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What size and type of piston to buy?

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Hi there guys, oky i took my top with barrel to the bearing works today. Some good and bad news - depends on your answers... not sure yet..

Oky they measured the barrel. At the top its 66.50mm and bottom 66.45mm. See i think the honing cased it to become a little bigger at the top.

Now i asked the guy working at bearing works what he thinks, with not to much assurance he said a standard piston will work. But he recommends they bore it.

One side:

I want to keep it as standard as possible (stock piston is cheaper)

Other side: I don't want to take chances. Good news bore will only cost 25$.

So lets say you guys think its better to bore because standard wont do the trick, what size should i go with? Wat type of piston? I see alot of

Namura pistons on ebay... how are they?

Oh and what's the pros and cons of forged vs cast pistons?

This project is killing me at this stage, but who cares... she (Honda cr250 1999) is worth it.

Your advise and knowledge will be much appreciated!

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I would definitely have it bored assuming it's a sleeved cylinder , I run Namura pistons in the last 2 cr 250s i built no problems, Still riding a 01' with a Namura . Cast pistons expand quicker than forged pistons they are not as strong but it came from honda with a cast piston and you are not running nitrous , are ya'? Just make sure to take your piston to the machine shop and have it bored to that piston for cylinder wall clearance , check ring end gap , i got mine set at .08 but .10 will be fine with a double ring it's not going to blow by with a single. Make sure all your ports are chamfered . All clearances correct and assembled properly you won't have any problems. http://www.ebay.com/itm/66-84mm-PISTON-GASKET-KIT-HONDA-CR250-CR-250-1997-98-99-00-01-/251518042810?pt=Motorcycles_Parts_Accessories&hash=item3a8fa4c2ba&vxp=mtr

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I hope you mean take that cylinder to have it bored out for the piston beau lol. Do what beau said, good info.

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I hope you mean take that cylinder to have it bored out for the piston beau lol. Do what beau said, good info.

You know what i meant  ;)

1513730_892170374143260_3863979976978052793_n.jpg

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See pictures of varrel.. going to bire it, just have to find out whats the next best size to go for.. oh and sorry, i know now you cant measure a cylinder this way. Also see attached the gasket set and con rod i bought . Believe they will do the job? . :)

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I would definitely have it bored assuming it's a sleeved cylinder , I run Namura pistons in the last 2 cr 250s i built no problems, Still riding a 01' with a Namura . Cast pistons expand quicker than forged pistons they are not as strong but it came from honda with a cast piston and you are not running nitrous , are ya'? Just make sure to take your piston to the machine shop and have it bored to that piston for cylinder wall clearance , check ring end gap , i got mine set at .08 but .10 will be fine with a double ring it's not going to blow by with a single. Make sure all your ports are chamfered . All clearances correct and assembled properly you won't have any problems. http://www.ebay.com/itm/66-84mm-PISTON-GASKET-KIT-HONDA-CR250-CR-250-1997-98-99-00-01-/251518042810?pt=Motorcycles_Parts_Accessories&hash=item3a8fa4c2ba&vxp=mtr

see above

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See pictures of varrel.. going to bire it, just have to find out whats the next best size to go for.. oh and sorry, i know now you cant measure a cylinder this way. Also see attached the gasket set and con rod i bought . Believe they will do the job? . :)

I would go with that link i posted to make sure the cylinder was good and cleaned up, you will have to shave your power valve also if not it will hang a ring too. You can go with any name brand piston you like, if you choose a forged piston like wiseco you will just need to let the bike warm up longer before riding for heat expansion. Cast pistons just heat up and expand faster . Pro X pistons i don't like they are heavy and just look cheap made with sharp edges , Vertex is good , wiseco and Namura both are Molybdenium teflon coated for reduced wear and anodized domes for heat transfer. Namura is almost half price of wiseco if you're building a  race bike go with a forged wiseco , trails and playing in fields Namura and save some cash for v-force 3 reeds  :thumbsup: 

Edited by Beau 88

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See pictures of varrel.. going to bire it, just have to find out whats the next best size to go for.. oh and sorry, i know now you cant measure a cylinder this way. Also see attached the gasket set and con rod i bought . Believe they will do the job? . :)

Who ever builds that crankshaft best know what that are doing and have the right tools to do it. Cranks on these CRs are hollow and can be crushed easily . Good luck.  :ride:

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I would go with that link i posted to make sure the cylinder was good and cleaned up, you will have to shave your power valve also if not it will hang a ring too. You can go with any name brand piston you like, if you choose a forged piston like wiseco you will just need to let the bike warm up longer before riding for heat expansion. Cast pistons just heat up and expand faster . Pro X pistons i don't like they are heavy and just look cheap made with sharp edges , Vertex is good , wiseco and Namura both are Molybdenium teflon coated for reduced wear and anodized domes for heat transfer. Namura is almost half price of wiseco if you're building a race bike go with a forged wiseco , trails and playing in fields Namura and save some cash for v-force 3 reeds :thumbsup:

Its just a toy for me... Numura it is. Thanks for the advise.

Im still new to building bikes, doing it as a fun project. What do you mean by " have to shave your power valve also if not it will hang a ring too" ?

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Its just a toy for me... Numura it is. Thanks for the advise.

Im still new to building bikes, doing it as a fun project. What do you mean by " have to shave your power valve also if not it will hang a ring too" ?

Once you bore the cylinder the ends of the power valves will be sticking out into the cylinder bore . The ends of them will have to be ground down for piston clearance . Mine were in a cylinder that was cut .80 so they are cut way back for the stock bore i'm running , it decreases a small amount of low end power but not enough to notice . 

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Once you bore the cylinder the ends of the power valves will be sticking out into the cylinder bore . The ends of them will have to be ground down for piston clearance . Mine were in a cylinder that was cut .80 so they are cut way back for the stock bore i'm running , it decreases a small amount of low end power but not enough to notice .

Thanks so much!! Jee good people do still exist. Last thing before i start hunting for the correct piston The small end bearing.. mine still seems fine. Con rod is not extremely tight on it. You guys think i can still use it with new Wiseco that's on its way? Below is a few pictures

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 if you choose a forged piston like wiseco you will just need to let the bike warm up longer before riding for heat expansion. 

 

This is a key point, and it actually applies to both forged and cast pistons. Warm the engine up fully before riding.

 

 

 

Cast pistons just heat up and expand faster .

 

You have that backwards. Forged pistons will expand at a sightly faster rate due to slightly lower silicon content. But the truth is that with modern alloys and modern forging techniques there is very little difference in the expansion rates or amounts between forged and cast pistons.

 

 

Pro X pistons i don't like they are heavy and just look cheap made with sharp edges , Vertex is good , wiseco and Namura both are Molybdenium teflon coated for reduced wear and anodized domes for heat transfer.

 

Wiseco pistons are not coated, and Pro-X is just rebadged OEM.

 

 

 

 

Modern forged pistons do not expand significantly more than cast pistons. A typical 4032 (approx. 11-13% silicon) alloy forged piston will expand about 13% more than a typical high-silicon (15-18% silicon content) hypereutectic piston. 13% of 2/1000's of an inch is only .0003". That's three ten-thousandths of an inch. Not an engine killer at all.

 
Now years ago, forged pistons were much more likely to be made with 2618 alloy (less than 2% silicon), and yes, expansion was a problem. But with modern alloys and forging techniques, the amount of expansion differs very little between cast and forged pistons. The only real issue is the RATE of expansion. Forged pistons will expand FASTER than cast pistons. And since the cylinders in our bikes are made from the same alloys as the pistons, this means the forged piston can expand faster than the cylinder. But, bottom line, warm your bike up before riding and that issue disappears.
 
The higher silicon content of cast pistons is also their Achilles heel. The higher the silicon content, the more brittle the alloy becomes. That's why cast pistons tend to break skirts when they've been run too long. Just dropping a cast piston can break it, where dropping a forged piston will likely only dent it (although it will still be ruined).
 
 Both piston types have advantages and disadvantages, but at least in my opinion the advantages of forged pistons far outweigh their disadvantages.
 
Forged pistons are more durable than cast pistons will ever be. Cast pistons are very brittle, and are prone to skirt breakage as a result.
 
For riders that will take the time to do a proper break-in, and will discipline themselves enough to fully warm their engines before each and every ride, Wiseco pistons are a superior product to OEM. They are more durable under high-heat high-load conditions, less prone to skirt breakage from piston slap if the bore has a bit too much wear on it, and longer wearing than any cast piston will ever be.
 
Cast pistons don't require much in the way of special break-in, that's part of the reason they come as OEM equipment in two-strokes. Forged pistons, on the other hand, are a different story. A forged piston should be heat-cycled before it's run hard. 
 
 
 
To better understand why forged pistons are superior, you need to understand what makes a forged piston different than a cast piston. 
 
A cast piston is manufactured by pouring molten metal into a mold. The final shape is machined to it's final exacting tolerances. 
 
A forged piston is made by taking a chunk of metal, and essentially "beating" it into shape with a die-press under enormous pressure. Like casting, the final shape is achieved through precise machine work. 
 
The main difference between a cast and forged piston is the grain structure. A forged piston is beaten into shape, and as a result the metal stretches and compresses as the piston takes shape. The varied, elongated grain structure is like fiber reinforcing, and it makes for a very strong piston. Microscopic cracks don't readily propogate through the structure of a forged piston due to the high density and the irregularly spaced and sized grains. A cast piston, on the other hand, is made up of grains that are all the same size, because it starts out as a liquid that, after being poured into a mold, undergoes a controlled cool-down process that allows the metal to reach a near-perfect equilibrium right out of the mold. The highly regular grain size and distribution makes them more prone to crack propogation and failure. Forging produces a higher grain density than casting, making the part much more durable under high-heat, high-load conditions. As long as you are patient enough to break a forged piston in correctly, you will have a piston that is more durable under extreme conditions.
 
Modern forging techniques have all but eliminated the expansion problems of old that forged pistons were once known for.
 
(I'm shamelessly ripping this off from Grayracer, as he explained it well and there's no reason for me to re-type what he already expressed so perfectly)
 

Contemporary aluminum forging is not done by taking a warmed up lump of material and hammering it into shape as it once was, at least not with things like pistons. The reason for moving away from such processes involves the simple fact that in order to be formed in such a manner, the metal must be malleable enough to tolerate the forging process. Aluminum can, of course, be made suitably malleable, but there was a problem the sprang from the relatively high rate of thermal expansion of aluminum alloys relative to the iron bores they typically ran in.
 
In order to use an aluminum piston you have to have enough cold clearance to allow the piston to expand into its correct size as the engine warms. Adding silicon and some other materials to the alloy greatly reduces the thermal expansion and allows tighter cold fitting. The problem is that it also renders the alloy too hard to forge by older conventional means. The solution is a modern forging process that is somewhat a hybrid of casting and forging, something you might call pressure casting.
 
The alloy is brought to the edge of being molten, a carefully controlled transitional phase where the metal is not really solid anymore, but not quite a liquid, and is pressed into the forms used to create the rough shape of the blank piston. Pressure is increased as the metal's temperature is carefully reduced, and some impact is often applied while pressure is maintained. All this has the same effects on the grain structure as mentioned above, but allows the use of high silicon, low expansion alloys that result in pistons that can be fit snugly into their bores, avoiding the cold slap and other problems that used to go with forged pistons.
 
Casting processes have also been advanced tremendously over the last 30 years or so, mostly by advances in the control of metal temperatures and in the alloys used. Cast components are not necessarily weak or inferior anymore, depending somewhat on the metal being used and the purpose to which the finished part is put. Processes like investment casting (also called "lost wax" casting) can produce very strong, high quality blanks with near finished dimensions, but it's mostly used with steels.
Edited by Chokey
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Thanks so much!! Jee good people do still exist. Last thing before i start hunting for the correct piston The small end bearing.. mine still seems fine. Con rod is not extremely tight on it. You guys think i can still use it with new Wiseco that's on its way? Below is a few pictures

Yes they can be used again and people do it all the time but why ? I wouldn't risk all my new parts and work for 12 bucks. I never reuse them and actually keep a few spares. Speaking of reeds check those for cracks and i think the gap at the edges are .01 but check your service manual. 

Edited by Beau 88

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