CG125 piston slap

[Mikey7094]

I've posted about this before, but I've tried a number of things and I'm completely out of ideas now.

My CG has terrible piston slap - the cylinder is very rattly and on inspection of the barrel you can see the score marks at the bottom on the front and rear. The weird thing is that the cylinder was rebored about 300 miles ago, and apart from the score marks it looks fine, and there is very little play between cylinder and barrel. I showed the cylinder to my local bike garage and they thought it was good. I can't feel any play at big end or small end, and the cam looks in good condition to me.

What else is there to check? Is there something stupid that I've missed?

I have experimented with rebuilding the engine and a spare engine I bought, trying different combinations of crankshafts in the two engines, but I've always had this rattle. I've checked the crankshafts while out of the engine, and they seem fine, with no play in the big end bearings. Is it possible that I've somehow wrecked both crankcases during the rebuilds? If so, wouldn't I be able to feel play at the big end?

Might there be something else wrong with the cylinder barrel?

Could it be a problem with the actual frame of the bike?!

Please help!

Mike

[Nemo]

Able to post a video?
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Previously owned: Kawasaki KH125, Suzuki GP125, Suzuki GX 125, Honda CB125 TDC '83, Honda ntv 600, Kawasaki ER5, Kawasaki ZZR600, Aprilia RSV 1000 Mille
Now own: Ducati 848 evo

[Walloper]

It's not piston slap if it was rebored recently. (Assuming they fitted an o/size piston.)

Clutch, gearbox, rear cushion drive. (though I don't think th CG has one)
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[Old Git Racing]

Have you actually measured the piston to bore clearance rather than relying on other people telling you its correct?

OGR
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2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[Mikey7094]

No I haven't, but then I don't have an internal micrometer... I could try with a piece of card or something, but I guess it won't be very accurate.

Is there anywhere you can find the specifications for clearances with oversize pistons?

Sorry, I can't post a video because the barrel is now off the engine. I might take some photos of the bore, though, to show you what I mean.

How could the clutch make a rattle? And if it wasn't piston slap then how could I have the characteristic marks on the cylinder?

[lukamon]

low oil pressure? have you cleaned the oil pickup screen?
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[Mikey7094]

Thanks for the reply. I think I tried before but didn't get much of it out of the gauze - how would you suggest is best to do it?

Any other ideas?

[Old Git Racing]

If the piston is rocking in the bore and marking the cylinder, then it's either undersized or the bore is tapered, i.e wider at top than bottom or other way.
Get the piston in the bore and measure the clearance with feeler gauges, don't know what it should be but someone will somewhere on the net. measure at the top and at the bottom with no rings in and on the same bit of the piston, then turn it 90 degrees and measure again it might even be oval slightly.
You could cobble up your own internal micrometer by screwing two bolts into a threaded bar with lock nuts and set them so they just touch the cylinder walls at the top of the bore then check allaround and all the way down, measure the spread with a good set of vernier calipers, not 100% accurate but will give you an idea within 0.05mm.

OGR
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2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[Walloper]

[unitynotsocri...]

first of all make sure you are not swapping components from chinese copy engines , as the bore x stroke is different , and other bits .
If in doubt after measuring , then start with a new / rebored piston and barrel . Be wary of mixing early 6v cg parts with type 2 brazil and later .
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[Mikey7094]

Thanks for your responses.

I've had a look at the cylinder (without micrometer as of yet), and if anything it seems to be too tight, like there is a small amount of resistance in the cylinder. So my latest theory is... Maybe my machine shop man didn't bore it out enough? Is this likely? I've taken some photos, which can be found here (if the link works):

Piston and cylinder photos

(The photos might be a bit big - please tell me if they won't load.)

The first photo is my +0.25mm piston in its rebored cylinder, and there is negligible visible gap between the piston skirt and cylinder wall - I can't even fit my smallest (0.04mm) feeler gauge in there. The second photo is a spare cylinder I have, which hasn't been rebored and I don't know its mileage although I think it's within the wear limits. There is more of a gap between the skirt and cylinder, which is hopefully visible in the photo - the next photo shows a 0.04mm feeler gauge in the gap. This piston moves more freely in the bore.

So maybe the rebored cylinder is too tight, and it's causing the con rod to push/pull too hard, making the piston rock back and forth... Does this sound plausible?!

I also noticed that my oversize piston (which was a genuine one from David Silver Spares) has two fewer holes in the side than the spare, standard size piston (last two photos). To anyone who happens to have a complete encyclopaedic knowledge of CG engines, Is this normal and would it significantly affect lubrication?

Is there anything else that can affect lubrication and/or oil pressure?

Thanks

Mike

[Old Git Racing]

If you put a ring in the bore and measure the end gap what is it in thous.

OGR
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2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[Mikey7094]

I've measured the ring end gaps - to my nearest feeler gauge, they are 0.2mm, although one seems slightly smaller than the other, so maybe 0.15mm. The Haynes manual says the end gap should be 0.15 - 0.35mm, so I guess this is OK.

However it does say that the piston skirt outer diameter should be 56.45 - 56.48mm, while the cylinder bore diameter should be 56.50 - 56.51mm, suggesting the clearance between skirt and bore should be somewhere between 0.02 and 0.06mm. (The wear limit is a clearance of 0.25mm.) The fact that I can't fit a 0.04mm feeler gauge in the gap suggests is might be too tight.

I'll try and track down some micrometers and measure it properly.

Also, regarding fitting the piston rings, there are two compression rings, one with a tapered edge and one with a square edge. The manual says the square-edge one should go at the top, with the tapered-edge one underneath, but in the pictures they look like they've done it the other way round.

Does anyone know which way round is correct, or whether it makes any difference?

Mike

[unitynotsocri...]

it looks wrong ,try and check it for ovality , or the piston may be origionally mishapen .looking at the sht on the piston top it looks like its been eating oil and the scuff marks on the skirt are a bit ott for a rebored barrell / piston . In all fairness it could have seized / warped while being run in . If I ever use a rebore on an engine I always redo the valve guides and seals / valves . Reboring an old engine and not doing the head can leave the engine very vunerable to heat / compression and power loss issues making it more likely to sieze when running in .
Remember the pistons are alloy , and easily melted / distorted , as many bcf'r 2 t sniffers may tell you .
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS