Honda CG 125cc to 150cc conversion problem

[Guichaoui]

I have a honda 125 cc engine that's driving me crazy, the engine has 2000 miles on it, relatively new with no problems whatsoever and runs great. I got a 150cc kit for it (piston, cylinder, head,) the whole top end, the installation went smoothly without any problems, it has great compression and top range performance, the problem is, when i started it the first time i noticed a clunking sound coming from the top end, when rotating the engine, you hear the sound when the piston is at TDC and BDC, the sound is louder with the spark plug off,and even louder when the bike is running, the sound disappears above 4000 rpm, after disassembling it again, i noticed no signs of valves hitting the piston, no piston play, even with the old head it gives the same sound. When i installed the old cylinder with the new head, no weird sounds and it ran greatly, so one would say the problem is in the cylinder, after comparing the new and old cylinders, the only difference i could notice is the 150 cylinder is 2mm longer on the bottom, but its not touching the crank, and the new piston is relatively tighter when entering it into the cylinder, even with no rings.

Notes:

No rod play up and down, slight play left and right
Piston pin fits perfectly in the top rod bearing with no play
Piston fits snuggly in the bore, no play front to back, slight play left and right
Valves adjusted to spec

Any ideas would help, thank you.

[WD Forte]

As the new cylinder wall is a bit longer at the bottom have you examined the bottom inner lip of the bore to see if the con rod has been touching it when the piston is halfway up/down?

Personally I wouldn't bother trying to 'upgrade' a CG for moar powah
just keep it well serviced or buy a bigger bike
.
____________________
bikers smell of wee

[stinkwheel]

Sounds like it might be piston slap. Are there any witness marks on the skirt of the piston front and rear?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Guichaoui]

Solved actually it turned out somehow the piston is too snug in the bore, i tried it even without rings and it moved with difficulty as if the rings are installed, i just sanded down the skirts of the piston until it moved smoothly in the bore then installed the rings and put back and everything ran without a sound.

[stinkwheel]

[virus]

What Stinkwheel says.

Pistons are designed with mild ovality when cold to counteract the differing amounts of expansion caused by the thicker (i.e gudgeon pin webs) vs thinner parts (skirt) of the piston. The correct thing to do would have been to bore to suit. I don't think anyone uses a cheap Chinese 150 kit and expects major longevity but if I were you I'd be careful not to thrash it from cold or rev it too hard for too long once warm as theres a fair chance you've reduced the temperature range it will put up with now.
____________________
own: 81 xs1100g...
owned: 85 rat CG (sold), 91 GS500e (stolen), 84 gsx400f (scrapped), 81 z250 (siezed, siezed, scrapped), 83 cb250rs (sold), 84 gpz750r ratfighter (killed) 84gpz400 (sold), '80 cb650 ratfighter (wrote off) 95gsx6/12f ratfighter (killed) 91 xj900 (sold)
stinkwheel Well I just had my hands up a pigs fanny. Which makes your concerns pale into insignificance.

[Nobby the Bastard]

[Guichaoui]

Alright so i went out and got a new piston installed it and let it run for 15 minutes, the sound didn't go away, so i think i will try honing the cylinder and give it a test. But all this does not explain how a tight piston causes knoking sounds

[A100man]

[stinkwheel]

I can confirm Virus has blown up his fair share of big-bore CG125s in his time so he has the voice of experience.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Guichaoui]

Another thing i overlooked is that there is a notch cut out at the bottom of the cylinder it turned out that it aligns with an oil nozzle that squirts oil inside the cylinder to lubricate it, and the 150 one doesn't have it. Guess i have to find a way to grind a notch in there too

[jeffyjeff]

[Bhud]

Put it back to how it was. Original bore and piston.

[P.]

[virus]

[MarJay]

https://i.gifer.com/embedded/download/Ywqm.gif
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[stinkwheel]

[virus]

[Guichaoui]

UPDATE:

So i got a new piston and honed the cylinder, grinded away a notch in the same position as the original one, put it all together and it woked fine, i rode it for about 30 minutes and didn't notice anything wrong with it, for those who are wondering, i only did this because the parts for the 125cc top end are inexistent in my country, you couldn't even find a gasket set for it, i don't know why but this is the only thing i could do since the 150 parts are available everywhere.

[MCN]

The CG125 doesn't really suffer from lack of powah. It is absolutely fine for its task.
The secret to using small engines is learning about energy conservation.
Don't upgrade the engine.
Downgrade the brakes.
Slowing down is the worst thing you can do on a 125.
____________________
Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[A100man]

[lingeringstin...]

I've had some right dodgy after market Chonda pistons in my time recently. Yeah I've had them with the skirt too long so you had to grind them so they didn't come into contact with the flywheels at bottom, and I've had them be a bit too tall (like one mill or so) after some "engine modifications" so they clapped the head as they came up. Again, it was just a matter of grinding or sanding things to fit. To be honest I now use a pair of YBR125 pistons in my rebored Benly clone engine thing and they work fine.

[Guichaoui]

UPDATE:

Got 500 km on the bike today, changed oil, checked valvee clearance, the sound was getting quieter throughout this week but its still there.

Here is what it sounds like:
https://youtube.com/shorts/iP_0VJd5gF8?si=8ZIZ06_MpSJ_uI_I

The video is taken when the bike is at operating temp, been driven for 15 minutes, when its cold its the same.

Any help is greatly appreciated.

[A100man]

[Guichaoui]