied properly. Blend it into the rest of the transfer duct as far up the duct as you can go making sure the outer radius is nice and even. A smooth, but not polished, surface is fine. Dont sharpen any edges either.

Just be careful though, youll catch the bug like i did and it may never stop!
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