KMX 125 engine re-build.

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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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Re: KMX 125 engine re-build.

Post by stevo135+ » Wed Feb 05, 2020 1:14 am

Evening all.
I just thought I'd finally post an update on my KMX's top end re-build, after not having much time to work on the bike for a few months.

I sent BDK racing a new head gasket and they kindly machined it out exactly to 56.5mm which should give me a safe working clearance for my 55.94mm bore.

I removed the head and took a few measurements. Firstly I set the piston to TDC and checked the deck height. I'm not sure what is technically measured as deck height, as the dome of the piston does come above the deck height of the cylinder. But at the edge of the piston it's lower than the deck height by 0.3mm as accurately as I could measure with a straight edge and feeler gauges, which probably isn't the ideal method but it gave me an indication of the clearance available.

Then I measured the step in the head from the squish band to the head face using the same method, so again not ideal. But I found that a 0.6mm feeler gauge was a tight fit under my straight edge. So this gives me about 0.9mm piston to head clearance + 0.25mm for a compressed head gasket. This though approximate is a bit more than I thought, but it's a safe clearance and I've already had 0.25mm skimmed off the head. I wish I'd been able to accurately measure the head volume and work out the compression ratio, but I forgot to order a burette in time, and the domed piston makes it a bit difficult to accurately measure the combustion chamber volume?

Anyway I've re-assembled the head with my modified gasket, but I did have an issue that has bugged me too. The cylinder was bead blasted before it was re-plated and although the deck looked to be flat in each direction as measured by visual indication with a straight edge, it's a slightly textured surface compared to my skimmed head face. I wasn't 100% confident that the head gasket would seal against the less smooth cylinder face, so I smeared a very light coat of threebond gasket sealant onto the head gasket, which is something I don't like doing as coated head gaskets are supposed to be dry fit and not need any sealant. I have always had success with installing head gaskets dry in the past, so my advice to anyone is don't let anyone bead blast your cylinder if you don't want it ruined. I suppose I could have lapped it on some fine wet&dry glass paper, but it's not the ideal outcome after having a cylinder re-plated by a specialist.

I've torqued the head down now to the quoted 19ib/ft and left it to for the sealant to cure. I'm hoping that it will be OK now and I'll have no clearance issues or coolant leaks. I still don't like the way the head fits without any location dowels, but there's room in my opinion for small dowels to be machined and fitted in the future on this engine. The other idea I had is maybe to have some blind bushes machined for the KIPS valve posts that go into the head, and then use the outer surface of the bushes as locating dowels in bored out holes in the head. I suppose to do this accurately without any binding you'd have to locate the bushes in the cylinder block too, so it'd be more complicated than adding dowels else where.

Anyway I'm just bolting the rest of the bits back on and I'll let you know what happens after I give the bike it's first run.

Can I ask for your advice on preferred running in methods for a forged piston in a Nikasil bore? I was going to run it under no load until it gets warm enough to open the thermostat assuming I have no leaks, and then let it fully cool down and re-check the head fastener torque figures.

I read alot about heat cycles, but I don't really get what they do for the running in process? Am I not better off on the second engine start to warm the engine up and then ride the bike round the block a few laps to put it straight under load, but keeping it away from full throttle or high rpm, until I've repeated this process several times.

Should I use mineral two stroke oil for the running in process as I was planning on using Silkolene Pro2 which is what I've coated the piston and bore with during the re-build?

Thanks Steve.

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Re: KMX 125 engine re-build.

Post by stevo135+ » Wed Feb 05, 2020 1:28 am

Image

Picture above showing piston at TDC.

Image
Cylinder head.

Image
Measuring step in head to squish band with 0.6mm feeler gauge.
Sorry about the poor phone picture quality, but I thought they might show an idea of how it looks.

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Re: KMX 125 engine re-build.

Post by JanBros » Fri Feb 07, 2020 12:59 am

looking at your last pic, to me the diameter of the head is +/-54mm : 2 inch (from 4 to 6) + 3/8th of an inch after the 6 minus 2/8 after the 4 . but you say the head is 55.3. still, let's suppoese you measured right.

than you still continue to mis-calculate your squish clearance : you can not count the 0.6mm from the head as it's diameter is smaller than the diameter of your piston !!!!

on the edge of the piston, you don't have squish-clearance, you only have head-clearance as the squish in your head does not reach the edge of the bore.

so you only have 0.25 and 0.3mm = 0.55 head-clearance which means you are in GP-engine-territory. when all parts are new it will not be a problem. But GP engines are not supposed to run 10.000km as they are opened probably daily and have service interval's of about a tenth for the crank. I don't think you'll get to 5000km before the piston start's kissing the head.

If I where you, I didn't start the engine, but take it appart again , and either add a 0.2mm gaskett under the cylinder, or better : have the head cut so the combustionchamber matches the bore.
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Re: KMX 125 engine re-build.

Post by stevo135+ » Sat Feb 08, 2020 1:08 am

Hi JanBros.

Thanks for your comments on my engine re-build. What you are saying now makes sense, and is another oversight on my part. I can't be 100% about the diameter of the cylinder head combustion chamber or indeed its concentricity to the bore without removing it and measuring again.

It's frustrating as now I expect I've used up a modified head gasket ( I should have have 3-4 gaskets modified I guess) and I've got the scrape off all the sealant.

I see your point about the clearance issue on the outer edge of the piston, and how it can't work as it is. I'm torn though now between the two options in how I could fix it to make the engine reliable.

1, if I remove the cylinder with the head still torqued down I suppose I could get a thicker base gasket made for it. The current base gasket is a metal one that's 0.48mm thick. I expect a 0.7mm base gasket would be enough to give me a safe increase in piston clearance to the head. I'm assuming that this slight increase in cylinder height won't adversely affect the port timing as I don't want to reduce the performance or make the engine specs too unsuitable for my exhaust which is a fixed parameter.

2, The better but more involved option would be to remove the head and get it modified to the new bore size. This would depend on how central the combustion chamber is in the head. If its miles out then it wouldn't be worth bothering, or do I look for a used head with hopefully a more centralised combustion chamber?

If I do have the head modified then it's something my local car engine machine shop could do, as I'd assume they just need to machine outwards by around 2mm while following the angle of the squish band.

I'd need to get rid of all the sealant and get BDK to modify another head gasket for me, and probably a spare too for the next re-build. But really I should remove the cylinder too now as get it on a flat surface with some wet/dry paper and lap the top of the cylinder face perfectly smooth and flat, as its not as smooth as I'd like it currently.

Thanks for letting me know the issue and how to go about resolving it. Do you have any preference for how I should run in my set up once I've fixed the issue with the head/cylinder and piston clearance. I've heard some people say that I should be using cheap mineral two stroke oil for the first few runs to help the piston and rings bed in to the bore. I also don't know if heat cycles are desirable or not? To my mind I don't see how they help run in an engine compared to putting it under load and lots of on-off acceleration and braking?

Hopefully I'll be able to do a final engine build this time and then it'll be good for my 5000km service interval goal.

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Re: KMX 125 engine re-build.

Post by JanBros » Sun Feb 09, 2020 10:55 am

do it properly and have the head re-cut.

when you raise the cylinder, you won't have any squish-action : only very limited between the head and piston, your actual squishgap between combustion chamber and piston will become to big for good squishing.
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Re: KMX 125 engine re-build.

Post by stevo135+ » Sun Mar 01, 2020 12:35 am

OK, so I got two gaskets modified now so I have a spare for when it blows up or I get nervous after say 2000miles and decide to strip and inspect it all. I did remove the head and the combustion chamber dome is between 55.19 and 55.3mm in diameter. I've put a tiny chamfer on the step in the head with a needle file run around the circumference of the head, which makes it almost 56mm anyway.

I reassembled the head yesterday and after the sealant had dried, I decided to do a compression test today before I fit the exhaust and radiator. With an open throttle, after 6-7kicks I was getting 195psi. I did it again to check for repeatability and it was about the same 194-195psi.

It's supposed to a maximum of 178psi or 7.8:1 corrected, so I know it's pretty high in relation to the book figure. My plan now is to run it on 40:1 with Shell V-power and 3% octane booster added, which should give me at least 100RON I hope? I'm going to suck it and see, but I think this fuel blend will maybe be enough to keep it safe from detonation or that's my hope anyway.

My compression test apart from being on a cold engine, might be skewed by all the oil I used on the piston and bore, and I put 3-4drops down each side of the crank and on the big end bearing too, so there's probably a bit of oil taking up space in the crank case?

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Re: KMX 125 engine re-build.

Post by stevo135+ » Tue Jun 02, 2020 8:08 pm

Hi all.
I recently got hold of a measuring burette, so for something to do I took the head off my KMX that I still haven't got round to starting, due to waiting for custom throttle cables to be made up and some other little bits and pieces to arrive.

With the spark plug in I measured the head as 12cc exactly, and the head gasket I calculated to 0.6cc of fluid volume. I think I worked out the volume of the piston crown using a formula for a spherical cap as 3.7cc. Taking this off gives 8.9cc of head volume.

My effective swept volume from the top of the exhaust auxiliary ports (190 degrees duration) to the top of the cylinder is 63. 3cc from my calculation, but I'm not sure if your supposed to measure it from the main exhaust port that on a KMX closes earlier at 176 degree duration?

I know my calculations won't be that accurate as I didn't manage to work out the volume of the bit of space left in the cylinder as the piston edge comes up to 0.3mm below the deck surface at TDC and I used 0mm of deck clearance as I couldn't work out the volume of the shape left in the cylinder. So in effect my true compression will be slightly less than my figure I got which was 8.11:1.

Now I think that's OK as the quoted factory compression figure is 7.8:1 so I'm only a bit higher than standard compression ratio. But if the factory compression ratio is calculated using the main exhaust port and not the longer duration auxiliary ports then I get it to 8.33:1.

Thats about 0.53:1 more than the book quoteed factory figure. I know that to be precise I'd need to measure the volume of fluid at TDC with the head on, but I don't think there is room to get the burette in with the engine in the bike so it would end up a bit inaccurate anyway. And I know your not supposed to take the factory figures as gospel, and you should measure your own engine to take into account different piston crown thickness and gasket heights tolerances too.

But I figured that some idea of the compression ratio in my engine is better than none, and whatever figure I've ended up with it should actually be less in reality not more, which is safer.

It's noticeably harder to rotate over TDC on the kick start than it was before, but I know that's a bit meaningless of a statement too.

My question is would people be happy to run up to 8.33:1 effective compression ratio on say a KR1S for road use?

I will use 97 RON fuel and I've got a good deal on 3 litres of octane booster which will last me a couple of years at least using it at 3% concentration. I think that should give me close to a 100 RON fuel blend which I would think is fairly safe on a tuned two stroke that's not going to be totally thrashed all the time like a race or MX bike.

How high have people pushed the compression ratio on the KR1S and not had problems just out of interest?

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Re: KMX 125 engine re-build.

Post by stevo135+ » Tue Jun 02, 2020 8:21 pm

Oh and I forgot to include the geometric compression ratio, as I've heard that sometimes it's better to use than the effective ratio from the exhaust closing point.

I got 16.08:1 un-corrected compression ratio which sounds really high, but I have found the factory figure for some European MX 125cc engines is as high as 16.7:1.

And the 1986 KX 125 that my engine is based on has a factory compression ratio of 9.4:1 which I presume is the effective compression and not the geometric CR?

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Re: KMX 125 engine re-build.

Post by JanBros » Wed Jun 03, 2020 12:10 am

don't look at full-Mx bikes as a comparison. just as you wouldn't look at full-GP-2-strokes as a comparison.

nobody uses corrected compresiion ratio. jap's normaly give that number, not the geometric. but any tuner will tell you it's useless, they'll never taalk about corrected compression.
only looking at the compression ratio is pretty meaningless. an expanson-pipe is a presure-device : it works like a turbo : it pushes more mixture into the cylinder than its geometric volume so it raises the actual compression.

good squish is very important running high compression's. you need a tight squish-height : where there is no space, there can not be mixture that can detonate.

high compression's are not all good for power. High compression-engines uses more of the energy from the burning to push the piston down, meaning that there s less energy in the exhaust gases, and that means that the pipe will work not so good. It's even possible that your pipe will be to big : by the time the gases get to the belly almost all energy is just to suck on the cylinder, and no (or not enough) energy is left to push fresh mixture back.
dropping the compression will uses less energy to push the piston down, the pipe will work better, suck harder and push more fresh charge back into the cylinder resulting in a bigger bang the next time, and that biger bang ...

but it all hangs together. changing one thing that normaly is supposed to be good for power can result in a drop of power, as the rest of the engine ain't working well together with the change.

it's not easy. I've just tested 6 heads (up to 15.7:1) on my MX-moped, with 2 cylinders and 3 different pipes to get plenty of data. made 2 straight test-pipes, each one more power than what I had. made a curved copy of the best pipe and lost 15% of power ... :cry:
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