KMX 125 engine re-build.

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JanBros
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Re: KMX 125 engine re-build.

Post by JanBros » Tue Nov 05, 2019 11:00 pm

stevo135+ wrote:
Tue Nov 05, 2019 8:25 pm
just bore it out the 0.6mm that I need to match the bore. I think that would work.
how much is 55.3 + 0.6 ?
and how much is your bore ?

I don't think it will work ...
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Re: KMX 125 engine re-build.

Post by stevo135+ » Wed Nov 06, 2019 1:23 am

Hi. The piston size is 55.94mm, so I presumed that the head gasket ideally wants to be just a fraction bigger than the bore size. Wouldn't 56mm exactly be OK?

I was thinking that boring it out by such a small amount won't effect the fire ring area of the gasket and so I can't see why it couldn't be a reliable solution?

If it proves not to be a good solution then I still have the option of getting a good engineering shop or tuner to machine a series of O-ring grooves into the head, though I'd lose 0.25mm of my squish clearance then unless I lift the cylinder with a thicker base gasket.

Or maybe I can find a company that can make me a custom head gasket that's a bit bigger using the original one as a pattern. I'm keen to get this sorted out one way or another, but in an ideal world I won't have to remove the cylinder or raise it, as I'm worried doing that could mean my exhaust won't work as well as it should then.

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Re: KMX 125 engine re-build.

Post by 500bernie » Wed Nov 06, 2019 7:58 am

Just scribe it against the barrel and carefully take off the excess with a dremel it's not difficult (just messy, wear gloves and goggles).
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Re: KMX 125 engine re-build.

Post by JanBros » Wed Nov 06, 2019 7:09 pm

stevo135+ wrote:
Wed Nov 06, 2019 1:23 am
Hi. The piston size is 55.94mm, so I presumed that the head gasket ideally wants to be just a fraction bigger than the bore size. Wouldn't 56mm exactly be OK?
all you have to do is make the head gasket exactly the same diameter as the bore, not the same as the psiton as the piston rocks at the dead centers like I already told you .

That's why I said "don't do half work". If you're going to take away 0.3 al arround, why stop there and not take away that last 0.05mm ? it would makes no sense at all !
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Re: KMX 125 engine re-build.

Post by stevo135+ » Wed Nov 06, 2019 8:48 pm

I see what you mean now. Yes I'll make sure that I open the gasket ID to 56mm and then as you said I should be OK.

Just out of interest what is the highest effective compression ratio that people have run on the KR1'S and how did it affect the power curve?

I still keep seeing 125cc Motocross bikes with compression ratios in the 8-9:1 range, and assuming they run on super unleaded, there must be something in the way that they are designed or set up that allows such high compression ratios to be reliable?

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