Honda 125 twin crankshaft spacing?? UPDATE.

[Stalk]

Hello all
Mini stalk has a new bike, Honda CM125, 1982, with a worn 4 speed gearbox. He also has a CB125, 1984 with a good 5 speed gearbox but a completely screwed top end.

Now, I suspect that the CB has a 180 deg crank and the CM is a 360 deg crank (cb crank pins opposite, cm crank pins together). If this is the case then my plan of good head and barrels on good 5 speed box will not work as the camshafts will be different, and I will have to split the cases to carry the crankshaft over (assuming for a moment that it will fit)

I could go and take the plugs out and see if the pistons rise and fall together or not, but the CM's not here, it's dark and raining. So, anyone know about these crankshafts? Any clues would be appreciated.

Cheers,
Stalk

[Kickstart]

Hi

From vague memory (looking it up 20 years ago trying to source a replacement CB125 engine) I think you are right. CB had a different crank.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[stinkwheel]

So you want to fit a CM top end to a CB bottom end?

Think you're right, CM is 0 degree, CB is 180 degree.

Is the CB cam intact? Does if fit inthe CM head?

If so, you could run the CB gearbox and crank with the CM pistons, barrels and head withthe CB cam.

A CB will run fine on a single carb through a CM manifold (done this myself).

You need to have the same model crank and cam though.
____________________
“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.

[Stalk]

Thanks chaps! You two are the people that I was hoping would notice this thread.

Camshaft in cb is knackered, ministalk did a "rebuild" and managed to block an oil gallery, starving the lh cam of oil and it ate the head.

Having given it some thought, the gearbox noise & vibration is in first and second gear, ministalk says that the kickstart rattles at the same time and that it does not always work. So, if I pretend to myself that maybe the gearbox bearings are ok and that the kickstart ratchet is faulty. I might see how much of it I can get out without splitting the cases and see what happens.

Thanks again,
Cheers
Stalk.

[stinkwheel]

Splitting a horizontally split case is not as scary as it first seems. Tricky bits are pulling the flywheel and undoing the clutch (but you don't always need to, just leave it attached and lift it off).

If the engine is already screwed, it takes the pressure off. Check my thread on the GPz305 in show and tell.
____________________
“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.

[Stalk]

Mr Wheel,

The problem is that the boy has waited since may to get back on the road and is now as happy as a pig in shit that he has a bike, so I sort of don't want to be the one to spoil it. I think that if I get one part from one engine that for some reason won't fit the other, he will be back to push bike. I am trying to kid myself that there will be an easy way out. Splitting cases is no big deal, problem is all the other stuff we have to do on a day to day basis that gets in the way of fun.

Insomnia's a bitch.
Cheers,
Stalk.

[Pete.]

It's a shame he knacked the CB cam because the old 360-degree engine was more powerful than the later 180's. I do believe that CB125TDC bits should fit the CM lump. The TDC engine has a red stripe on the side of the head.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Kickstart]

Hi

I do have the remains of a CB125T bottom end (kickstart still attached) in the garage. May even have the remains of the top end but not a clue where.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[prawny1]

Would it not be easier to source a low mileage 125cc rebel engine and fit that?

That will give a five speed box and electric start (trying to remember if cm is kick only ),


cd250u engine fits straight in just needs exhaust, top engine stay bracket and the cm carb rejetting (or bigger single carb as twins from the cd won't fit cm frame), only good if your licenced though but it does make the cm a much better and safer bike to ride.

I think physicaly anything upto a 200 will fit without changing exhausts and what not, my dad put a rod through the block of his 200 engined cm thats why i fitted the cd lump, it was good for about 80 with the carb jetted up, we fitted the cd front end as well to give it a disc brake which was nice.

[Stalk]

Update: Bike stopped completley and had to be collected in the van, got it home and had a peek. Teeth missing from first gear cog, lots of bits of metal in sump. CM now stripped and cases split, CB engine out on bench waiting for morning. The plan is to put the CM crank, bores, pistons & head into the cb cases with the 5 speed box. Will also carry over the CM points and alternator so the sparks happen at the right time.

Kind of commited now so I will have to make it work.

Thanks for all the comments,
Cheers,
Stalk.

[stinkwheel]

I'd have gone with the CB ignition. I'd guess it will be a bit more advanced than the CM.

It'll have two timing pickups, you'll just need to pick the right one, the other will be 180 degrees out.

Easy swapped over again if you aren't happy with it.

Best of luck.

Proper honda flywheel pullers cost a tenner
____________________
“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.

[Stalk]

[Mini_Stalk]

[Stalk]

Update, Both engines stripped, cleaned up CB cases and fitted up with 5 speed gearshafts and selectors, refitted clutch, oil pump, clutch cover, generator, pistons. Go to slide the CM barrels on, hang on…. Won’t fit.

Err….. 53mm pistons? Quick bit of calculation, 185. That explains why the cc cast on the back of the barrels has been scrubbed off with an angle grinder.

Stomped inside for a cup of tea, had a look, pulled it all apart again.

Problems, CB125 engine buggered, CM185 barrels will only fit CM cases and CB 5 speed gears will not go in CM cases. Time for another tea.

**Bodge mode on**
The first gear from the 5 speed box is the same diameter as the broken first gear but it has one more tooth, the corresponding cog on the input shaft has the same amount of teeth… That will do. Face on “new” first gear cog is slightly more recessed than the old. Find some spacers, used one of the old camshaft thrust washers and a circlip to get the cog close enough to the syncro hub so it did not jump out of gear. Job done.

The next bit was amazing. The only new gasket I started with was where the crankcase half gasket. The clutch cover gasket, barrel base gasket and head gasket had been in the CB for about 30 minutes run time before ministalk realized he had screwed the cam. The plan was to take a chance with these, but I now need head and barrel gaskets for a 185. So it was decided to press the gamble button.

Cases went back together using the old new gasket, cut the 125 gasket to cut the 185 barrel. Re-use the old 185 head gasket. Explained to ministalk that this was probably not going to work (he is skint at the minute) and that I could not use gasket sealer for fear of blocking an oilway. Bolt it all up, set valve clearances and timing, spin engine over on bench with top off and plugs out till I see oil flowing from cam bearing. Top on and sling it in.

Started second spin, no oil leaks (yet), goes rather well! No one is more surprised than me that this has worked.

So, it’s not a rat bike, but could this be counted as a rat engine?

Cheers,
Stalk.

[Pete.]

Are you sure that the 1st-gear cog had the exact same diameter but 1 tooth more? If that's the case then I would expect the pitch to be different. If that's the case you should have swapped both the driven and driving gear for 1st or suffer meshing problems and premature wear.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Stalk]

Geri,
Problem was that the meshing gear was part of the shaft that could not be swapped as it would not have fitted the cases. Lots of checking went into making sure it woild not bind and a lot of blind luck. It works, but how long it lasts is another thing. At least mini stalk gets to ride a bit longer whilst he looks at his options.
Thanks for your interest.
Cheers,
Stalk.

[Pete.]

Yes, it's very common for the driving gear of 1st to be cut into the shaft. It's possible that they used the same pitch on a different PCD, in fact some aftermarket gearbox clusters do just that (as well as over-drive oil pump gears. That's why I asked if the diameter was exactly the same.
Does give you a chance to source some further spares in the meantime
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good