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Hole on intake side of piston. Is it needed?

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Hey guys,

My buddy has an 03 cr125 and ordered a new piston for it. However, when the piston came it we noticed that on the intake side it had no hole like the stock piston did have.

But, this is an OEM honda cr125 piston. SO my question is, do the newer than 03 or previous year 125 have pistons that dont have a hole on the intake side? Will it hurt the bike if he runs this piston? Also, this piston is domed on the top instead of being flat like the top of the stock piston.

Thanks, Dave.

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the window n the piston is nothing to worry about..............on the other hand, the domw may well cause a serious problem. A falttop and a dome piston each require theri own head cut and configuration. You may loose performance running a flat top in a dmoe cut head, butif you run a dome pistno in a head cut for a flat top, you could potenially cause some damage. Worse case scenario would be if you are running a flattop cut head with a pretty narrow squish clearance, a dome piston could potentially hit the cylinder head. If it doesn't hit the cylinder head you could still be cutting the squish down so narrow at a point that dramatically reduces gas flow to the preimeter of the cylinder.

Send it back to wherever he got it and get a piston that matches what he had, as I assume that if he had a flattop in it, then the head must be cut for a flattop piston.

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the window n the piston is nothing to worry about..............on the other hand, the domw may well cause a serious problem. A falttop and a dome piston each require theri own head cut and configuration. You may loose performance running a flat top in a dmoe cut head, butif you run a dome pistno in a head cut for a flat top, you could potenially cause some damage. Worse case scenario would be if you are running a flattop cut head with a pretty narrow squish clearance, a dome piston could potentially hit the cylinder head. If it doesn't hit the cylinder head you could still be cutting the squish down so narrow at a point that dramatically reduces gas flow to the preimeter of the cylinder.

Send it back to wherever he got it and get a piston that matches what he had, as I assume that if he had a flattop in it, then the head must be cut for a flattop piston.

Thanks for the detailed post.

We have ran the bike with the piston and it is running fine right now with no knocking or pinging heard. I'm going to keep a watch on the plug colour to see if its running lean or not.

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If you have a domed piston installed with a head cut for a flattop piston, I highly recommend you change it right away. You WIll suffer damage. Heed what Charlie had to say, or I can predict what your next post will be, then we can say,"we told ya so"! Tdub

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If you have a domed piston installed with a head cut for a flattop piston, I highly recommend you change it right away. You WIll suffer damage. Heed what Charlie had to say, or I can predict what your next post will be, then we can say,"we told ya so"! Tdub

What kind of damage will happen?

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What kind of damage will happen?

It's all about clearances....after engines warm up metal expands and you start to gain clearances in some places and loose clearances in others. At high rpms it gets worse. You take a chance on hitting the plug and or head with the piston, being that it is domed. That is if your head is made for a flat piston. The piston could hit, then yahooo look out! :thumbsup:

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Do you hav a set of calipers? If not get a pair, even a cheap pair will help. then get some solder. Check your squish. DO you know how to do that?? Just stick the solder in the plug hole and push it out to the side until you hit the cylinder wall. Then run the piston up to TDC. and squish the solder. pull it out and measure its thickness and also in this case you will want to look at the shape of the solder. With a dome piston in a head cut for a flat top, what you may see in the solder is a > shape to it. it will be wider at the very tip than it is a few mm in from the tip. This is because pistn height at the edge of the piston will be the same in either one, but as the dome risesup from the edge, it will come closer to head. SO your solder will be wider at its furthest edge and then narrow downas it gets to the point that the dome is rising and cutting down the clearance. I've seen them where a dome piston was put in a falt cut engien and the squish at one point was so narrow that the solder brok when we tried to measure it!!

Even if you don't have a set of calpiers to measure it, at least stick a piece of solder in it and check its shape. Looking at the piston crown and plug will do you no good. it will be at the very perimeter of the piston where the damage will occur. THe plug and crown may look perfectly fine!

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