What oem piston will be a good overbore for my 81' CB125S ?
- Thread starter grcamna5
- Start date
CALfeRacer
Fat man on a little bike
Re: What oem piston will be a good 'drop-in' overbore for my 81' CB125S ?
Mine is a 1979
Mine is a 1979
crazypj
Split personality, I fake being smart
Re: What oem piston will be a good 'drop-in' overbore for my 81' CB125S ?
You have to machine top of CB750 piston.
Use same spec as 750 piston in CB500f. (same bore, 56mm)
Need access to a lathe or milling machine
It will not be possible to beat the price of eBay kit
XL185 cylinder is too tall for 125 crank
Lifan may be pushrod motor not OHC?
You have to machine top of CB750 piston.
Use same spec as 750 piston in CB500f. (same bore, 56mm)
Need access to a lathe or milling machine
It will not be possible to beat the price of eBay kit
XL185 cylinder is too tall for 125 crank
Lifan may be pushrod motor not OHC?
Re: What oem piston will be a good 'drop-in' overbore for my 81' CB125S ?
ok PJ,
the ebay kit,was that the one mentioned in an earlier post to this thread ? I was speaking to a buddy who does lot's of CB750 work and he Knows a bunch;I'm going to send him my(spare) cylinder(81-82')and he thinks it might be possible to install a piston from a DOHC CB750(the 4 valve engine)as he uses them for the single overhead cam engines,they're 62mm.The cylinder liner is another thing that he'll take a look at..
ok PJ,
the ebay kit,was that the one mentioned in an earlier post to this thread ? I was speaking to a buddy who does lot's of CB750 work and he Knows a bunch;I'm going to send him my(spare) cylinder(81-82')and he thinks it might be possible to install a piston from a DOHC CB750(the 4 valve engine)as he uses them for the single overhead cam engines,they're 62mm.The cylinder liner is another thing that he'll take a look at..
Re: What oem piston will be a good 'drop-in' overbore for my 81' CB125S ?
Yeah,
this 81' 124cc cylinder has the sleeve and when I look at the bottom,it's a lot of cast iron around the outside dimension of the liner up inside the aluminum 'outer' finned part,much more than the 122cc older style.I look from the top and it does kind of look like they molded/cast the aluminum 'outer' finned part around the inner liner or something.. I bet LA Sleeve co. would be able to re-sleeve it for a price..
I am determined to do this one way or another.
crazypj said:62mm won't fit in stock liner, 61mm is max (been there, done that
You have to bore the 125 liner out of cylinder as they were cast in place
Yeah,
this 81' 124cc cylinder has the sleeve and when I look at the bottom,it's a lot of cast iron around the outside dimension of the liner up inside the aluminum 'outer' finned part,much more than the 122cc older style.I look from the top and it does kind of look like they molded/cast the aluminum 'outer' finned part around the inner liner or something.. I bet LA Sleeve co. would be able to re-sleeve it for a price..
I am determined to do this one way or another.
Attachments
Re: What oem piston will be a good 'drop-in' overbore for my 81' CB125S ?
I could find an XL185 piston and measure the distance from the crown to the wrist pin and see how much taller it is than the oem 81' CB125S piston.. I think it's(XL185) a 'flat top' & I'd like to find a slightly crowned piston similar to my oem CB125S piston..
The stroke difference between the CB125S(49.5mm) and the XL185(57.8mm) is 8.3mm and the XL185 piston(63mm bore) and cylinder would also work if I could find the taller(8.3mm taller from the wrist pin to the edge of the crown) piston crown to keep the compression numbers(9.4:1) close to my stock CB125S.The cam chain for the CB125S is 98 links while the XL185 is 100 links and I would swap them.I know it'll work but it depends on the right piston
edit: the 'right' piston..,now there's a thought.
This is my stock piston for the 81' CB125S and the distance from the top of the wrist pin hole to the top of the edge of the crown is .609in.
Any of you have an XL185 piston you could measure for me ?
thanks
crazypj said:
I could find an XL185 piston and measure the distance from the crown to the wrist pin and see how much taller it is than the oem 81' CB125S piston.. I think it's(XL185) a 'flat top' & I'd like to find a slightly crowned piston similar to my oem CB125S piston..
The stroke difference between the CB125S(49.5mm) and the XL185(57.8mm) is 8.3mm and the XL185 piston(63mm bore) and cylinder would also work if I could find the taller(8.3mm taller from the wrist pin to the edge of the crown) piston crown to keep the compression numbers(9.4:1) close to my stock CB125S.The cam chain for the CB125S is 98 links while the XL185 is 100 links and I would swap them.I know it'll work but it depends on the right piston
edit: the 'right' piston..,now there's a thought.
This is my stock piston for the 81' CB125S and the distance from the top of the wrist pin hole to the top of the edge of the crown is .609in.
Any of you have an XL185 piston you could measure for me ?
thanks
Attachments
Tune-A-Fish
BOTM LOSER Proudly Deplorable
Tune-A-Fish
BOTM LOSER Proudly Deplorable
Re: What oem piston will be a good 'drop-in' overbore for my 81' CB125S ?
XL185 1979
I will sell the motor already rebuilt with all electronics and wire harness for $450
XL185 1979
I will sell the motor already rebuilt with all electronics and wire harness for $450
Tune-A-Fish
BOTM LOSER Proudly Deplorable
Re: What oem piston will be a good 'drop-in' overbore for my 81' CB125S ?
Dude thanks for all that tech. I already have an 83' XL200R parts bike w/ engine I'm presently(& slowly$..) in the process of rebuilding and a good wiring harness,etc.
Thanks a lot for offering me the bike & rebuild w/ parts.
I see now from what you've showed me in measurements that the stock XL185 cylinder really would be too tall,that's good to know what won't work.
I measured the spigot/hole in my engine(81' CB125S)today and it's 70mm so I think doing the older CB750 cylinder liner/piston(61mm) into my stock outer alum. jug w/ some machining will work.I like the 49.5mm stroke of the CB125 for good top end power & the XL200R engine will be 'on the back burner' until I get a few more $ to do it up all the way.I'm sending my spare cylinder and old piston to a CB750 expert in CO. and he's going to look at the collection of pistons he has to see what'll work for me. thanks again man
I'm going to take a pic of the bike once I re-install the original top end w/ fresh valve guides,honed cyl. and std. piston,new rings etc. I just got to get this one on the road w/o issues. More power will be nice later and will be better for long rides.
Dude thanks for all that tech.
I already have an 83' XL200R parts bike w/ engine I'm presently(& slowly$..) in the process of rebuilding and a good wiring harness,etc.Thanks a lot for offering me the bike & rebuild w/ parts.
I see now from what you've showed me in measurements that the stock XL185 cylinder really would be too tall,that's good to know what won't work.
I measured the spigot/hole in my engine(81' CB125S)today and it's 70mm
so I think doing the older CB750 cylinder liner/piston(61mm) into my stock outer alum. jug w/ some machining will work.I like the 49.5mm stroke of the CB125 for good top end power & the XL200R engine will be 'on the back burner' until I get a few more $ to do it up all the way.I'm sending my spare cylinder and old piston to a CB750 expert in CO. and he's going to look at the collection of pistons he has to see what'll work for me. thanks again man I'm going to take a pic of the bike once I re-install the original top end w/ fresh valve guides,honed cyl. and std. piston,new rings etc. I just got to get this one on the road w/o issues. More power will be nice later and will be better for long rides.
crazypj
Split personality, I fake being smart
Re: What oem piston will be a good 'drop-in' overbore for my 81' CB125S ?
You never, ever measure the top of piston to get size.
The top is way smaller to allow for the greater expansion
Measure 8~10mm up from bottom of skirt
If you use a stock CB750 liner you may as well go to 63mm
You never, ever measure the top of piston to get size.
The top is way smaller to allow for the greater expansion
Measure 8~10mm up from bottom of skirt
If you use a stock CB750 liner you may as well go to 63mm
Tune-A-Fish
BOTM LOSER Proudly Deplorable
Re: What oem piston will be a good 'drop-in' overbore for my 81' CB125S ?
He was asking for top of pin to top of dome, I threw in a few extra... I just measured the top so i guess I "do" measure the top The bore is not tapered like a forged piston, so he can use the measure as a reference when looking for his piston and dig deep if he gets close. Stock cb750 is 61mm never perfect so 59-62 is normal junk.
He was asking for top of pin to top of dome, I threw in a few extra... I just measured the top so i guess I "do" measure the top
The bore is not tapered like a forged piston, so he can use the measure as a reference when looking for his piston and dig deep if he gets close. Stock cb750 is 61mm never perfect so 59-62 is normal junk.
crazypj
Split personality, I fake being smart
Re: What oem piston will be a good 'drop-in' overbore for my 81' CB125S ?
Piston is usually 1~2mm smaller at top and gets progressively 'wider'
If you roll it on a flat surface you will 'see' the taper
Extra measurements are never a problem but it isn't where you would measure for piston size
If you do correct accurate measurement, on most Honda pistons it will be 0.04mm undersize (eg, 61.00 piston measures 60.96mm)
I think pin is bigger diameter on XR/XL185 piston (but I haven't had one since 1980~81 ;D )
Piston is usually 1~2mm smaller at top and gets progressively 'wider'
If you roll it on a flat surface you will 'see' the taper
Extra measurements are never a problem but it isn't where you would measure for piston size
If you do correct accurate measurement, on most Honda pistons it will be 0.04mm undersize (eg, 61.00 piston measures 60.96mm)
I think pin is bigger diameter on XR/XL185 piston (but I haven't had one since 1980~81 ;D )
