CG Road to 250CC

[ego]

Hello Folks

After following, reading around the forum, I decided to create this thread in order to benefit from your useful advices.

So, a few months ago, I just bought my first chinese cg clone. Did a few customizations on it matching my favor “cafe racer/scrambler look”.

Here you got some photos of it.
https://photos.app.goo.gl/TDpf1RcSpMT71uHJ6

It is based on a Loncin block 125cc. As I know, the cylinder length is 69mm. I don’t really know the wight but the piston pin is 15mm.

My plan is to bore it up to 67mm on stock block. I found a 250cc head and a piston, cylinder set 67mm with 69mm length. Can I get this work on my stock crank? Will it handle it? Did some research but wasn’t able to find examples like that. What do you guys think would this be lasting on stock block? What do I have to upgrade next to make it safe? What size of carb would feed it?

Thanks in advance for your help.

Ride safe
ego

[XLsior]

what is the case opening diameter or cylinder skirt outside diameter?


Finding a bigger piston with a 15mm pin isn;t to big an issue aliexpress usually has those options if you look...


The big issue is whether or not a bigger jug will actually fit in a CG125 case.


TBH it would be better to just find a CG250 complete engine and do a total swap rather than trying to modify a smaller displacement engine.

[ego]

Originally Posted by XLsior

what is the case opening diameter or cylinder skirt outside diameter?


Finding a bigger piston with a 15mm pin isn;t to big an issue aliexpress usually has those options if you look...


The big issue is whether or not a bigger jug will actually fit in a CG125 case.


TBH it would be better to just find a CG250 complete engine and do a total swap rather than trying to modify a smaller displacement engine.

The country, in which I drive, finding a proper cg250 engine is nearly impossible. And you wont be able to pass inspection if you do. While giving it more engine displacement isn’t a problem.

Didn’t disassemble it before but 65.5mm fits without any modifications, so 67mm would fit, even if some little modifications are required. Making it fit isn’t the problem I believe.

[XLsior]

Well I just thought I'd mention it.



If the skirt and studs line up and fit then the 67mm piston with a 15mm pin should be possible to find.

[Megadan]

The only other unknown factor that needs to be accounted for is jug deck height. If you overbore a 125/150cc jug then it's no issue. The 200 and 250 jugs are taller.

Then you will also be tossing a different piston into the mix that could alter that spec as well, and if that different piston has a different crown height or dome height to the original it can also affect clearances on both the 125/150 jugs, and the bigger ones.

This may be your best option. It has the right 68mm sleeve bore size, is a 65.5mm bore and has a 15mm wrist pin.
https://www.aliexpress.us/item/32568...TQ5Ni42MC4wLjA.

Anything bigger than 65.5mm will be a CG250 sleeve diameter, which is 73mm and would require the case halves to be bored. If you try to bore any of the smaller jugs out to 67mm your skirt thickness at the sleeve would be .5mm, which is way too thin.

[ego]

Originally Posted by Megadan

The only other unknown factor that needs to be accounted for is jug deck height. If you overbore a 125/150cc jug then it's no issue. The 200 and 250 jugs are taller.

Then you will also be tossing a different piston into the mix that could alter that spec as well, and if that different piston has a different crown height or dome height to the original it can also affect clearances on both the 125/150 jugs, and the bigger ones.

This may be your best option. It has the right 68mm sleeve bore size, is a 65.5mm bore and has a 15mm wrist pin.
https://www.aliexpress.us/item/32568...TQ5Ni42MC4wLjA.

Anything bigger than 65.5mm will be a CG250 sleeve diameter, which is 73mm and would require the case halves to be bored. If you try to bore any of the smaller jugs out to 67mm your skirt thickness at the sleeve would be .5mm, which is way too thin.

Actually, I found a set that looks exactly like the one you just linked. Piston dome, crown exactly the same. Though it is a 67mm bore and 69 height. So skirt thinness wouldn’t be a problem I believe. I could also bore the case halves no worry about that.

Other than that. Do you recommend these roller valve arms sold on AliExpress? Are they better than the flat ones? And would an oil cooler be necessary on this build? If yes, oil pump upgrade needed?

[delzy]

Transplant the CB250R 4 valve Zongshen 172FMM-6 22hp out of the box. You can get it for under $600 from alibaba. Mine's in transit.

[TominMO]

The kit that Megadan listed says it is for 150cc motors. I'm thinking that if it would fit a 125, they would have said so. Probably there is something that makes it not work in the 125. Such as the stud spacing.

It might save you a lot of hassle to get a base or head gasket for the 250 and see if the stud holes line up with your 125 studs. Or get someone to measure their CG250's stud distances so you can check that against yours. I fear that you are trying to put a "big block" cylinder on a "small block" case. I also suspect that the 125 crank was never intended to handle the 250 power.

Something like this would give you 150cc and be simple, cheap and designed to fit your motor:
https://www.aliexpress.us/item/22518...Cquery_from%3A

And a CG150 head: https://www.aliexpress.us/item/32568...Cquery_from%3A

[Megadan]

Originally Posted by TominMO

The kit that Megadan listed says it is for 150cc motors. I'm thinking that if it would fit a 125, they would have said so. Probably there is something that makes it not work in the 125. Such as the stud spacing.

I had to dig a little, but you are correct. Thanks for catching that. The CG125 is 85mm bolt spacing on the diagonal, the 150-300cc engines are 90mm on the diagonal. The 125 also has a 64.5mm sleeve bore in the cases and the 150 is 68.5mm.

Basically, the best you can do with the 125 engines is a 150cc big bore kit wth a 62mm piston. Otherwise, you need to start with at least a CG150.

[ego]

In my opinion the 4 valved head is too expensive for this build. That’s why I am planning to use a 250 polished and port matched head. Maybe I will use a custom camshaft too. My only question is, will the stock crank and gearbox handle it. Newer opened a case by before. Thats why I am asking these things. Will there be any overheating issues? Is an oil cooler necessary? What else do I have to upgrade to make it work reliable.
Last but not least which size carb do you prefer on this build?

[Megadan]

Originally Posted by ego

My only question is, will the stock crank and gearbox handle it. Newer opened a case by before. Thats why I am asking these things. Will there be any overheating issues? Is an oil cooler necessary? What else do I have to upgrade to make it work reliable.
Last but not least which size carb do you prefer on this build?

My Hawk is basically rocking a "stage 3" Motocult big bore kit, which is over 20hp at the rear tire, a 6-7hp increase from stock, which is nearly 50% more power. It has enough compression/cylinder pressure that you can feel it vibrate the ground at idle. Crank is just fine, so I wouldn't worry about that unless there is a manufacturing defect that can cause a problem.

I run a decently large oil cooler, and it doesn't overheat at all. I would say that if you do want to hop up the engine like this, an oil cooler is definitely a good idea. At the very least one of the 65ml coolers that uses the valve cover oil ports. This will drop cooled oil right on the valves and head, and will keep the rest of the oil at least a bit cooler too. I have a 125ml cooler that draws from the sump and sends the cooled oil straight into the pump to cool the entire oil supply. It's a rather large, but also very very effective cooler.

Side effect of a good oil cooler is that it also makes your oil last lot longer, and helps prevent clutch fade.

[jsumd]

That is a very good looking bike! I'm afraid I won't be much help on the rest haha