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How to find what sized piston i need with an overbored cylinder

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I have a new cylinder that i bought to rebuild my 1993 kawasaki kx250 with, because the old cylinder was destroyed. The guy said the cylinder is overbored a little but never gave us numbers. The new one i bought came with a piston thats been used and i would like to replace. I believe the piston that came with the cylinder is the correct size, but it very well could be off and i have no way of knowing. So how do i properly measure the cylinder bore, and how do i use this measurement to find what size piston I need to get?

 

thanks,

Kaelan

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The cylinder will have a slight taper to it if it was bored correctly. The area where it will be its smallest will be at the top of the cylinder by the head. Use an apropriately sized inside micrometer. Most machine shops will have one. NEVER use calipers. Not only will you get a slightly inaccurate result but depending on how the cylinder is plated you rist scoring it. I put a little bit of 2 stroke oil or lightweight oil on the (pawls, pins?) Of the micrometer to avoid scratching. Its also important to do that to the cylinder when you install the piston and rings or you will scratch the cylinder quite badly before the system builds the oil pressure to properly lubricate the cylinder.

The other thing to look out for is ordering the right rings. And you need to watch out for achieving the right minimum cylinder volume proportional to the new volume of the cylinder. This is relatively easy as you just need the distance of piston crown to the center of the wrist pin to be the same in most cases. (Unless you also want to mess with the compression ratio to run a different fuel and make more or less power). The cases where this is not true is when the geometry of the piston drastically changes or of the head. Do you have a head that will fit this cylinder? If the head is significantly overbore then you run the risk of decreasing the compression ratio by keeping the smae piston 'height' as most cylinder heads are domed or some other shape to accomodate valves. Most people dont scale the head geometry with the cylinder geometry so on significantly overbored engines the effective minimum cylinder volume compared to the maximum does not stay constant between pre-bore and post-bore resulting in performance loss and sometimes knock.

Aside from that it is generally easier to go to a engine centric machine shop amd have them give you the recomendations. On my last overbore I cnc-ed a piston I designed and it ended exactly as you would expect... Horribly. There is a lot more to piston design and cylinder design than most people realise. Sometimes when you go to a non-engine-centric machine shop amd ask them to overbore they do a piss poor job and the cylinder geometry is wrong and they did not put a fake head on the block to simulate the distortion caused by the screws holding things down or compensate for things like different thermal expansion in airs that have more or less heat. So it might be worth taking the cylinder to a proper engine shop to get their oppion on the prior job. Just nake sure you go to an honest one or they will try and sell you services you dont need.

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On 8/24/2018 at 7:15 PM, broken.biker said:

The cylinder will have a slight taper to it if it was bored correctly. The area where it will be its smallest will be at the top of the cylinder by the head. Use an apropriately sized inside micrometer. Most machine shops will have one. NEVER use calipers. Not only will you get a slightly inaccurate result but depending on how the cylinder is plated you rist scoring it. I put a little bit of 2 stroke oil or lightweight oil on the (pawls, pins?) Of the micrometer to avoid scratching. Its also important to do that to the cylinder when you install the piston and rings or you will scratch the cylinder quite badly before the system builds the oil pressure to properly lubricate the cylinder.

The other thing to look out for is ordering the right rings. And you need to watch out for achieving the right minimum cylinder volume proportional to the new volume of the cylinder. This is relatively easy as you just need the distance of piston crown to the center of the wrist pin to be the same in most cases. (Unless you also want to mess with the compression ratio to run a different fuel and make more or less power). The cases where this is not true is when the geometry of the piston drastically changes or of the head. Do you have a head that will fit this cylinder? If the head is significantly overbore then you run the risk of decreasing the compression ratio by keeping the smae piston 'height' as most cylinder heads are domed or some other shape to accomodate valves. Most people dont scale the head geometry with the cylinder geometry so on significantly overbored engines the effective minimum cylinder volume compared to the maximum does not stay constant between pre-bore and post-bore resulting in performance loss and sometimes knock.

Aside from that it is generally easier to go to a engine centric machine shop amd have them give you the recomendations. On my last overbore I cnc-ed a piston I designed and it ended exactly as you would expect... Horribly. There is a lot more to piston design and cylinder design than most people realise. Sometimes when you go to a non-engine-centric machine shop amd ask them to overbore they do a piss poor job and the cylinder geometry is wrong and they did not put a fake head on the block to simulate the distortion caused by the screws holding things down or compensate for things like different thermal expansion in airs that have more or less heat. So it might be worth taking the cylinder to a proper engine shop to get their oppion on the prior job. Just nake sure you go to an honest one or they will try and sell you services you dont need.

Thanks for all the info, sounds like you know what your talking about. I bought this cylinder and it was previously re sleeved, and its not really super overbored it just has a very slightly larger diameter because of the resleeve. I didnt know what else to say other than overbored, or people would just tell me to buy a stock piston and thats not the answer im looking for. But based on what you said, I should just bring my cylinder over to a machine shop and ask them what size piston ill be needing? And yes I do have a stock head that fits being that my cylinder is not overbored to an extreme. 

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