Introduction: Honda Rebel 125 Upgrads Honda Cb125tdc Engine Rebuild With Some Mods

with this step by step im going to tell you how to upgrade the rebel engine from its standard 11 hp to about 19 to 25 hp 

this is not as simple as putting a big bore kit on it 

i will tell you what you need and what thing to upgrade on the engine 

this will go into depth how the engine works 

i only know what i know by hours of research on this engine and wondering why there is no performance parts available for it

please note the there isnt a 180 crank that will fit the cmx250 as the 250 has a 53mm stroke and the cb crank im using only has a 41mm stroke however saying that this is one way you can make a crank that will work but it will be impossible to get the crank aligned properly without specialist equipment 

if you have any questions on doing this project im always hear to help 

Step 1: Why I Desided to Upgrade My Rebel

i went to a bike meet and got talking to a couple that knew these engine and they told me that i could get twice the power out of my bike by changing a few thing so then when i got home i had a look on ebay for different bikes that had the same engine and i found that the rebel had the same crank casing as the cm125 cb125 cd185 cmx250 cd200 and a few more so then i looked up the stats for each engine and notice that out of them all the cb125 t engine had the most power to cc 
so i had a look at different engine components and noticed the internals for the cb125 engine where a bit different in there layout 

the cb125 engine had the so called 180 deg crank
where as the rebel engine had the 360 deg crank

Step 2: Gearing and Stuff

so then i started to think would i be able to put this engine in my bike and it turns out that i could all i would have to do was change the cdi unit  on my bike doing this alown i would have got 17 hp thats a 6 hp increse

i eventually found a half built engine of a cb125 and i striped it down co i could check all the components and replace the bearing 

i worked out the gear ratios for both the rebel and the cb 

rebel  gear ratios 

1st                  2.846   *     
2nd                1.777   *     
3rd                 1.333   *     
4th                 1.083   *     
5th                 0.913   *     

finial reduction 3.000

cb gear ratios

1st                  2.769   
2nd                1.882
3rd                 1.450
4th                 1.217
5th                 1.083

finial reduction 2.666

after finding the ratios i  found the the rebels ratios where longer 
apart from the 1st gear of the cb gear box

Step 3: Bottom Half Engine Rebuild

first i made sure all parts where clean and not damaged 

second i instaled the bearings int the crank case

third i instaled the crank from the cb engine and tightned the bolts down after i gave it a quick spin to make sure it all spins free make sure you fit the cam chain on before you put the crank in 

forth i instaled the rebels gear box and shifting forks made sure that i could get all gears 

fith i gave every think a oiling /new gasket on /then put the other half of the casing on 

sixth i instaled the gear slecter rod and fitted the clutch basket fitted with a new clutch plats with stronger springs /fitted the oil pump /fitted the gears on the crank and bolted the side casing on

first  on the generator side fist i fitted the netrule switch and rutid the wire to where it should go 

second fitted the new oil seal / new gasket and spacer plate 

third i fitted the rotor and starter clutch on the crank 

forth i fitted the starter motor and chain 

fith i bought a staror ans i didn't get one with then engine the rebels wouldn't fit they are completed a different size to each other and fitted the pulse pick up 

sixth bolted the casing on job done on the bottom half

Step 4: Top End Rebuild

there is a few mods you can do to the cylinder head most are optional but i wanted to squeeze every bit of power i could out of the engine 

so hear we go this is what i have done to mine 

 i found a big bore kit on ebay that fits this engine its only an 18cc increase  it has 47mm piston instead of the standard 44mm   if you do decided to keep an the old cylinders its probably advisable to replace the piston rings and deglaze the cylinder before you put it on 

another ting i wonder why honda didn't use bearings on the cam shaft instead on the bushes they use probably down to cost so i had a look and i found some bearings the same size as the bushes they are a bit pricey  but its worth it 

first i put the pistons on the con rods and secured the gudgeon pins 

second i fitted the gaskets and the cylinder block 

third i striped the head so i could ported and polished it to increase the air flow relap the valves and put the springs on 

forth i put the new head gasket on followed by the head 

fifth i fitted the cam shaft with the new bearings and lined all the markers up with the timing and put the cam chain on the sprocket 

sixth i put the rocker arms on and bolted the head down 

seventh a bolted the rocker cover on job done the engine is now complete 

there is some more mods you can do 

you can get the cam lobs changed but only by a professional as it will need to be grinded to the correct shape 

you can get a cylinder block from the cd185 engine but you will have to make something to take up the 1 mm gap on the gudgeon pins and adjust the crank case  

or the other thing would be getting forged pistons / forged con rods  and titanium valves all of which i haven't been able to find any where so it would all be custom made and cost a bomb but if you did this you can then turbo the engine 

i will be doing this in the future but at the moment i don't have the fund to do so

i will add pictures of the top end rebuild next week when i have all the parts 

Step 5: What Parts I Have Bought to Be Able to Do This With Prices

cb125 half built engine                                                   *       £18.69  $28.32

cb cam shaft                                                                     *       £20       $30.31

cb timing sproket                                                             *       £8         $ 12.12

cb stator                                                                             *      £20       $30.31  

cb tdc cdi     i fount that you can use the
original rebel cdi because ether way
your going to have to get twin pick ups                         *      £16.96 $25.70

ca oil magnetic sump plug                                             *      £5.99    $9.08            Part Number: K2567-810

ca125 air filter                                                                    *      £10.42  $15.79        ebay item no 281004739594

ca125 spark plugs  X2 part no CR6HIX                        *      £12.96  $19.64       part number CR6HIX

cb 125 gaskets                                                                  *      £9           $13.64      

cb125 piston rings                                                            *      £21         $31.83

cmx250 carbs                                                                     *      £100      $151.54

cmx250 inlet rubbers X2                                                   *      £17.12   $25.94

cmx250 inlet rubber                                                           *      £17.53    $26.56

ca125 spark plug caps   X2                                              *      £9.90       $15                part number XD05F-R

clutch kit                                                                               *      £46.49     $70.47

camshaft bearings X2                                                       *      £35           $ 53.05         part no RNA4902-XL

hope this help 

Step 6: Just Finished My Bike

hi i have just finished the engine rebuild on my bike and i can say it wast easy 
most of my problems was the engine over fueled as the new cmx 250 carbs need more air i went out last night and got 70 when it wasn't running at is optimum to day im gong to try and put a k&n on it to see if that will give me more power and stop the over fueling

good news i ended up taking the side of the air box filter still in and i have got 75 on my clock but i had a friend clock me and he said i was doing 78-84 mph 

i know i said it was over fueling yesterday but now the side is off the air box its getting a bit to much air so im going to increase the carbs jets to see if it will give me more power and stop it from running lean 

my target  is to get to 90 + mph

i will check back in a few days and tell you the result