View Full Version : CR125 Swapped RS50's - all the info in here.

10-18-2014, 02:17 AM
Well I thought I would start a new thread since the old one is all sorts of random stuff, and I couldn't edit the title

Now that they are alive and running, as well as track proven I figure I will use this to document a parts list so that anyone who wants to do this swap can make it as turn key as possible.

Pics, parts, part numbers and website links, and all the other good stuff are sure to follow..

11-21-2014, 01:10 PM
So I've been over here working in Saudi Arabia, and it's been quite busy. I apologize for the long delay.

I'd first like to start off by saying that I did this swap because I was having major reliability issues with the AM6 transmission, and I wanted to have a bike with a little more oomph. The RS50's size and weight reminds me of the Honda RS125, and I always envisioned it as the perfect 125cc candidate. Apparently I was right, as even the new Aprilia RS4-125 uses the same Benelli frame as our 99-05 RS50's, albeit with a 4 stroke 125 as opposed to a 50cc 2 stroke.

This IS a swap that you can keep your lighting coil power with, but I do not have a solution to use the stock clocks/ gauge cluster tachometer, because it uses a tachometer cable. If you were to use newer clocks / gauge cluster with an electronic tachometer, you could probably use one from a newer RS50. What I did was switch to a high performance display like those on MotoGP bikes, which gave me data logging, gear position indicator, multiple temperature indicators, lap times, warnings, etc. It isn't for everybody, but since I am tracking my bikes, it made sense.

So, basics:

1) The engine. The engine I based it around was the Honda CR125R power plant from the years 1991-1999.

Ideally I would say you want a 1992-1996, because those have the stator and electronics that you have a lighting coil for, as well as having the 6 speed transmission, and some other parts here and there that are easily upgradeable in the shifter part world. I do not know which cases cross over exactly, but I can tell you I've built 1990 cases up with 1998 parts using some shims in the transmission, as well as putting a 1994 transmission into 1999 cases, which turns a 5 speed 1999 engine into a six speed. Again, you'll need some shims. If you are NOT well versed in assembling transmissions, checking tolerances, etc., I would not recommend getting a 5 speed engine from 99. Therefore, I recommend the 1992-1996 CR125 motor, as it will be the most bolt-on easy one to use.

The engines normally go for a couple hundred dollars. Cheaper than if you were to do a full build on an AM6. If you do a search on eBay, you should be able to find one for around 500 dollars, including CDI, ignition coil, and the whole nine yards.
The other route you could go is purchase an entire donor CR125 motorcycle. I did that personally. You can end up selling the bike for parts to gain some money back. A 1990-1996 CR125 should go for somewhere around 800-1000 on craigslist. You can probably sell it back minus the engine for 350.00, or part everything out on eBay for a bit more, if you have the time. Personally, I don’t have the time, so I would probably just do a curb alert on craigslist and call it a day.

11-21-2014, 03:59 PM
want to see pics, but how did it turn out?

11-22-2014, 12:52 AM
lots of pics in my other thread. theyre coming though as i compile the other part numbers in between working

11-26-2014, 04:37 PM
I managed to pick up a Honda RS125 Engine with Loom and stator for a decent price in my opinion. It's currently setup with a cart head and managed to get the radiator in the deal (may work with project). Thought to be a 93-94 model year but unsure. I know its not going to be a low budget build for sure as I already spend $1500 for the MA titled 99 RS50 and 1k on the engine,loom,carb and radiator. Can anyone help me identify the year based on the serial # KH007 and pictures. And Matt.... I will be seeking your guidance in this build as I am not much of a fabricator and work on Toyota's for a living, if you would do a east coast 2 smoker a big favor. The build you have completed inspired me for this project ( Bay Area 2 stokes ).


11-26-2014, 04:41 PM

11-27-2014, 01:46 PM
NF4 RS125 motor - > very similar to a CR125 motor that I am using. I am not sure what all will work, but I know the cylinders swap over. I would love to take some measurements and see if my cradle swap kit jig would work with that motor.

Lets continue though

2) The bolt on parts to make it all work. For me, it was a list of Aprilia parts, Honda RS125 parts, shifter kart parts, and even a few custom parts that, at this point, are off-the-shelf.

The first thing any street bike needs to think about is a charging system. Many people have performed these swaps on RS50s, but how many have working lights and charging systems that can power the RS50’s headlights quite easily while also charging the battery?

One of the main reasons behind the 1990-1996 CR125 was the availability of programmable ignition AND a lighting coil equipped stator! It is available from electrex UK.

The sprockets! The front sprocket for me is from swedetech or sharkshifter. You can have anything from a 14 to a 19 tooth front sprocket, that gives you lots of gearing options. I run an 18 or 19 front and that gives me anything from 47-54 rear, and I have a ton of gearing options that way without messing up the antisquat too horribly much.


The rear sprocket is from Rebel Gears. Just input your stuff, and they already have the program for an RS50 with a 428 rear chain.


As a side note… I won’t recommend this but I can confirm that a 428 chain is so close to a 420, that a worn rs50 rear sprocket will work fine on a 428 chain. By the time it is worn down a bit, the larger rollers of the 428 will fit fine. This is good if you are trying to figure out clearances / sizing / chain length / etc.

The radiator I used is genuine Aprilia RS125

The intake boot you need to use is off an NF4 RS125, or you use the Swedetech part.

These are actually really hard to find. You can find the OEM rs125 intake sometimes for pretty cheap also, but they are rare.
You can use the normal CR125 15 degree intake, but it will make fitting an air filter a little , ok, a lot more difficult, because the carb goes right into the frame.

11-29-2014, 12:54 AM
For the controls, the CR125 uses a cable clutch just like the RS50. I ended up using the CR125 clutch lever and put that right on the RS50’s handlebar. The clutch lever, clutch cable, the whole nine yards works that way, no questions asked.

I did the same thing for the throttle assembly. I used the throttle and throttle cable that I was using on my CR125. I am betting that if you have an aftermarket throttle or something for use with a Mikuni TM24, then the carburetor and throttle cable will work fine on the CR125. Since I had the donor stuff, it was easy for me to do. If you don’t have donor bikes, again, Ebay is your best bet.

One of the things that you’ll have to address is the single cooling hose going into the single radiator on the RS125 radiator, but the CR125 utilizing dual radiators. Again, the solution is the karting world, as their shifter karts run single radiators.


That will pretty much fix you up for the cheapest, but you’ll have a lot of hose clamps and fittings. Another, cleaner solution is to convert your cylinder head to a single outlet: Taken from EKNClassic

“You will need to heat the head and then un screw both fittings. Use a six point, 19mm box end wrench or you will just round off the corners. Next you will retap the holes to 3/8" pipe thread. This will allow you to use an automotive type hose barb fitting. Use a NAPA #660-1523 fitting. Plug the left side with a 3/8 pipe plug (allen style). Now you can use a single 3/4" hose which is the same size as the typical 125 radiator inlet. Again, NAPA has hose with pre formed 90 degree bends on one end. 18" is #9816;”

I gotta throw my $0.02 in that ekartingnews used to be one of the greatest websites around, but the owner switched over to a completely worthless bbs wordpress contraption and got rid of the old awesome vbb/ubb/board and as a result, it’s now worthless. RIP the old ekartingnews.

This is what a typical single outlet cooling system looks like:

The single outlet makes it a lot easier to go with the RS125 radiator. I would recommend doing that.

I ended up going with a billet cylinder head from phatheadz racing, so that I can experiment with different cylinder head profiles eventually.


12-02-2014, 10:12 AM
Good stuff! Keep it coming!

MrGouldy to you
12-07-2014, 04:05 PM
get on mate looks good , and sounds the bollox ive just brought a rs 50 frame to do a project from the ground up , picked up a engine today with stator, cdi , etc for £0 as was of a mate its a dt125re so even better its elcy start so i haven't got to worry about kick start clearance and also as you mention my engine has a charging system more than capable of powering the rs light etc and servo if i go down that route , this is the second time im doing this but this time its stay fared so i can fool some bigger boys , front end is sat here waiting (rs 125) , i have a rs 50 rear end here also as well as a yzfr 125 swing arm a spindle etc so again i have options , rs 50 chassis is a great bike , i dont know how it compares to the rs 125 chassis i know of course there's a few differences but suspect the rake trial not to be too far of ,