switch 125 small cast iron cyl. for aluminum

[Steve Genter]

What all do you have to change beside the cyl.& head on a early model 125 to convert over to the aluminum barrel.
           Thanks for any help, Steve Genter.

 

[john durrill]

steve,
 i did this in 1970,
trying to remember exactly what we did. we did it ar Rick G's shop in half a day one sunday. you would need the cylinder , head, head bolts and studs. if you have a head pipe that slips into the expansion chamber then all you would need is a head pipe and exhaust flange for the alloy barrel.
 you would have to rejet the carb
thats all i can think for now ,
 John D.

 

[Mark Annan]

The conversion from Iron cylinder to Alloy cylinder is pretty easy, assuming you have all the parts.  You will need all of the alloy cylinder parts (cylinder, matching piston, head, head and base gaskets, long studs, nuts, and washers).  Remove the iron parts (head, cylinder, piston).  You will need to replace the short studs that hold on the iron cylinder to the crankcase with the long ones that hold on the alloy cylinder.  You will need to use the special long nuts and washers that hold on the alloy cylinder and head.  Install the piston, slide on the base gasket, cylinder, head gasket, and head, then tighten up the nuts.  As far as the engine is concerned you are done.

To get it to work on the motorcycle you will need to change your exhaust setup.  The iron cyl. exhaust bolts on with 2 bolts.  The alloy type uses a small stub that bolts onto the cylinder and then the exhaust slips into this and is held on with 2 springs.  The best solution is to get the correct exhaust pipe.  If you can find the parts you can cut off the end of your current (iron type) pipe and weld on the end for alloy cylinder type exhaust.  

You most likely will want to change your carb also.  Most of the iron cylinder bikes I have seen use a 26mm carb.  Otherwise you will only need to change the mounting bushing in the carb.  The iron cylinder carb spigot is smaller than the alloy cylinder spigot.  To get the full benefit you will want to rejet the carb to the best alloy cyl. specs.

 

[Steve Genter]

Hey Mark, thanks for all the info on the cylinder swap.
              Steve Genter.