some Chinese engine information

[lingeringstin...]

I've been taking apart some Chinese twin engines and have discovered a few things.

For starters the Honda twin "Benly" clones are almost identical between the 125 and 250. They use the same engine cases, clutch, alternator, starter and they look almost identical except the 125 has a physically smaller top end.

The 125 engine studs are shorter than the 250 ones.

The 125 has a 41mm stroke and uses 13mm gudgeon pins. The 250 has a 53mm stroke and uses 15mm gudgeon pins. As far as I can tell either crank will probably bolt in to either bottom end.

So if you were converting a 125 into a 250 you'd need the 250 crank, cam chain gubbins, barrels and pistons, engine studs and head (or use the 125 head for high compression).

I am not sure if the 125 and 250 gearboxes are the same. I had already built my 250 engine before I acquired a 125 engine for spares and didn't have the opportunity to compare the gearboxes. But after seeing how similar the two engines were in every other respect I decided to build 125 as a spare engine but with a few design changes.

The 250 engine I recently rebuilt is a 253FMM. I've subsequently taken apart two different Chinese 125 engines now, one was an AJS Regal Raptor DD244FMI and the other one was a Jinlun 125 JL244FMI. The AJS had a 360 crank and the Jinlun had a 180 crank. Other than that they were identical.

In the photo below the head on the left is from an AJS Regal Raptor 125 engine. The inlets are about 22mm. The head on the right is from a Jinlun 125 twin, unknown model, and the inlets are about 19mm. Other than that they are the same.

https://s28.postimg.org/nf7wt7t99/111.jpg



The bolt pattern on the 125 heads is exactly the same as the 250, so you should be able to bung a 125 head onto a 250 motor to make higher compression. I don't see why this wouldn't work, the 250 pistons are basically flat top so it should work fine. I am not sure if the 125 and 250 cams are the same profile but they use the same tappets and maybe even the same valves. I haven't been able to check into this yet but I do know either cam fits either head. Both the 125 and 250 heads appear to be exactly the same height so the 250 cam chain and cam chain tensioner should work fine for putting a 125 head on a 250 engine.

125 head on 250 barrels:
https://s27.postimg.org/eau0gcvqb/image.jpg



The 250 barrels actually fit onto the 125 bottom ends. They just slid straight onto the AJS 125 bottom end, and were just slightly tight on the Jinlun bottom end but it's only by a very small bit that could be dealt with in a few minutes by some heavy sandpaper. Either head should fit but the 250 barrels are 15mm taller than the 125 barrels, so to fit them to a 125 bottom end (using the 125 crank) you'd need to shorten the 250 barrels by 15mm to cope with the shorter crank stroke of the 125. And this is where it gets interestingly complicated.

I intend to make a hybrid engine using a 125 bottom end with a 360 crank and am going to fit shortened 250 barrels to it, bored out to 56mm and using YX140 pit bike pistons because they have 13mm gudgeon pins to fit the 125 crank. I will use the AJS head with the larger inlet ports.

I'm going with the 360 crank on the 125 bottom end simply because I'm using a single carb and I've read somewhere that a 180 crank is a bit harder to get to run right with a single carb due to the funny inlet timing of the 180 crank.

This should make about a 200cc short stroke twin engine something akin to a Honda 185/200 Benly. I know it sounds like a lot of work when I've already got a perfectly good 250 engine sitting there but I am just doing this to prove it can be done. In theory I should get about 17 or 18 horsepower out of it. The 250 long stroke engine only makes about the same horsepower so there probably won't be much difference between them. They will both have identical motor mounts and use the same carb and exhaust so either one can be fitted to my bike without changing anything, they'll just bolt right in. This should be an interesting experiment. Pics to follow as I get to it.

Well, somebody's got to do it...

[Rogerborg]

Thanks for posting this. So many times it's been asked, so few times answered.
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GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[lingeringstin...]

I know, it's terribly unnecessarily complicated since there seems to be precious little actual information out there about what's what in the world of China bikes.

So far my limited experience with the standard 125 and 250 Honda twin "Benly" clone engines (often referred to as CA, CMX or CD engines when looking for Chinese spare parts online) is that they share the same size and shape bottom end meaning either engine should fit into the same frame. That could be useful if for instance somebody had a 125 and they wanted to make it a 250. They could just bolt in a 250 engine and as far as I can tell virtually everything else hooks right up the same so no fiddling with wires and stuff. It might appeal to some people to get the 125 they want and do their "L" plate business on that and then fit a 250 engine once they pass their test.

Where to actually get engines is another matter. I keep coming across internet sites that offer engines and bits for sale directly from China, often only offering bulk orders, but there's also places like ChineseMotorcyclePartsOnline (CMPO) if you just need to maintain a China bike but there's very little technical information available. There's probably some good Chinese bike forums out there but I haven't had time to look for them yet.

Actual Honda Rebel and Benly spares also seem to work in my limited experience, so parts availability is probably pretty good. I've found the Honda Rebel 250 repair manual works perfectly fine for the Chinese copy 250 engine.

The bigger problem with Chinese bikes seems to be the rather poor quality of everything else about the bikes. They do rot badly in every way. But I'm only using the engines and they actually seem to be built alright so far. Not GREAT, but pretty interesting from the standpoint that as far as the Honda twin clones go pretty much everything seems to be interchangeable should you need the usual parts for them. It's good that they all share starters and alternators and clutches. Even the gasket sets appear to be identical apart from the head gasket between the 125's and the 250's.

Basically my experiments are born from my idiotic curiosity to see if I can change things just for the hell of it. I only started messing around with Chinese twin engines because they were pretty much the only thing that would fit my bike due to the stupid frame design of my MZ. For most people it's probably more expense and hassle than it's worth to start mixing and matching 125 and 250 bits just to come up with something in between. If you have a 125 and you want more power just fit a standard 250 engine in it's place. I'm just lucky that I came across a couple of really dirt cheap engines to poke about with and I don't mind doing it just to see what happens. I'll post pictures of anything interesting along the way.

I've already built a Chinese 250 engine to fit to my MZ frame and once I finish my 200cc spare abomination I'll probably be sick of the things but at least I know that I can fit a variety of cheap Chinese engines in the same spot so long as I stick with the Honda twin clones.

[stinkwheel]

Haw have you got round the fact that the final drive comes off the opposite side on the MZ? Just flipped the rear wheel? If you have, does the brake cam-on ok?
____________________
“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.

[jjdugen]

Love this thread! Nothing more satisfying than making things that shouldn't work....well, work!
I'm not sure of the inter-changeability, Teff's probably your man, but.
The late 60's early 70's 175 / 200cc Hondas make quite satisfactory power outputs, maybe look to the cams / carbs from those genus of this engine. Both types of crank used, plenty sold, though how many remain is anyones guess, I think Teff has most of them!
Keep us up to date, this is very enjoyable.

Cheers.
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The CBR900RR has been sold. Aprilia Falco worms its way into my heart.
Try Soi 23 on Amazon for a good read.... Self promotion? Moi?

[Polarbear]

It is interesting from a mechanics point of view but Chinese 125's and 250's?

Go sit in a corner with Linuxyeti and dribble over the damn things
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Triumph Trophy Launch Edition

[lingeringstin...]

[lingeringstin...]

[lingeringstin...]

I just thought I should mention this as there's bound to be somebody out there thinking this is useful Chinese bike information.

The 125 and 250 heads use different exhaust collars. The exhaust stud spacing on the 125 is closer together than on the 250, and they use different size exhaust gaskets. You have to use the right exhaust collars for the engine size.

I didn't think of it at first because the Quadzilla 250 two-into-one exhaust I have will fit either head. This is because the actual spacing of the exhaust ports is the same, which makes sense as the heads have the same bolt pattern and are pretty much identical apart from actual size and combustion area. Not sure about the valves yet but at a glance they look the same size.

The quad bike exhaust I have will fit either head so long as I have the right collars for the engine it's going on, which are easy to get as it's all Honda copy stuff and readily available.

ALSO...

I still think there's some difference between the 125 and 250 gearboxes but since I don't have a 250 apart at the moment I can't say how they differ.

This occurred to me again when I was trying to order an output shaft seal and forgot what size they are so I looked up a parts drawing to see if I could find the measurements.

The gearbox parts diagram here:

https://www.fowlersparts.co.uk/parts/view/91201030033

...says it's for all kinds of Honda 125, 185 and 200 engines but no mention of any 250's so I suspect there must be some slight difference in the gearboxes but I also suspect that with minimal modification (or none at all) either gearbox will probably fit either bottom end as they share the same dimensions for engine cases and use the same clutch, gear shifter and oil pump.

I suspect it's all down to slightly different gearing between the 125 and 250 but that isn't really an issue for me since sprockets on my amalgamated contraption are easily available in all kinds of sizes.

I'll be seeing the nice man at the machine shop soon to have him shave down and bore out some 250 barrels so i can put them on a 125 bottom end to make some kind of Chinese 200 twin engine for no apparent reason whatsoever. I still have some parts to get for the build so we'll see how it goes once I've got all the bits together.

And then of course there's still all the wiring to do and the motor mounts to make and the footpegs and pedals adapting and the exhaust making and the carb-sticking-on somehow and the battery getting and the butt scratching and the "Eureka!" shouting and the inevitable dammits, shites and feck I broke it's.

Hammertime!

[c_dug]

Out of interest, do you insure your bike as standard MZ?
____________________
I am a bellend, I am a man of constant sorrow, I am a gummy bear, I am a rock.

[Tankie]

[lingeringstin...]

Believe it or not the insurance people don't seem to care about any modification or engine changes etc. I've spoke to them over the years and so long as nothing I do to it makes it better or faster they really don't care. I think it's considered so old, so pointless and so underpowered as to be of basically no interest to them. It's never been "performance enhanced", on the contrary actually, and I suppose it's not considered in any danger of being stolen because, well WHO would? It's basically hideous, worth exactly fuck all so the only thing they are concerned about is if I attack somebody's shiny new Range Rover with it and scratch the paint, which sadly in today's world would totally write off the Range Rover.

[lingeringstin...]

Just thought I'd point this out in case anybody cares.

As far as I've experienced it the Honda and Chinese cranks and cams are interchangeable in the engines, BUT...

SOME Honda twin engines appear to use a double row cam chain and all the Chinese Honda clones I've come across seem to all use a single row cam chain.

What this means is if you're using a Honda crank you'll PROBABLY need a Honda cam chain and a Honda cam sprocket. If you're using the Chinese crank you'll need to use the Chinese cam chain and cam sprocket.

It's just a matter of matching the cam chain to whichever sprocket is on the crank end. Also the 125 and 250 cam chains are different length so don't get them mixed up.

The Honda stuff looks better than the Chinese stuff and the prices reflect the differences in quality. I haven't run either engine to excess (yet) so I can't vouch for how well each one holds up, but due to the design of both cranks I would expect that under normal use with acceptable maintenance the little ends on the con rods will probably be kaput before you wear out a cam chain.

Unfortunately the stock cam bearings are a bit shit in both engines and they will probably be the first thing to go anyway.