replacing Honda CM 125 Conrods with CB125 Conrods?

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

Can you replace the CM125 Conrods with the honda CB125 Conrods on the Honda CM125?
Also must it be from the same year of the bike? (1984)

Finding it really hard to find cm 125 parts! Confused

Their either not what I need or overpriced for the condition!

Really appreciate the help and advice

Thanks! Mr. Green

Hokum · Spanner Monkey Joined: 26 Apr 2012 Karma :

I think Tef will reply in 3....2....1....
____________________
Sane: To know when you are doing something crazy.

gavcarter · Could Be A Chat Bot Joined: 28 Mar 2009 Karma :

I tried to check microfiche for you to see if part numbers matched but crankshafts are sold as complete unit with rods Mad

gavcarter · Could Be A Chat Bot Joined: 28 Mar 2009 Karma :

The complete crankshaft unit for a 1982 cm125 also fits the 1985 cm125 plus many later years - not sure if 1984 was different or just followed on....
a CA125 rebel 125 also shares the same part number for the crankshaft unit so rods should be identical - but AGAIN 1984 is not listed - it jumps from 1982 to 1985.....
https://www.mototoro.com/search.aspx?Keyword=13000402702

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

gavcarter · Could Be A Chat Bot Joined: 28 Mar 2009 Karma :

it would be quicker to pull the rods from both engine and compare with a micrometer than read Tef's post, but f**k me the bloke is useful!!!

I enjoy reading his posts to be fair, usually like a mini-book on the subject, very informative Thumbs Up

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

gavcarter · Could Be A Chat Bot Joined: 28 Mar 2009 Karma :

They actually worked out cheapest for some parts on my bike, other sites are reporting the crankshaft as discontinued or contact for price Laughing

In respect to the models using same parts - i Honestly have no clue Embarassed

Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

The cranks are sold as an assembly, because... they is an assembly, and not user serviceable. This is a hint....

And begs the question, why the question? Why do you want to know if CM rods are the same as CB rods....not user serviceable, you probably shouldn't be trying to 'service' them, whether bits of the assembly are the same or not.

How do you expect the answer to help you, or solve your problem..... which one must presume is that the big ends on your CM crank be fucked.

'Fix' as suggested by the parts listings is to get another whole crank assembly....

Getting a crank reconditioned by one of the specialists is approx £300, ISTR.... approx £150 a journal, so double what they would charge to do a pressed up crank on a two-stroke single..... doesn't seem too unreasonable given they have to critically balance and align the pins.

And its NOT a job I would try at home with a vice and a coppa hammer... have seen old boys do old Brits bikes like that.... B-U-T... something of a dark art, and old brit-bikes didn't potentially rev to 14,000 rpm, and still had a big question mark over how long it would be till they cranks shook themselves to bits.... typical shedologist might make one hold together long enough to be nursed to a few classic meets... I wouldn't have much faith in one done by any but a 'master of the art' proving reliable in an every day rider.

And as, for the same price as a pro-recon, I could probably buy two or three complete CB's to 'break'... (or necromance!) I tend to treat CB cranks as disposable assemblies....

What's up with the CB crank? where did it come from? Do you have the rest of the engine it lived in?

My suggested solution is to build up a CB spec motor around a the 'good' CB crank. You'll need the CB cam, to get 180 crank matching valve timing, and you'll need CB ignition to fire 180 sparks; but you'll gain at least 30% more power for the doing of along the way.

If you have CB crank, and the matching bits to build around it from a CB engine, dont really matter which cases you choose to use; and you probably already have most of the bits needed.

If you need to go source another crank? Well, as hinted; these little benleys aren't really in either short supply or commanding premium prices; you might source a good crank assembly from a breaker or e-bay for maybe £100-£150, but you could equally get a whole shed of a bike for that, you could break for the crank...

..if not the whole engine.... and if whole engine was serviceable... need not be a 125 Benley, avoiding the anomoloy of the CB's 180 crank... all the other's are 360', and come in 175, 185, 200 & 234cc displacements; and are pretty much bolt-for-bolt drop in interchangeable.

Which takes us back to your last posts... where you had gear selector problems, and when you pulled the cases apart; motor was pretty much a complete dodo; with knackered selector mech, chipped cogs? (I cant remember) and some disturbing motor-damage, given you had done a complete top end rebuild prior to the shift niggle....

So where are you at? and What are you trying to do? And what's changed since last time? (Like where did that CB crank come from?)
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
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?'

gavcarter · Could Be A Chat Bot Joined: 28 Mar 2009 Karma :

Hurrah for bed-time reading!!

I honestly didnt think of swapping the rods at home - I was just thinking what other bikes might have the same crank....

Robby · Dirty Old Man Joined: 16 May 2002 Karma :

I'm assuming it's a pressed-up crank, so you can't just unbolt the conrods - the whole crank assembly has to come apart.

If your engine is knackered to the point of ruining the crank, get another engine. If the rest of the bike is in equally sorry state, get rid of it, a secondhand Chinese 125 would be better.

With Chinese 125s on the market there is very little reason to bother spending any money on stuff that was cheap in early 1980s. The Chinese stuff has a reputation for being unreliable, but that is improving rapidly. 30 year old Japanese 125s will not reliable - they've had 30 years worth of 17 year olds trying to fix them. Parts are also rather hard to come by, and as they start to become "classics", people will buy up good parts and stockpile them.

Don't waste money on it. Sell it to Tef.

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

Hm, I thought you could split the crankshaft and just slide off the conrods?

I didnt know that if one thing gets knackered on the crankshaft you have to replace the whole thing. Shocked

I have a kind of complete CB engine in the house now with just the rotor cover missing.
Its a seized engine that Ive had for 2 years now, given to me cuz they didnt know what to do with it.

I tried set it up on my cm, but it just wouldnt start with alot of worn and need replacing parts.

So going back to my cm 125 engine, I need to replace the return spring on the gearbox to sort out my gear lever.

I had that piston seize on the conrod that I had to saw off, and the pin that goes through the conrod to secure the piston is stuck on the conrod because the pin expanded from the heat so the pins not going anywhere.

This is why I need another conrod, so you suggest I just get the whole assembly of the crankshaft? Theres one going on ebay but It says 1982-3, hence my other question, can I use a part a different year to my bike? Plus I've asked the guy some questions the other day and though he can submit price offers to me, he wont answer my questions? Neutral

I've just got another piston set but cant do anything yet because of the conrod. Rolling Eyes

I have quite an attachment to my cm125, it was my first bike so thats probably why, I've got a cm250 so I can still ride but I dont want to get rid of my cm125 or sell it! I just want to fix it and get it back on the road! It's a weird emotional attachment, maybe because I'm female? aha

Its been re-chromed, repainted, no rust spots, everything on the bike apart from those stated on the engine now works fine because I spent alot of time fixing her up.

Its just the engine, I say its worth it fixing it, Ive spent enough money for the parts why stop now? Its so close to being a normal working bike then a death machine!

Thanks for all the help and advice! Mr. Green

but I'm still feeling like I'm in the same spot aha Neutral

I'm in a bit of a state as you could imagine aha

Thanks guyes, looking forward to your replies! Smile

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

Would it work or fit if I put the cb125 whole crankshaft in the cm125?
And what would I need to change if it did?

I hope someones tested this before or something so I can get a yes or no so I can look more into other ideas?

Thanks Mr. Green

Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

Heres the pic of the crankshaft that I'm watching. Will this do?

Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

Cant see the cam sprocket, but looks to have the cam-chain attached, and it appears to be a Hi-Vo chain.
If it comes with the Hi-Vo chain and you can get them to supply, or can source separately, the corresponding cam-sprocket.... and possibly the cam-chain tensioner blades and mech.. then could be useable....it would appear to be a 360 crank.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
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?'

bypass2 · Spanner Monkey Joined: 12 Sep 2013 Karma :

I have got a cm 125 engine but how can you tell what year it is and if it was the same as yours. or were they the same

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

Morning people,

Theyve replied and confirmed its a 360 crank and hi-vo, I'm gonna have to ask them if they can supply or source the corresponding cam-sprocket and the cam-chain tensioner blades and mech. And then swap all that over, so more work for me Laughing

So if this had the roller cam-chain it would be good to go, if only it was that simple. Rolling Eyes

Ive decided I'm gonna keep searching, now I'm more aware of what kind of crankshaft I'm looking for, and if I'm lucky enough, maybe an engine might come out of it for my patience Very Happy

Thanks for the help and if anyone has this engine for sale or knows anyone who does I'm interested haha As long as its working unless given all details about it Laughing

Thanks for answering my questions Tef.

Thanks everyone else

hannadarling · Two Stroke Sniffer Joined: 13 Apr 2014 Karma :

I am so glad I didn't buy the crankshaft in the end as I have found and bought a Honda cm 125 with a great working engine for £160!! Just because the guy had loads of projects to do and needs the spare cash I guess. Theres afew bits missing on the bike but who cares? I got an engine with spare parts Mr. Green