How can I easily determine what piston size I need?

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Bitteeinit
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How can I easily determine what piston size I need?

Post by Bitteeinit »

Sorry for the newbie question. I'm getting a new top-end (on a bike with little in terms of documented history) but was wondering what piston size I should get. Standard (66?), or +0.5? Should I just get the same as the current piston whatever it is? I'm not sure if this cylinder has been sleeved or not. I'll go check tomorrow morning (magnet test). If there's no damage to the cylinder, then it's safe to assume I can just measure the old piston and get the same size? If the cylinder is damaged, then I'd have to get it either honed or bored and get 66+0.5? I know these engines are plated, but I doubt anyone here can plate these (South East Asia) and I have a strong feeling it might already be sleeved. If there's serious damage (which I didn't notice when I had quick glance today), then I should just go 0.5 over the current size? The previous owner told me something about the piston being +0.25 (told me to get +0.5...) which is kind of weird. Is +0.25 even a size?

Sorry for sounding like an idiot.
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Julien D
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Re: How can I easily determine what piston size I need?

Post by Julien D »

If it's sleeved and doesn't need a bore, you must replace with the same size that comes out. If it's sleeved and damaged it will need a bore up to the next size with appropriate piston. You can't just slap an oversize piston in a damaged cylinder without boring it out.
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pumpguy
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How can I easily determine what piston size I need?

Post by pumpguy »

If your cylinder has a sleeve, it will be obvious. When you look at the inlet and transfer ports, you will clearly see the difference between the sleeve and the aluminum cylinder block.

If your cylinder is damaged and has a sleeve, as Julien D says, you will need to bore to the next oversize. It's always best to get the piston first, then give the new piston and bad cylinder to the shop that's going to do the work. That way they can size the cylinder bore to the new piston with the correct clearance. The manual calls for cylinder to piston clearance of 0.079 to 0.089 mm for the 200, 0.095 - 0.105 mm for the 220.

The standard bore for the H series KDX 200 is 66.026 - 66.041 mm. The KDX 220 standard bore is 69.025 - 69.040 mm. If your cylinder has significantly different dimensions, it has been fitted with an oversize piston.

Hope this helps.
Bitteeinit
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Re: How can I easily determine what piston size I need?

Post by Bitteeinit »

Thanks guys. For sure I understand that I would need to rebore.
The standard bore for the H series KDX 200 is 66.026 - 66.041 mm. The KDX 220 standard bore is 69.025 - 69.040 mm. If your cylinder has significantly different dimensions, it has been fitted with an oversize piston.
Ah! That explains why the guy said it was .025! How can I measure the inside of the cylinder? I used calipers to measure the piston but didn't measure the cylinder yet because I don't have a bore gauge (and I'm not sure the local shop has one either).

I checked it out this morning (forgot to bring a magnet), but I'm pretty sure this is what you mentioned about it being obvious... Seems it's definitely been sleeved already. The piston also seemed to be 66.5, not a 66 (maybe even 66.6?!). They only had a normal old-school caliper, but I think my reading is correct? Do you figure I should get a 67 and rebore the sleeve, or stick with the same size (66.5)? The sleeve looked a bit scratched so I don't know if it would need a rebore or just a honing. If one only gets a honing can you stick with the same sized piston or does one move up anyways? How to I know if the bore is "damaged"? I honestly don't know how smooth the insides of the cylinder sleeve should be.

Obviously the piston is done... :shock:
Does the sleeve look too thin already?

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Sleeved?
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So what are my options at the moment? If I buy too small then I'd have to resleeve? No easy task I take it. Please correct me if I'm wrong regarding my options:
1) Buy a 67 wiseco piston/ring kit. Rebore the sleeve/cylinder and put the engine back together again (new gasket set?).
2) Buy a standard sized 66.0 piston and resleeve? Or is resleeving out of the question/impossible?

Thanks for any input. I'm about to get someone to bring me over a new piston/ring set but want to make sure I've got this right because it's not like I can just send t back to exchange it.
pumpguy
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How can I easily determine what piston size I need?

Post by pumpguy »

Yep, it's been sleeved. If you don't have access to Inside Diameter Micrometers or a set of telescope gauges, there's still a way you can measure your bore diameter quite accurately.

Take a good stout nail, say 10d size, and cut the head off. The nail should be longer than your bore's diameter. Then round off the ends with a file. You can do the rounding off quite easily by chucking the nail in a drill press or pistol drill and with the drill running, hold the end of the nail against a file. Continue to remove metal from the end of the nail until the nail will just rock through the bore across the diameter. Now measure the length of your nail, and that's your bore diameter at the place you measured.

Hope this helps. Good luck.
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