Topic: CB125 Head On C100

[Mars]

I recently got a cb100 and know someone with a cb125 head so I figured I'd ask around and see if anyone knew if I could simply throw the whole 125 head on my 100. Any input is appreciated. Thanks!

[mlinder]

Bore is like, 4 or 5mm bigger for 125 head. You'll have a mushroom shaped combustion chamber, and no quench or squish band.

Bad idea, if it even fits.

[kirkn]

What vintage?

If you're talking about the 69 ~ 75 or 76 single-piece heads, yep, they're fully interchangeable with no real issues.

I'm currently running a '72 SL125 on my '75 CL100 motor with no issues at all.  And MANY folks are running the 125 heads with their slightly bigger valves on 100's and competing in the vintage 100cc MX class.

Just my two cents...

[mlinder]

stock bore?

What are the bores on those two you are running, kirk?

[Coyote13]

Don't quote me, but I think the later SL/CB/CL 125 heads should fit the early SL/CB/CL 100

There are a few resources that discuss these little buggers in pretty good depth, this is a good place to start: http://www.thumpertalk.com/forum/archive/index.php/t-476527.html

There are MANY similarities AND differences between different models and year, just do your research and you'll be alright. Good luck bud

[kirkn]

stock bore?

What are the bores on those two you are running, kirk?

Stock head off the SL125 (cleaned up and lightly ported) plopped on the stock bore of the CL100.

And I misspoke on the model years; both of them are '72s. 

[kirkn]

Don't quote me, but I think the later SL/CB/CL 125 heads should fit the early SL/CB/CL 125

There are a few resources that discuss these little buggers in pretty good depth, this is a good place to start: http://www.thumpertalk.com/forum/archive/index.php/t-476527.html

There are MANY similarities AND differences between different models and year, just do your research and you'll be alright. Good luck bud

The later two-piece heads do indeed fit the earlier motors, but a few extra parts need to go, too.  The cam in the two-piece heads sits a bit closer to the head gasket surface than in the one-piece head, so the shorter cam chain and both shorter chain tensioner pieces are required as well.

And the later 2-piece head has larger valves and much better stock ports than the earlier 1-piece heads.  The 2-piece heads are generally not legal in the vintage 100cc MX class.  Gotta run the 1-piece head.

Yep, lotsa info out there...

[mlinder]

stock bore?

What are the bores on those two you are running, kirk?

Stock head off the SL125 (cleaned up and lightly ported) plopped on the stock bore of the CL100.

And I misspoke on the model years; both of them are '72s.  

So you don't find the 6mm difference between the cylinder bore, and the outside of the quench band on the head to be an issue?

[kirkn]

You mean am I philosophically opposed to it??  Or do you mean does it cause any running issues? 

As for running issues, no, it doesn't cause any problems that I can detect.  I've put 500 commuter miles on the motor with no issues whatever so far.  No detonation, no loss of power, no issues.

As for philosophically, I'm being a smart-aleck, of course.  But I'm not sure there's any actual difference in the combustion chambers between the 100 heads and the 125 heads at all.  I have 4 or 5 of these one-piece heads from various motors - 100s, 125s, SLs, CLs, CBs and all the combustion chambers, valves, and ports all look completely identical between them all...

[mlinder]

Interesting. Would you be willing to mic the OD of the combustion chamber and the ID of the cylinder?

You don't have to, I'm just interested.

[kirkn]

Well, I recorded the ID of the cylinder at the time I rebuilt the motor but I'm not sure if I kept the data once the motor went back together.  I'll look.  But, to get you in the ballpark, it was a stock bore '72 CL100 motor.  So, within a few thousandths for wear, that's what it was.

As for the combustion chamber OD, I'll measure up every head I still have open.  The SL125 head is obviously back on the CL100 motor, but I have 3 or 4 others I'll measure.

[mlinder]

Sweet. Interested to see!

[Coyote13]

Kirkn you have any 125 heads that would fit an early ('70 or '71 CB100)?

You might have a potential buyer if ya do 

[Coyote13]

And not directly related to the question at hand, but if you need a little "inspiration" this guy's build is pretty cool

http://www.caferacer.net/forum/topic.asp?TOPIC_ID=11025

[kirkn]

There are a few resources that discuss these little buggers in pretty good depth, this is a good place to start: http://www.thumpertalk.com/forum/archive/index.php/t-476527.html

Ha!  I just went back and followed your link - that's ME!  :-)  I'm KDN over on Thumpertalk.

Geez, was it THAT long ago that I started that?  2007?  Makin' me feel old... 

[Coyote13]

Haha that's crazy man! I've been through that link SO many times in preparation for bringing my little 100 back to life, just never seem to have the time to do it. I will have my day!!!!

[jaguar]

the head to get is the XR200 two piece head.  biggest valves and best runners.

the 125 and 100 heads can all be swapped around as needed.  as long as they are pre 76
some have larger valves then others. have a good listing somewhere

oldrice.com has some great info on these motors.

[kirkn]

the head to get is the XR200 two piece head.  biggest valves and best runners.

the 125 and 100 heads can all be swapped around as needed.  as long as they are pre 76
some have larger valves then others. have a good listing somewhere

oldrice.com has some great info on these motors.

That's where I got a lot of my own info - oldrice.com

I've read, though, that the 200's head is wayyy to big in the chamber for the 100s.  Drops the compression down to, like, 5:1 or some extreme number...  I seem to remember reading that on The XL Forum, which is linked from the oldrice site.

And the post-76, pre-79 or so heads can be swapped on, too, as long as the shorter components are used, too.  Somewhere in the late 70s/early 80s tho, significant changes were made and those heads don't swap on to the earlier motors, or so I've read.

[jaguar]

yeah i wouldnt do the swap and then keep the stock piston...
your already there right?

Hell after that send it off to prowoll for some crank work
Tigh cams and the whole lot....

then MAYBE just MAYBE you will make 17hp.
im looking for 20hp out of mine at 172cc short stroke high compresson madness

[Sam Green Racing]

Hi Jag, how's the little lSR doing?

Ya can stick any head on these little singles how about a DOHC Yamaha FZR600

http://forums.sohc4.net/index.php?topic=35474.msg745377#msg745377

Sam.

[jaguar]

I'm at school now(aero engineer) so work has been slow
Jug is being resleeved to take an xr200 sleeve and forged HC piston
Talking to tigh about cams and needle bearings
Collecting trans gears(mix of cb sl and tl seems to make a nice box)
Msd ignition
Few other hopes and dreams like making a Ti rod...
Goal was to run maxton at about 80mph in the spring.  But now thinking next fall...

[Sam Green Racing]

It's all money and waiting but it will happen for you. I was 25 when I broke my first record, the next 3 came when I was 29. 34 years later, money no object and the best I could do was equal the Brtish best, some you win, some you loose then you run out of money and start saving all over again.
You do know that the guy who runs Maxton is a member on here.

Sam.

[jaguar]

well if im 25 and your at the "money no object" point then i think we should team up and brake some records!
wanta start with the MG175-4 class?  soft record.....
i used a mobile police speed cart/sign to play with gearing and such on a semi stock motor,  got in the low 60mph range...
when can you ever test and tune a bike with government stuff and still only be 5MPH outside of the law?

we have a full metal shop here on campus and im just starting to learn how to use lathes CNC and the rapid prototype tools......but cant really use them to much for my stuff