How to break in new piston/rings on a CB125S

nrcb125

nrcb125

Been Around the Block
I recently finished my first top end rebuild on my '74 CB125S. I can't wait to kick the little bastard over, but I'm not sure how to break in the new parts.

Some articles have said to not go full throttle and be extra careful, some have said to pretty much ride normally and check the oil more frequently. Also there was discussion about using conventional oil over synthetic. It's my understanding synthetic has better wear properties which may not be good for break in.

Because it is such a teeny little engine, it's hard to ride it without really working the revs. Not sure how to go about this whole process.

What are your thoughts or recommendations?
 
Conventional oil for the 1st 1000 miles. Don't exceed half throttle and 60% of redline for the first 500 miles, and always keep the rpms fluctuating. Do NOT hold any single RPM for any extended time, including idle. For the next 500 you can start getting on it, but keep it to 3/4 throttle or less and do not redline. After 1000 miles, change the oil and you're good to go.
 
Grip it and rip it! Ride it like you stole it! OR... if you just got it new how would you ride it? Just by nature unless you go from driveway to I-95 and set the cruise control you will fuctuate power/rpm so no biggy really... use a good standard oil for a "while" and then IMO still use a good standard oil... unless your motorcycle has modern tolerances and materials, the use of a (fake or ester) syn won't be any benefit because you still need to change it often. The best advice would be to NOT get it too hot or ride it into a lake. The cam is not new so you are seating rings is all and that as long as you ringed it right and the hole is honed correct with a good hatch... you will be juuuust fine.
 
and retorque the head after the first heat cycle make it a short one ,like a quick shakedown ride a few miles at most...let it cool doesn't need to be stone cold a few hours is fine.then retorque to factory specs
 
It's a small bike as you said, I would avoid going over 40mph during the first 100 miles and 45mph for the next 400. As High On Octane recommended, take it easy on the throttle for that period. Avoid over-reving/ over working the machine especially for extended periods of time, but don't baby it too much ether. Short bursts of 1/2 - 3/4 throttle are just what it will need...being too gentle will never seat the rings.

Don't wring its neck! A friend of mine who has recently rebuilt his Honda Benley (150cc) took it right out the 100 series highways blasting the little bike along full throttle at 60mph...now it smokes like a freight train. I've seen this issue before, even on much bigger machines, I strongly suspect he glazed the cylinders.
 
Dont know if it would apply to the older bikes...but, I bought a new 750 Suzuki in 01 and bounced it off the limiter with 13 miles on it...it ended up being the fastest GSXR in the whole area...luck maybe...I dont know...lol...My buddy bought one just like it and mine would spank its ass any way shape or form...He even had work done and I could still outrun him...
 
the new stuff is so precision that the rings dont break in like the olden days they fit and seel perfect from the gitgo
 
1fasgsxr said:
Dont know if it would apply to the older bikes...but, I bought a new 750 Suzuki in 01 and bounced it off the limiter with 13 miles on it...it ended up being the fastest GSXR in the whole area...luck maybe...I dont know...lol...My buddy bought one just like it and mine would spank its ass any way shape or form...He even had work done and I could still outrun him...
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Your cams were likely speced with a bit more lift than the other bikes were. It happens quite a bit in large production numbers. While it's usually not very much difference at all, if a set of cams in 1 bike were ground just a hair under spec, and another bike was just over factory spec, there could be as much as .005-.008" difference in lift. Obviously, the bike with the slightly bigger cams is going to have the edge.So, it's not that uncommon for this type of thing to happen, just most people never notice it. Hitting redline off the showroom floor would have no effect on performance unless it was a negative effect from spinning a bearing or seizing the piston.
 
Only reason modern stuff has a "break in period" is to get people to eat up warranty miles.....
 
High On Octane said:
Your cams were likely speced with a bit more lift than the other bikes were. It happens quite a bit in large production numbers. While it's usually not very much difference at all, if a set of cams in 1 bike were ground just a hair under spec, and another bike was just over factory spec, there could be as much as .005-.008" difference in lift. Obviously, the bike with the slightly bigger cams is going to have the edge.So, it's not that uncommon for this type of thing to happen, just most people never notice it. Hitting redline off the showroom floor would have no effect on performance unless it was a negative effect from spinning a bearing or seizing the piston.
Click to expand...
My bike had more compression than his...his high numbers were around 185..my high cylinder was over 200 on the compression tester 210 if i remember right..Had a guy that wanted to tear into my bike to see why it was so much faster...that was not going to happen..and slightly different cams would not make it that much faster than his...it would take a big profile difference to amount to the number of bikes I could put on him.
 
You know, I was fortunate enough to spend a few years in aviation as a crewman (grunt broom pusher) very young, I spent a short time with an old school dude that was an AP/IA and did all the 1500hr OH work on specifically Continental 210 pusher power with a variable pitch prop setup to change blade pitch (feather)... all that said, this guy believed if it was gonna break it was gonna break at break in and when he fired those new engines he pre-flighted them as if they had 500 hrs wringing them out and washing the props like an air brake over and over. I remember him saying that the rings need to seat at operating temps or the thing will under perform in the long run.

This was one of the experiences that causes me to keep my tools. I would of never paid attention, but ole Newhouser was a stubby prick and he asked me to help him hang oil drain buckets because he couldn't reach without a stool to latch the twisted safety wire hook to the underside of the engine ;D somehow we became buddies (I was his gopher) and I learned a bunch that year.

So grip it and rip it! break it in under normal intended use temps and change the oil after a few hours ride time, maybe you can gain a pony or two even. :eek:
 
1fasgsxr said:
My bike had more compression than his...his high numbers were around 185..my high cylinder was over 200 on the compression tester 210 if i remember right..Had a guy that wanted to tear into my bike to see why it was so much faster...that was not going to happen..and slightly different cams would not make it that much faster than his...it would take a big profile difference to amount to the number of bikes I could put on him.
Click to expand...

To much compression like that is noted in the manual as a bad thing.......
 
1fasgsxr said:
My bike had more compression than his...his high numbers were around 185..my high cylinder was over 200 on the compression tester 210 if i remember right..Had a guy that wanted to tear into my bike to see why it was so much faster...that was not going to happen..and slightly different cams would not make it that much faster than his...it would take a big profile difference to amount to the number of bikes I could put on him.
Click to expand...
it is more likely that from the factory your clamms were dead on the sweet spot timing wise ....they wouldn't have any more lift that is a dimension that a factory can easily keep in spec with no variences
 
I should have checked the cam timing but never wanted to open any of the covers up ....lol...I should have kept it. It may have been just one of those...the guy who built the engine may have got the best batch of parts and made a runner that day...a Tuesday bike... ;D
 
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