what's the difference in these cams?

[lingeringstin...]

I've got a choice of two seemingly different cams that will fit the 253FMM engine I'm building. They both apparently fit but they cost slightly different. I can't find any info about the lobes on them. Just wondering if anybody knows the difference or where I can find info.

The cams are either for CM/CBT125-150 or CA250. If they cost different then there must be something different between them but I don't know what that is and I can't find any information about them.

This may be relevant as I have an idea to fit 56.5mm flat top pistons instead of the standard 53mm dished pistons to get higher compression. I think there's enough liner to bore out that far but just wondering about valves in case I have to modify something.





I do wish it was easier to find real undiluted mechanical info on obsolete bike engines. Asking on things like Honda Nerd forums generally gets answers like "how dare you mess with the perfect design of a sacred Honda engine" or things like "it's all Chinese shit and you're wasting your time because it will instantly explode unless you fit a 30 year old Japanese engine with a hundred thousand miles on it you got off Ebay from a dodgy bloke in Slough".

[Teflon-Mike]

All Japan built CB125's were 180 timed.
The Shinray/Honda derivative 'should' have been too; B-U-T.. you get into the minefeild of China market-models and I believe that some clones did get 360 motors.
It could be either 180 or 360 timed, f from China, if listed for the CB.. but odds are it's 180.. though that wouldn't fit a CM rebel, that should be 360 timed!
Clocking up a couple of chinky cams, seems they have never had the tooling or tolerancing to make either the 'hot' 309 cam from the full power 180 timed 125's or even the reduced lift and dwell 'reduced effect' cam for the 125TD-C/E. 180 timed cams appear to have been based on the later 125-TD-J motor, that had hi-vo cam-chain and CV carbs, and a cam that had even softer timing, possibly even the more common 360 motor's lower duration lift and dwell, to suit the CV carbs.
All the 360 timed motors seem to have used that softer cam, that does have less duration, lift and dwell.. and Chinks have ether exploited worn tooling or deliberately softened the profile a tad more to aid with emissions issues.

I dropped a Chinky CB125T cam into a CB125 motor and it was a pig to set up, and just didn't respond to the factory carb settings as every other motor has; when tweeked best as; it did pull the higher revs a CB should, but reluctantly, and was rather asthmatc throughout.

So, your guess is as good as mine whether its for a 180 or 360 timed motor, but if from China, best guess would suggest it's most likely a soft profile, regardless, and not likely to give you any more oomph than what you got, if that's what you is after.
____________________
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Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[lingeringstin...]

Yeah they were on a Chinese website so they're bound to be for whatever the clones are nowadays. I'm not too bothered really as the cam in mine looks fine. I have no idea if the quadbike engines use a different cam. My engine was supposed to be a SMC250 from some quadbike but most of them seem to use a single carb that looks a bit like a CG carb, but my engine definitely came with the two rubber inlets for the pair of CV carbs like what you'd find on a Jinlun or something, so I would suspect if there is any difference in the cams I've got whichever one they put in that engine.

The pistons I was considering look like this:

https://i66.tinypic.com/2zsz3op.jpg

https://i68.tinypic.com/25s6jbm.jpg


Look to me like they'd fit pretty well. Same gudgeon pin size and the top is only like half a mil taller, which could be dealt with. They have just a tad longer piston skirt but that's easily fixed. The indents for the valves look like they might just work. Anyway they're cheap enough so I might get some to try.