CB 125 Cylinder & Piston on a CB 100 Bottom?

AlphaDogChoppers

Science is true whether you believe in it or not.
The CB100 and CB125 both have the same stroke. There is a good chance that it will work. One way to find out...<G>
 

whynot

~~~If it ain't raining, I'm riding~~{iii}?~~prost~
Hey, thanks dog.....................................

My thoughts exactly...........................pull the trigger and order it and find out.
 

DirtyFrank

Been Around the Block
Let me know how it works. I have a 125 and have been wanting to order the 145 cylinder and piston from that seller.
 

Kanticoy

Mad Science!
DTT BOTM WINNER
You're going to run into grudgeon pin size differences depending in the year 125 you use. One is 17mm, the other is 19mm iirc.
 

whynot

~~~If it ain't raining, I'm riding~~{iii}?~~prost~
So, in other words, that CB125 kit will not fit on a CB100 connecting rod as a bolt on?

So, I just bought something that is worthless to me?
 

AlphaDogChoppers

Science is true whether you believe in it or not.
budlite282 said:
So, I just bought something that is worthless to me?

Probably not an insurmountable issue. It may require a little machine shop work to fit the new wrist pin to your old connecting rod.
 

nrcb125

Been Around the Block
budlite282 said:
So, I just bought something that is worthless to me?

If it hasn't shipped it's not too late to email the seller and say you don't need it anymore. I've got a complete 1980 CB125 engine I don't need. You're welcome to come pick it up if you'd like. PM me if you're interested.
 

Kanticoy

Mad Science!
DTT BOTM WINNER
You could always get the con rod to match? It would require cracking the cases though. Not a big deal on the 100/125s.
 

whynot

~~~If it ain't raining, I'm riding~~{iii}?~~prost~
Kanticoy said:
You could always get the con rod to match? It would require cracking the cases though. Not a big deal on the 100/125s.

So apparently the cranks are the same, so all I need is a cb125 con rod?

Along with this kit I ordered on fleabay. I then so be good to go.

Then bearing, and the list just keeps on growing.

Sheeesh, this little cb100 project is growing more and more.
 

surffly

Over 1,000 Posts
How about you look at how the connecting rod is attached to the crank.
You can not swap rods..well you can but this involves pressing apart the crank and rebalancing it. Not really worth it as they mad MILLIONS of these bikes so the correct part is not hard to find.

Tones of good reading around that show how to mix and match parts to make cheap big bores and such.
 

AlphaDogChoppers

Science is true whether you believe in it or not.
budlite282 said:
so all I need is a cb125 con rod?

It's entirely possible that the only difference is the bushing in the connecting rod small-end. Like was pointed out, changing the connecting rod is kind of a big deal, because the crank is pressed together. It's worth exploring the possibility of simply reaming out the bushing to the correct size.
 

surffly

Over 1,000 Posts
Don't make this harder then it needs to be.
I'll assume if you have to ask about some things that other things are over ones head. Not mean, just is what it is.
Willing to bet that the OP doesn't have access to the tools needed to enlarge the wrist pin, even if it could be. So now we are talking about finding and paying someone to modify a used part...and for what? Just find a 125cc motor, bolt in a big bore(some use cb750 parts) and bolt the heap into the bike.
You can make a faily hot single with no more machine work then boring the cylinder
 

whynot

~~~If it ain't raining, I'm riding~~{iii}?~~prost~
Hey Fly, you hit it on the head......this is above me, that's why I'm asking questions.

Buying anything and everything would take the fun out of learning to do it yourself.

That's the easy way out. Sort of defeats the purpose of forums.

Anyways, you are right, best for me to just trash it and give it up.

Or you could get off your high horse and explain how to do the last 2 statements you made.

And yes, I do have access to the machines you speak of, but, I have work ethics that prohibit me from taking advantage of my employer, unless given permission to use these machines.

However, I did cancel that order, and decided to go with Honda's over bored piston, rings, and have the cylinder bored out.


BTW, I do like your little 125, I have that same tank on my cb100.
 

surffly

Over 1,000 Posts
No high horse at all.

Thing is that the web is funny.
Web is filled with all kinds of info that is out there to be searched...or asked on forums.
Other school of thought is to just buy things and check. Looking around and crossing part numbers will show that the 100 and 125 have different wrist pins. The parts list or manual will also show how much of a pain in the but swapping rods would be. My point is that reinventing the wheel might be a journey of learning and all that. But why not have a journey that has better odds of being sussesful?

Work smarter not harder. Work of the sake of work is lame in my opinion.

Seek out the oldrice website and forum. Read every article PDF they have. Most are dirt but most of the same hop up tricks work on the street.
Get the parts and factory manual, both are found free on the web. This will help show you the internal gearing differences cb,sl,xl,cl,tl. Most can be mixed and matched to make a nice street gear box. As with most old Hondas the head is the weak link, search out xr200 head swaps. The 150cc big bore kits from back in the day are little more then a cb750 piston. Resist the idea of 175+ cc bores as they are tough to live with and tune. Unless you find powroll and get them to make you a stroker crank. Also don't fall for the $20 mikuni carbs, they are shit knock offs.

I love my CL very much. Bought it from the original owner with 1100 miles. Now at 3500, mostly from picking up beer and take out.
I have also built a few singles.

Check out oldrice and the articals.
What is the goal with bike and the engine? How do you plan on using it?
It sounds all well and good to have big bore this and race that, but the truth is you will only net modest gains and still be slow. So making small improvements that fit the character of the bike will be easier to live with and more enjoyable
 

whynot

~~~If it ain't raining, I'm riding~~{iii}?~~prost~
Main goal, and main goal only........

Get it running, don't care about going fast, just want a bike I can get on, ride on bike nites, poker runs, and club rides.

And on nice and warm days, ride to work and back.

Can't stand to drive, rather ride.

Want it to be an ice breaker , conversational item.

Seems, I'm the only vintage Honda rider/enthusiastic here.

It's a nice feeling to go to the garage and have options.....and I want very much for this CB100 be one of those options.
 

surffly

Over 1,000 Posts
Deffinatly not the only Honda guy.

Your best bet then is to just rebuild the 100 to factory.
If you want after that find a 125 and build it up a bit for some more power.
Hell just a stock 125 motor is a nice upgrade in a 100.

Watch the exhaust and intake, these bikes hate short pipes and pods.

If you want messing around, you are better off with the 125. I have some OEM cb750 pistons around if you want to explore that option.
 

whynot

~~~If it ain't raining, I'm riding~~{iii}?~~prost~
So, my decision is to go as standard as possible.

But the cylinder looks as if it may need to go up one size.

So, I ordered a .50 over bore piston and rings and will deliver the jug, piston and rings to a machine shop.

Question is, will I need to have anything done to the head to match this.

The valve seats are shot for sure, wouldn't even hold oil. Valves too probably.

Looking for those and possibly guides on the web.
 

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