KMX 125 engine re-build.

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KMX 125 engine re-build.

Post by stevo135+ » Sat Oct 26, 2019 6:20 pm

Hi all.

I have recently got my KMX 125 back together and I'm just doing the coolant plumbing for the bigger custom radiator I had made.

I went with the 2mm oversize cylinder and piston kit that had been bored out and re-plated to suit a Wossner 56mm forged piston, as the price was cheaper than new kawasaki parts and the ports have been lightly matched and cleaned up at the port windows too.

I found the std head gasket to be 55.3mm bore and with the piston only 0.6mm larger in diameter it seemed to have no problems with clearance or the piston touching the gasket. (The piston with a 0.5mm standard base gasket does not quite come flush to the top of the cylinder.)

Now the next thing I did was measure the std squish clearance which was 1.32mm as accurately as I could measure it with solder and then plasticine at each side and front and rear of the piston crown.

I've now fitted a head that's been skimmed by 0.25mm and a new genuine head gasket. The old head gasket measured 0.25mm compressed. My squish clearance now seems to be 0.98-0.99mm which I thought was a good number?

I've got a drizzle of oil around the piston, and big and small end bearings, which might push up the compression a touch, but when I fit a spark plug and kick the motor over the compression feels very high on the kick start. I can still turn the motor over using my hand to operate the kick start but it's really hard.

I just wanted some advice from people before I run it, as Ive been told that a 2mm oversize piston and a skimmed head will push up the compression significantly. I didn't cc measure the head volume as I didn't have any accurate liquid measuring cylinder or burette available.

I've fitted a BR9ES plug which is what the manual recommends for fast riding, and I will use super unleaded fuel only. I'm starting to worry as if my compression is that much higher than standard which is supposed to be 7.8:1 corrected, then am I likely to damage the motor before I can run it in?

I will be using a programmable CDI that the dyno guy/tuner is going to fit for me, but to start with I'm running a factory CDI. The manual says it advances to 26.5degrees BTDC at 4000rpm, which might be too much advance for a bored out motor with a skimmed head?

Should I fit a BR10 grade plug to start with and I thought about mixing some octane booster with super unleaded fuel if that would help?

I've got a new 30mm flat slide carb on it too, which I have copied the jetting spec from a Suzuki RG125FUN that uses the same carb. I've gone up two sizes on both the pilot jet and main jet as a starting point.

My aim is to get the bike running, do a heat cycle and then start it from cold and after warming up do a 5-10min ride round the block before re-torqing the head and cylinder nuts. I'd like to put around 100miles on it before I take it to the dyno and get it set up properly and the new CDI fitted and mapped. The only trouble is that apart from the bottom end nothing is standard so I could be miles out with the jetting and running too much compression maybe?

Any advice will be much appreciated!

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Re: KMX 125 engine re-build.

Post by JanBros » Sun Oct 27, 2019 12:10 am

first and most important question for me would be : what is the deckheight ?
i.e. the distance from the top of the cylinder to the edge of the piston in TDC.
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Re: KMX 125 engine re-build.

Post by KR-1R » Sun Oct 27, 2019 1:34 pm

.
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what was the piston bore clearance for your forged piston
do a basic mechanical compression test to second indicate what the CR might be

get Maccas/GasketGuru to do a thick base gasket to compensate for skimmed head
prob 0.25mm thicker elliminates the swept volume comp change from +2 oversize

what does Martin do to modify 300cc KR head gaskets?

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Re: KMX 125 engine re-build.

Post by stevo135+ » Mon Oct 28, 2019 12:33 am

Hi, Thanks to JanBros and KR-1R for your input and advice it's much appreciated!

First off I can't be 100% accurate about the deck height as I didn't specifically measure it, being caught up with the squish measurement. I would estimate it to be about 0.35mm with the head face to squish band clearance to be around 0.4mm, which gives the 0.74mm squish clearance I measured with no head gasket.

The piston is 55.94mm diameter and it has a 0.06mm bore clearance. I have ordered a new compression tester as the one I had will not fit the plug hole with the engine in the frame. The manual quotes a compression figure of up to 178psi for a standard engine. I'm expecting mine to be higher than this, but maybe it'll not be too much higher? Will testing the compression pressure with a cold engine be accurate enough for comparison or should I warm it up first to operating temperature?

If I end up having to add a second or thicker custom base gasket to correct the compression ratio, then Id be worried that it might affect the port heights and maybe this will reduce performance or cause a mismatch between the porting and the pipe design? If this is the case I could at more expense look for a good used standard cylinder and buy a new standard piston to use, which might be OK with the skimmed head?

I know it's a completely different thing, but I've noticed that some 125cc Motocross bikes run with a higher corrected compression ratio of 8.5:1 or higher, so what do they do to make this possible seeing as they run on unleaded? I also wondered if my new CDI could be programed to a safe curve to suit my engine spec, or is it better to make the engine suitable for a certain advance curve instead?

I'm not looking for the ultimate performance, but I'd like to see if I can get to around 24bhp at the rear wheel, as a goal but only if its possible to keep it reliable for 2-3000miles between re-build?

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Re: KMX 125 engine re-build.

Post by 500bernie » Tue Oct 29, 2019 12:40 am

Hi Danny
The 300 head gaskets are manually opened up to 61mm using a dremel. I wouldn't leave a head gasket overhanging in the cylinders, tempting fate.
Cheers
Bernie
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Re: KMX 125 engine re-build.

Post by JanBros » Tue Oct 29, 2019 12:59 am

500bernie wrote:
Tue Oct 29, 2019 12:40 am
Hi Danny
The 300 head gaskets are manually opened up to 61mm using a dremel. I wouldn't leave a head gasket overhanging in the cylinders, tempting fate.
Cheers
Bernie
neither would I, if the deckheight is only 0.35 !
the squishgap needs tobe a gap because parts bend. cranks bend to at high revs so the piston goes higher compared to turning it by hand. as the minimal safe squish gap (for race engines which are opend a lot more and fitted with new parts much more often), it is 1% of the stroke.

so your 0.35 will never be enough. make the hole in the gasket as big as the bore, or use thicker base)gasket, but then your scuish is ruined again ...

and do I read your post correct, that only the head is skimmed, but the combustion chamber hasn't been widenned to match the bore ? another thing I wouldn't do.

hasn't anyone told you tuning can't be done cheap ?
;-)
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Re: KMX 125 engine re-build.

Post by KR-1R » Tue Oct 29, 2019 3:09 am

.
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I had a KX250, Mark Boyle who i bought it from used to run only one ring on the piston, so when that wore the loss in compression would become much more apparent (than if it had 2 rings) - letting you know to change the ring again.

ignition curves...
http://kr-1s.co.uk/forum/viewtopic.php? ... 381#p55381

Standard KR-1 vs KR-1s ignition (CDI) curves
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Re: KMX 125 engine re-build.

Post by stevo135+ » Tue Oct 29, 2019 8:09 pm

Thanks again for some great advice folks.

I'm torn now between trying to fix the head and gasket size/diameter issues and just putting it back together and seeing how it runs with high compression and closer than std squish. Before doing that though I'll do a compression pressure test at the weekend and see how much higher than the book figure it is.

Or option B, is accepting that the cylinder needs to come back off and a thicker base gasket fitted, then open out the combustion chamber in the head to increase the head volume and allow me to still run closer to 1mm of squish clearance but with lower compresion.

Thanks for the ignition diagram and the difference it shows in advance for using a BR10 grade plug too.

If I don't lower the compression ratio noticeably, then I think I'd probably need to run a BR10 plug and either octane booster in super unleaded or some sort of race fuel which just isn't practical for a little trail bike I'll ride round the back lanes in the summer.

Regards the head combustion chamber diameter, well its not 56mm but its just over 55mm I think like the gasket is. I did talk to a guy on here called James about the head before I built the motor, and he said that a 2mm overbore will need the head to be fitted with dowels to locate it, and possibly even O-ring seal grooves machined into it. I asked BDK about this and they said use a genuine head gasket and try it, as converting a KMX to O-rings isn't straightforward due to the outer head shape and the fact that the KIPS valves would need O-rings too. It would be expensive and not guaranteed to be better than a decent gasket was what I took away from it.

I bolted the head to the cylinder before I built the top end, just to see how well it matched with the bore. What I found was that the combustion chamber dome was not perfectly central in the bore and there was no overhang on one side and nearly 1mm on the other side. The five head bolts are new, but you can bolt the head down with at tiny bit of movement or slop in the way the head and cylinder fit together, so you can either get the combustion chamber centred in the cylinder and over hang the edge of the cylinder mating face or vice versa.

You aren't going to be able to get rid of this without dowling the head for a location or having the combustion chamber welded up and re-machined to get it perfecy central to the plug hole and cylinder.

I just put it down to Kawasaki's manufacturing tolerance being a bit wide, and also that because my engine is based on the 1985 KX125 motocross engine that's 10bhp more in a higher state of tune, that if the tolerances on that engine aren't perfect, and it not having any dowels to locate the head, then it'd be OK on a de-tuned KMX.

I would like to know what the perfect solution is to guarantee a reliable KMX engine between re-build intervals, as I'm not trying to save money there's no problem in what it costs, but I wanted to save time and too much assembly and dissassembley as I'm limited in time, and I want to ride the bike next summer as I've only been able to do 500 miles on it in 5 years, due to a long restoration, waiting for parts and waiting for an exhaust to be made. I just want one summer season of using it as much as possible, and I don't mind low interval re-builds 2000-3000miles? I just don't want to be constantly fiddling with it and having to fix issues or make changes and modifications to make it work, instead of using it whenever the weather is nice and I have a bit of time off work.

P. S. how would one make a perfect circle larger with a dremmel tool, as I couldn't think of a way of doing this, and I'd never get it perfect doing it by hand. I didn't know if you'd bond a head gasket to a scrap head and stick it in a lathe or Mill to bore it out?

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Re: KMX 125 engine re-build.

Post by JanBros » Tue Oct 29, 2019 10:52 pm

stevo135+ wrote:
Tue Oct 29, 2019 8:09 pm
Thanks for the ignition diagram and the difference it shows in advance for using a BR10 grade plug too.
I am misreading you, or you misread the ignition curves.
but the difference has nothing t do with the plugs, it's just programmed that way into the CDI : the KR1 has a different curve vs the KR1S . an S uses a 10-plug because it produces more power and therefore more heat.

about your head : you can perfectly O-ring your head. just an O-ring arround the combustionchamber ill work just fine, the water and KIPS you seal wth silicon sealant. I have run my KR like that for years before I had the CNC-head made. Never had any leaks.

Again : the most important thing you must be 100% sure of is : the piston may not touch the head gasket ! so your deckheight should at least be what you would use as squish-height (about 0.7mm). If you have less deck height, bolt the gasket onto the cylinder and dremel/file/... the gasket to the same size as the bore.

then If your deckheight and the gasket-thickness is the same as the minimal safe squishheight you want to use (+/- 0.7 ?), you are good to go as the piston will not be able to touch the head. if it is less, you willalso have to re-cut the combustion chamber to the bore-diameter.

my last worry would be the compression ratio, as standard 2-stroke engines are seriousle down on that part, and a good squish helps preventing detonation .
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Re: KMX 125 engine re-build.

Post by stevo135+ » Tue Oct 29, 2019 11:53 pm

Thanks Janbros.

I did misread the ignition advance charts, but I see now that one curve is the KR1 and the other is for the KR1S, and that they peak at different rpm.

My engine should not rev much past 10'000rpm as the pipe builder designed the exhaust to peak around 9600-9800rpm.

Thanks for the advice, once I've done a compression pressure test I'll take the head off and re-check and measure everything, including the gasket diameter and the squish clearance. I'll check the deck height with a straight edge and feeler gauges to be 100% certain of the clearance. Then if I need to open out the head gasket I will bolt it to an old cylinder and carefully use a sanding drum or file to enlarge it equally around its circumference. It won't need much material taking off if it's 1mm smaller than the bore size at worst case scenario.

Once that is done I think if the deck height and squish are reasonable, I'm going to give it a go at running it and keep an eye on it. I'll definitely use a colder plug and mix a measured quantity of octane booster with Shell V-power fuel, and I'm hoping my jetting will be too rich also initially.

If it seems to run OK then, and I don't see any of signs of metal particles on the spark plug after 50miles or so then I'll be happy to take it to my dyno guy for setting up and fitting the Ignitec CDI.

I've tried to uprate things to help the engine cope with more power, so for example I'm using a custom made alloy radiator which has an uprated larger core with more tubes, and I've fitted an upgraded ignition coil which seems to produce a really strong blue spark compared to the original coil.

I'd like to get the head dowelled and O-ringed in the future, maybe on the first piston change, but I think it'd be worth seeing if my set up works first and if it can be set up to be reliable.

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Re: KMX 125 engine re-build.

Post by James P » Sun Nov 03, 2019 7:19 am

stevo135+ wrote:
Tue Oct 29, 2019 8:09 pm
I did talk to a guy on here called James about the head before I built the motor, and he said that a 2mm overbore will need the head to be fitted with dowels to locate it, and possibly even O-ring seal grooves machined into it. I asked BDK about this and they said use a genuine head gasket and try it, as converting a KMX to O-rings isn't straightforward due to the outer head shape and the fact that the KIPS valves would need O-rings too. It would be expensive and not guaranteed to be better than a decent gasket was what I took away from it.

I bolted the head to the cylinder before I built the top end, just to see how well it matched with the bore. What I found was that the combustion chamber dome was not perfectly central in the bore and there was no overhang on one side and nearly 1mm on the other side. The five head bolts are new, but you can bolt the head down with at tiny bit of movement or slop in the way the head and cylinder fit together, so you can either get the combustion chamber centred in the cylinder and over hang the edge of the cylinder mating face or vice versa.
Steve,

I'm not sure whether you mean me when you refer to "a guy on here called James", as I do remember discussing several aspects of your intended conversion with you some time ago.

Other respondents have raised some good points so far, but as far as the diameter of the outer edge of the squish band and its alignment with the bore go, I would definitely recommend taking the trouble to ensure they are a match.
If you are lucky, the stud hole centres in the cylinder head may match those in the cylinder...it just being the case that the stud holes in the head are a bit too large. You can check this by having (at least two) special 'test' studs manufactured (thread on one end only, the other end being a nice snug fit in the stud holes of the head). Screw these special test studs into opposite positions in the cylinder and then loosely fit the head. Looking up into the bore, you should be able to see whether the squish band is concentric with the bore. If it is, all you should have to do is have two special steel sleeves/bushes made (inside diameter only slightly larger than the standard head studs) and make counterbores in the head to fit them in the same stud holes you used to perform the initial check. This modification will allow only minimal movement, meaning that the head will go back on in almost exactly the same place every time and you won't get the 1mm overhang you are experiencing now.
I did this same modification when I built my SDR engine (see the third message under this thread: http://diffrentstrokers.com/phpBB2/viewtopic.php?t=2308).
Once you have completed the above modification to your satisfaction, you can increase the diameter of the outer edge of the squish band to 56mm. I think this would be best done in a lathe (not too difficult if the spark plug is central). Of course, I don't know exactly what the existing edge looks like, but you may be able to continue the radius/profile of the squish band 0.5mm further 'outwards' to give you the diameter you want.

If you can't get a head gasket to suit the new larger bore, it may be better to omit the gasket and use one or more O-rings. However, if you are keen enough, it may be possible to modify a standard head gasket by (somehow) mounting it on a rotary table on a milling machine and using a high-speed cutter to gradually and carefully open the diameter to 56mm - I would not advise attempting anything like this by hand!

Briefly concerning compression ratio, combustion chamber volume and squish clearance; Once your squish band has the correct diameter and you know your head will fit on the studs with minimal play, you can set the squish clearance to your ideal figure (using gaskets, spacers, skimming or whatever method is most suitable). Once this is done, check the volume of the combustion chamber and use that figure to calculate compression ratio. If you need more volume, I recommend removing material from the combustion chamber only (try not to make the squish band narrower...and try not to reduce the length of the spark plug passage!). In doing so, also make sure you aren't reducing the thickness of the casting too much! Check the combustion chamber volume periodically as you go, to make sure you are still heading in the right direction.
When you think everything is correct, test assemble the top end. With the piston at TDC and the cylinder vertical, measure the volume of fluid it takes to fill the chamber to the bottom of the spark plug hole (if you will be using a head gasket, you can omit it for this check and just add its volume by calculation later). Whatever fluid volume you end up with (including the volume of any head gasket), use it to calculate your compression ratio.

Theory suggests that an 'inefficient' expansion chamber will allow an engine to tolerate higher compression ratios and this is seemingly supported somewhat in practice by hearsay. However, I can't really recommend deliberately designing such a pipe just to offset a known engine 'deficiency'. My experience in designing and building two-stroke expansion chambers is practically nil, so it may be worth having a word with your pipe builder in that regard.

If this all seems like a lot of trouble, you're right! It will also be fairly costly if you haven't got access to machine tools. If you can obtain a standard-bore cylinder and new piston at reasonable cost (and just do a 'standard' engine rebuild), that may suit you better in the long term.

I do vaguely recall you were considering a 200cc conversion as an alternative - is that still a possibility?

Whatever you decide to do, please let us know 8) .

Regards,
James

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Re: KMX 125 engine re-build.

Post by JanBros » Sun Nov 03, 2019 1:58 pm

with good squish, the only "deto-problem's" you realy have to worry about are with part throttle and low(er) rev's
Frits OvermarsPart-throttle deto at high revs (that is what you use in a burn-out) has nothing to do with ignition timing. At high revs the blowdown time area is still sufficient to let the cylinder pressure drop below the crankcase pressure by the time the transfers open, at full throttle.
But at part throttle, the crankcase pressure is a lot lower, lower even than the cylinder pressure, so exhaust gas will enter the transfer ducts, polluting and heating up the fresh mixture. This mixture will then show its displeasure by detonating.
One last word about ignition timing: part throttle, or reduced cylinder filling in general, allows for more ignition advance.
jan thiel

"I am 100% convinced our engine could have run for 6 hours at max power without seizing.
The problems arise when you close the throttle, or run part throttle!
The piston is mainly cooled by the transfer flow.
And at part throttle there is less transfer flow, causing detonation (auto ignition)
The entering fresh charge is ignited by the remaining, hot, burned gases!
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Re: KMX 125 engine re-build.

Post by stevo135+ » Mon Nov 04, 2019 9:33 pm

Thanks again all for the input and advice, and yes James P I do remember talking to you about the 56mm bore kit I found for sale which is what I went with in the end.
Just to put it into perspective the kit including postage for an old exchange cylinder was £343. The cost of a brand new cylinder alone with no piston from Kawasaki was £394. That may explain my decision and bearing in mind the piston in the kit was a forged Wossener item not an OEM cast one.

I've not had time yet to take the head back off to re-measure anything, but I will! And I will open up the head gasket diameter to suit the bore, as I've had a few ideas on how to do this neatly. I did grab a spare brand new head gasket and measure its diameter accurately though. The std gasket was an average of 55.3mm diameter and the piston is only 55.9mm so as you can see there's very little to be removed 0.3mm from each side isn't much material or alot of work to achieve either IMO. Also the uncompressed head gasket is 0.32mm thick so I'm working on 0.25mm for a compressed head gasket, but it might be a bit more?

Once I've bored the head gasket to the right size and checked the piston deck height is OK which I think it will be, then I've decided that I'm putting the engine back together and running it. The squish clearance is 0.98-0.99mm and so there is no way the piston will hit the head even if the squish band and combustion chamber dome are not centrally aligned with the bore. From what I read kawasaki didn't always build the KR1 engine to perfect parts tolerances, and I've aligned the head so it matches the cylinder exactly on the outside surface.

You are right that the head seems to be able to move in the bolt hole bores slightly. If I do have a 1mm lip of flat surface before the squish band then I don't think it should be my job to correct Kawasaki's casting tolerances, and though I'd really like to have head dowels, if Kawasaki didn't provide them as standard then in theory they should not be needed either, even if I think it was an oversight personally.

From what JanBros has said I'm not really worried about the compression ratio, as I'll just use 400ml of octane booster per 5litres of 97/98RON fuel. The new ignition could be mapped to a more gentle advance curve if required, like the way you build in on boost ignition retard in a four stroke turbo engine for example. If I was using the standard CDI I wouldn't have this facility available.

And as regards an inefficient exhaust, I've spent over £300 on an expansion chamber that was designed using the latest two stroke simulation software, by one of the UK's well known pipe builders whos pipes are well known for getting very strong power gains on other bikes. My pipe as I understand it was designed to get the most possible performance out of a standard cylinder port map. The only difference is that my new cylinder has all the port windows well matched to each other, and I have a 2mm increase in cylinder bore size. Im not going to have any other pipes made it'd be madness to do so. This is also why I'm not converting the motor to 200cc as I'd need a pipe for it and I'm not keen on having another one built.

Regards doing all the work and the best practice, such as perfect matching the concentricity of the cylinder head dome and squish band to the bore size, and O-ringing the head, and fitting locating dowels, it's not something I can do myself, and any specialist would want the whole motor to work with not just the head and my word on dimensions and what needs doing. I'd agree with you on all the above if I was going for a significant increase in bore size (150cc+) but I'm increasing the capacity by just under 12cc on a 125, so I don't think that 12cc increase warrents £££'s of machine work personally, and especially because one specialist said O-ringing the head might not be a better solution on the KMX due to the KIPS valves needing sealing rings and probably copper washers for the head bolts too.

I'd still be happy to swap my 56mm top end for a brand new cylinder and piston and rings, but not at £££'s more cost than what I've already spent. I can add up over £1000 on this engine already and it's only a 125! The economics don't stack up if I need to spend that sort of cash again on parts, machine work and another custom one off exhaust. I've got most of 2x KMX 200 engines in my garage too, unfortunately I've not got a KMX 200 exhaust, so going that route isn't currently an option for me.

I will sort out my head gasket and try to take some pictures at the weekend, and keep you updated on my progress. I just no longer see the increased compression ratio issue as a rod breaking certainty, so I'll have to try it out. Ive not had the bike running for 18 months and i need to at least try and get it ready for 2020.

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Re: KMX 125 engine re-build.

Post by JanBros » Tue Nov 05, 2019 12:32 pm

I did grab a spare brand new head gasket and measure its diameter accurately though. The std gasket was an average of 55.3mm diameter and the piston is only 55.9mm so as you can see there's very little to be removed 0.3mm from each side isn't much material or alot of work to achieve either IMO
don't do half work , do it good the first time !

you forget that piston's rock at the dead centers, match the gasket to the bore !
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Re: KMX 125 engine re-build.

Post by stevo135+ » Tue Nov 05, 2019 8:25 pm

Hi JanBros.

Yes that is definitely my intention, and one idea I've had on how to do this is to wrap a 2" mandrel sanding drum in emery paper to the size of the head gasket, and then gently spin it in the bore of the head gasket to just bore it out the 0.6mm that I need to match the bore. I think that would work.

The other method I thought of was to hole cut two steel sheets to 56mm and sandwich the gasket between them bolting it together with short bolts. I'd then run a half round file round the circumference of the gasket to open it up slightly.

What are people's thoughts on using a gasket sealer on the head gasket surfaces, as I've always fitted them dry when possible with no sealant, but I've read others recommended a smear of sealant on both sides of the gasket?

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