KMX oversized piston.

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stevo135+
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KMX oversized piston.

Post by stevo135+ » Sat Jul 14, 2018 11:39 pm

Hi all.

I've been trying to source a re-plated Nicasil cylinder and piston kit for my KMX125 to replace the existing cylinder which had been sleeved with a liner when the engine was built.
I've been offered a re-conditioned cylinder for a reasonable price thats been re-plated and comes with a Wosener forged piston kit, but it's a 56mm bore 133cc cylinder and does not come with a matching head. Is it likely that I will need to have the squish band in my head enlarged to 56mm before fitting if I go ahead with this cylinder kit and measuring the compression ratio and squish clearance, or am I better to look for a used std 54mm bore cylinder that I can send away for re-plating instead?

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Re: KMX oversized piston.

Post by Gerrit » Mon Jul 16, 2018 11:08 am

You will have to add the cost of modifying the head to the cost of the bigger bore cyinder and piston unless you can do the work yourself, plus the cost of a non-standard head gasket if it isn't included with the 56 mm cylinder and piston. This may or may not be cheaper than finding a standard bore cylinder and having that replated, plus the cost of the piston of course if you don't have a spare.
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James P
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Re: KMX oversized piston.

Post by James P » Mon Jul 16, 2018 2:18 pm

Gerrit wrote:
Mon Jul 16, 2018 11:08 am
You will have to add the cost of modifying the head to the cost of the bigger bore cyinder and piston unless you can do the work yourself, plus the cost of a non-standard head gasket if it isn't included with the 56 mm cylinder and piston. This may or may not be cheaper than finding a standard bore cylinder and having that replated, plus the cost of the piston of course if you don't have a spare.
What Gerrit said...but first check the outside diameter of the squish band on the existing head - they are sometimes a little larger than the standard bore size (the same may apply to the standard head gasket). Does the KMX have dowels to make sure the head is located concentrically every time it is fitted? If not, it would be wise to make such an arrangement, as the "safety margin" will shrink to practically nothing with the larger bore.
Also check whether the dome radius on the 56mm piston is the same as that on the original 54mm piston - it may complicate things if the are different (but probably nothing that careful machining can't sort out). If you will be machining the head anyway and finance allows, you may like to consider omitting the head gasket in favour of O-rings.

Is there a particular reason why you don't want to use the sleeved cylinder? Although sleeved cylinders are generally regarded as inferior to plated ones, it may be of no consequence if the job has been done correctly. Using what you've already got is usually the cheapest option, but it depends how much time/trouble/money you want to spend. Let us know how you get on!

Regards,
James

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JanBros
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Re: KMX oversized piston.

Post by JanBros » Mon Jul 16, 2018 10:58 pm

James P wrote:
Mon Jul 16, 2018 2:18 pm

Is there a particular reason why you don't want to use the sleeved cylinder? Although sleeved cylinders are generally regarded as inferior to plated ones, it may be of no consequence if the job has been done correctly.
a sleved one is always inferior, not matter how good the job has been done. because of the lesser heat dissipation between sleeve and cylinder.
don't think it will matter much in one "underpowered" KMX though.
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Re: KMX oversized piston.

Post by stevo135+ » Tue Jul 17, 2018 12:04 am

Hi thanks for all the replies and advice about my conundrum.
Firstly the 133cc cylinder I've seen for sale does come with new head and Base gaskets to suit. Im not sure if these are specially made for the larger cylinder or if they are modified original gaskets. I'll have to check this out.
I guess as said I need to add up the cost of having a firm enlarge the squish band in the head and match the profile to the piston etc, as I can't do this work myself. If it adds much more cost then I think I'd be better looking for a used 125cc cylinder to send away for a re-plate and hopefully it'll not be much more expensive.

The KMX head has no dowels fitted just 5bolts, and I guess there is the option of having an O-ring conversion machined into it. I think there's space for an inner and outer O-ring, as the head casting is the same physical size as the KMX 200. Would dowels and O-ringing be the best long term solution? As im intended on keeping this bike indefinitely.

And regarding the liner in the existing cylinder, we only found this was fitted when Kenny was developing the pipe. But as said depending on how well the liner was matched to the ports and the fact that the heat dissipation will be less, I've been told I could be losing 10-15% power. I figured that seeing as Kenny went to alot of trouble to design the ultimate pipe, and I've got a programmable ignition to fit, that I might as well try and get the most performance out of it.

The other issue is that my Base line dyno run showed about 18.7bhp with the current cylinder and all std except a Fresco pipe. The power drops like a stone just before 9000rpm, so we think that the KIPS valves are either not opening correctly, or that it's restricted still using the B-model KIPS spring which opens the valves at higher rpm than the unrestricted A-model. I plan on changing all the KIPS governor parts to A-spec when I do the top end re-build, so I know that it's the unrestricted set up.

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Re: KMX oversized piston.

Post by James P » Wed Jul 18, 2018 2:26 pm

JanBros wrote:
Mon Jul 16, 2018 10:58 pm
a sleved one is always inferior, not matter how good the job has been done. because of the lesser heat dissipation between sleeve and cylinder.
don't think it will matter much in one "underpowered" KMX though.
Yes Jan - I agree. As you point out, the cooling "inferiority" of a sleeve may not be noticeable on the KMX. Many earlier bikes used iron bores and seem to cope OK (although they were mostly cast-in sleeves instead of pressed-in sleeves...).

Steve; Does the KMX125 share many/any engine components with the KMX200? If so, could you get a better result using standard 200cc parts (i.e. same power and/or torque as modified 125, but a wider spread with less stress)...? I'm not sure whether the 200cc crankshaft will fit the 125cc crankcase, or whether the cylinders will interchange, but such a conversion may give you a more satisfactory result.
I'm not sure what other modifications you have in mind besides the pipe and ignition, but if you are trying to get as much performance from the 125cc engine as possible, you will likely have to de-restrict the air intake system (possibly also fit a bigger carb), modify the cylinder head, the cylinder porting and would be well advised to fit dowels if there is any slop in the cylinder head mounting which could cause the eccentricity.

Alternatively, you may like to consider using the sleeved cylinder for development purposes and see how it performs. If you were only going to discard it anyway, you haven't much to lose by trying it out - it may well be fine. Just make sure the ports match from the cylinder to the sleeve, that the porting is standard or at least suitable, that the piston clearance is correct and that it doesn't have any obvious defects.

If you are still considering buying the big-bore cylinder, you may like to ask; Why was it bored out? Is the piston clearance correct? Is the porting standard? Has it been modified in any other way?

For your info on the cylinder head dowel and O-ring conversions; I did these when I built my SDR. See the photos on this page: http://diffrentstrokers.com/phpBB2/view ... sc&start=0

Regards,
James

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Re: KMX oversized piston.

Post by stevo135+ » Wed Jul 18, 2018 10:50 pm

Hi James.
Thanks for your advice and the link to your head O-ring conversion project too. That looks like a great idea for my bike with the dowel upgrade too.

I've got a lot to think about then I guess. I understand alot of bikes like the Yamaha DT use linered cylinders and run very well, so as said Nicasil might not be needed on a KMX. I just thought because they had it from new that it would have been better and maybe my bike is less powerful than a std one? 2bhp or so is alot to give up on a 125 relatively.

Believe it or not I do have a fresh re-plated and professionally ported 200 cylinder and modified head sat on a shelf ready. The 200 fits the 125 cases and the crank is the same 54.5mm part too. Its just stuff like the carb, radiator and clutch that are different, and I've got upgrades for all those ready too.
The only problem is if I fit the 200 top end on this engine, the custom pipe I've got won't really work, and I've not got a 200 original pipe as they are rare as hens teeth now.

I think the 56mm big bore cylinder conversion I saw was done to get rid of damage after a seizure as there is a Wosner piston to fit a 56mm bore. Im not even sure if I should be using a forged aftermarket piston or if I'm causing more issues for myself? I've not really planned ahead very well and just been buying uprated parts that I thought would be a good idea.

I think I'll have to contact a firm to see about getting the 125 head O-ring conversion fitted and some dowels put it like you say, as that should be a good future upgrade regardless and mean I wouldn't have to worry about head gaskets when re-building the top end next time and checking the squish clearance etc.

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James P
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Re: KMX oversized piston.

Post by James P » Sat Aug 04, 2018 11:12 am

stevo135+ wrote:
Wed Jul 18, 2018 10:50 pm
The 200 fits the 125 cases and the crank is the same 54.5mm part too. Its just stuff like the carb, radiator and clutch that are different, and I've got upgrades for all those ready too.
The only problem is if I fit the 200 top end on this engine, the custom pipe I've got won't really work, and I've not got a 200 original pipe as they are rare as hens teeth now.
How about that? I didn't know the 125 and 200 shared the same crankshaft! As you have all the 200cc parts already, you could either use them or keep them in reserve if the 56mm 125 set-up doesn't work out. I don't suppose your pipe builder could modify the pipe to suit the 200cc set-up...?

Whatever option you choose, let us know how it turns out!

Regards,
James

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