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CG125 cylinder + piston woes

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treeno
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Joined: 26 Dec 2012
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PostPosted: 14:19 - 19 Jul 2013    Post subject: CG125 cylinder + piston woes Reply with quote

Hey!

Long story short I replaced the piston and barrel on my 1997 CG125 and the new piston is more domed than the old one and is also taller. This results in the piston poking out of the end of the barrel by ~1mm. I have a feeling this means that someone in the past has replaced the piston and cylinder head with one from the newer models which according to what I've read (link below) had flatter and slightly shorter pistons and a non-recessed cylinder head.

http://hondacg125.awardspace.com/how_to_increase_engine_size.htm

Anyway, can anyone confirm that the older models had a recessed cylinder head which would allow for the piston poking out of the barrel and stop it from bumping into the head as it does currently?

Ta!
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unitynotsocri...
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PostPosted: 20:08 - 19 Jul 2013    Post subject: Reply with quote

difficult one ....Measure the size of the pistons , check the barrel , and compare with spec . You may have an odd chinese barrel or piston , check the ohc models to compare .
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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
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treeno
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PostPosted: 11:52 - 20 Jul 2013    Post subject: Reply with quote

unitynotsocrippledatmo wrote:
difficult one ....Measure the size of the pistons , check the barrel , and compare with spec . You may have an odd chinese barrel or piston , check the ohc models to compare .


I've actually had two pistons delivered as I thought the first may have been a dud. They are both 1mm taller from the centre of the gudgeon pin hole to the top of the Crown. Hard to find exact dimensions of the original piston but it fits the description. The only thing I haven't changed in the top end is the cylinder head now. If you have a look at the picture of the underside of the head in that link, it does look like that. Unless it is the crank or the control length having been changed but I somehow doubt it?

Cheers
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treeno
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PostPosted: 20:12 - 21 Jul 2013    Post subject: Reply with quote

Bump... Anyone have any knowledge on this?
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unitynotsocri...
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PostPosted: 22:30 - 21 Jul 2013    Post subject: Reply with quote

The different gudegeon pin sizes . You may have a later model piston .

One way to check its ok is to put a thin layer of grease or bluetack on the piston , put the barrel , piston and head on and tighten the head bolts.....

then turn it over through a few good turns .Disassemble the head and check the valves are'nt touching the piston crown by marks in the bluetack / grease / paint. You may end up with a higher compression cg , they are tuned lowish anyway .

differences in the skirt should be checked , and rings / oilway patterns / casting marks to indentify it visually / by measurement on ebay parts etc .
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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
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treeno
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PostPosted: 00:03 - 22 Jul 2013    Post subject: Reply with quote

unitynotsocrippledatmo wrote:
The different gudegeon pin sizes . You may have a later model piston .

One way to check its ok is to put a thin layer of grease or bluetack on the piston , put the barrel , piston and head on and tighten the head bolts.....

then turn it over through a few good turns .Disassemble the head and check the valves are'nt touching the piston crown by marks in the bluetack / grease / paint. You may end up with a higher compression cg , they are tuned lowish anyway .

differences in the skirt should be checked , and rings / oilway patterns / casting marks to indentify it visually / by measurement on ebay parts etc .


Thanks for the tips! The gudgeon pin is the old style 15mm, but apparently they changed that to 13mm in 2004 (but changed to a flat piston in 2000) so I guess that puts the parts in between that date range.

From what I can see the edge around the combustion chamber on the cylinder head is where the piston is bumping. The edge is flush with the bottom of the cylinder head therefore doesn't allow the piston to go all the way up. I will give the blu-tac method a shot! I have also been trying to identify the year of the barrel and cylinder head but it is proving quite difficult.

The new piston looks very similar to the one in the first photo (as did the first one I ordered, I thought it was wrong at first!) and the original has the same shape and size as the second one but is not completely flat on the top like that one.
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treeno
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PostPosted: 00:10 - 22 Jul 2013    Post subject: Reply with quote

Some further evidence for my theory is the piston from the 01-04 model which I have just found:

http://www.ebay.co.uk/itm/NEW-HONDA-CG125-CG-125-PISTON-KIT-RINGS-01-04-STD-/230443913117?pt=UK_Motorcycle_Parts_13&hash=item35a7870f9d

This looks exactly like the original, however having changed the barrel and bought a new original piston I would quite like to finish the job off and get an original head if that will fix it! Although that one strangely has a 13mm pin. Confused
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unitynotsocri...
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PostPosted: 11:07 - 22 Jul 2013    Post subject: Reply with quote

I reckon the flat piston is wrong ,or a really early jap one . The domed one looks right . Dry fit them ,use old gasket on head to check .
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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
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treeno
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PostPosted: 10:57 - 19 Oct 2013    Post subject: Reply with quote

Sorry to resurrect an old thread but I thought this would best go here. Does anyone see any issues with using a completely different piston? I found a piston from a pit bike that is shorted with the same bore, so should fix this problem that has stumped me for months. Is it fine to do that?
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unitynotsocri...
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PostPosted: 13:48 - 19 Oct 2013    Post subject: Reply with quote

If the skirt is not too long or short and the small end fits ok .
Check for oilway paths from conrod and behind rings are up to it .

Most of all Accuratley........measure the piston bore clearance.
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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
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