Honda CR125 1991(dyno video + charts) + CR125 1992 + YZ125 1996

You should be able to straighten that left radiator pretty easy, I just lay the bent side on a flat bench, with a channel lock plyers on the radiator edge and the lower jaw under the bench top squeeze slowly and evenly across the entire outer brace surface till straight, then work on the fins to reopen with a small flat blade screwdriver. Enjoy your build.

NiekL wrote:

Hi Guys, i thought it was time to share my project bike, a Honda CR125 from 1992 build in the 1991 colorways.

I'm a bike mechanic for a living for over 13 years , 29 years old from Holland.I always had the idea to build myself a nice vintage bike. Here in Holland we mainly have deeper sand compared to the tracks all over the rest off the world, so finding a good base bike is a little more difficult!

The main reason i chose a Honda is because i really like the quality of the CR twostrokes from the 90's. When working on modern bike engines, i see less and less material being used for making engine cases and bike frames. When wrenching on the 'older' Honda engines i always thought there is a reason why the 90's cr models are still riding around this much.

The main target i had was build a nice vintage racebike, it had to look like new, at the same time it still needed to stay a under 4k all in budget bike

Found this bike as a good base:





So i started stripping the bike

Had to cut out the swingarm axle after trying every trick i had.



The previous owner rode with a swingarm nut that was nog tightened, so the swingarm axle hole was beatened! I decided to search for a frame that was all healthy, found one from Dirk Timmer. After that i got the frame checked by a good welder and a guy who knows where these frames have there weak spots.

Thanks for the interest, updates coming shortly!

Frame blasted,welded, and updated at the footpeg assembly points so the new footpeg pins and footpegs fit like a new bike.




Back from the powdercoater.
Meanwhile i got some nice titanium parts for building up the frame: Pro circuit YZ125 footpegs, rear brake pedal bolt, footpeg pins, raptor shift and brake pedal tip andchain adjuster bolts

very cool

wolfy0067 wrote:

You should be able to straighten that left radiator pretty easy, I just lay the bent side on a flat bench, with a channel lock plyers on the radiator edge and the lower jaw under the bench top squeeze slowly and evenly across the entire outer brace surface till straight, then work on the fins to reopen with a small flat blade screwdriver. Enjoy your build.

Yes thanks, i will make that work. I did straighten some nasty bent radiators before, oem radiators can have a lot of bending and still come out fine in the end after repair.

The intention was to build a peak cr125 in 1991 colorway, but i saw more and more peak bikes being build so i decided to wait with that idea and change to the oem 1991 look. I got some parts for the peak theme already at that time: grey airbox, replica plastics (not the real acerbids), some pro circuit parts, stickers etc.



Got all the bolts,nuts,axles and hardware plated and sorted out for the frame and engine. It took me some hours to sort it all out on size ( this was for more then one bike).







Found a CR125 1991 fuel tank wich was in a good enough state for me!




Some bikes i build:

Nice work. I think it was a smart decision to build an oem looking 91. I'm not into Hondas but that is my favorite and if I ever build one it will be a 91 look. You have the tank so the rest should be easy.

Nice project, but why not 92 plastics? I know a bit hard to find but so good looking!

Banana_oil wrote:

Nice project, but why not 92 plastics? I know a bit hard to find but so good looking!

I just like the orange/red 1991 color a lot more than pink.

Now that i got a good frame that was ready to build up i first started to tear the engine down. The parts hunt can be a little more difficult with the older bikes.



Honda cr two stroke engine cases often tend to have a little damage near the front sprocket side. This one was all ok! I didn't want to save money on the engine, it just needed to be real good! When i picked this bike up i thought the engine sounded fresh. With tearing it down i wasn't unhappy at all by the overal condition of the parts.



I could see and feel the clutch was used, but all like normal wear. But you can tell it when a 25yrs old bike is mainted or not, this one was. The clutch felt good for keeping it spare for the races. But i decided to go for a complete new clutch. Nino Fenaroli got me into plating my new aluminum clutch package: the weight is a lot less compared to the steel versions wich will give a nice throttle response, and by plating them the wear should be minimal.





Engine all stripped. The gearbox had some wear on the dogs, usable for another 30-40 hours running time. Use or dont use: no, i will get that gearbox complete later on for spare. I found a gearbox that had roughly 15 hours riding time on, perfect!

I discussed with some college mechanics about the crankshaft, i know the older type crankshaft CAN break, i don't mean they always do but i had some examples seen before. The steel stuffing can give problems when replacing the connection rod isn't done the right way somewhere in the life of the crank. I decided to go with the CR125 2005-2007 crank with solid crank webs. This is a plug and play update. With these modifications i had a solid lower engine.





New clutch housing

I had some spare cases for the engine, this got me to the idea to modify one set to the more 'actual' two stroke case design. (Like a YZ125 for example). The YZ125 has a narrow inlet hole at the reed case compared to the later two strokes. Good or not, i just wanted to make 1 set modified cases and keep one set oem. This for me is a good option to learn some more about gasflow in a twostroke. I did some engines before, but not the older two stroke cases. When u never try you never learn!

The stock cases:




Yamah YZ125 cases:


Honda HRC CR125 cases:
The rear transfer port from the YZ125 and the HRC CR125 have a cut out in the lower part after the reed to get the rear transfer port feeded. This can be seen at the right side red marked circle.



The v-force reed i was going to use had a restricted part at the upper part of the reed because of the narrow inlet window:





With that in mind i went to work. Starting with the basic refining of the cases by removing rough edges and materials that came with the engine after the production proces of the cases. This wil improve gasflow for sure.



After that i did the 'hrc' mod.


I finished the cases off with some bead blasting, yes this doesnt look better, but the main reason is that want the oil/fuelmix to create a oil film on the surface of the cases to increase airflow.



Cases ready after painting them together with all the crankcase covers:

Coming on great! 1991 bikes are awesome looking, but i'm currently converting my 92 Peak bike into 92 Factory style. I love the pink/red colour haha!

I really like that Raptor titanium gear shift tip, didn't know they made them. Does it fit right on?

Great work, would be nice to know before and after dyno numbers, pity it's not possible.
What head and piston combo you are going to use? 90-91 domed, 92 flat or 93 and up flat top ?

chuckdavies wrote:

Coming on great! 1991 bikes are awesome looking, but i'm currently converting my 92 Peak bike into 92 Factory style. I love the pink/red colour haha!

I really like that Raptor titanium gear shift tip, didn't know they made them. Does it fit right on?

Chuck noooo haha! But factory 92 is awesome also.

I used a brake tip and a gear shift tip from a Beta. So KTM would be about the same i think?
It needs some work, you need to grind down the size of the gear shift lever head.

125mx.fi wrote:

Great work, would be nice to know before and after dyno numbers, pity it's not possible.
What head and piston combo you are going to use? 90-91 domed, 92 flat or 93 and up flat top ?

Thanks!
I have a dyno availible at my work.
I'm using the flat top cylinder 90-91 with a flat type piston and a milled/cut head. Got a new cylinder also from a later year 93-99 so i will try that also.

NiekL wrote:

Thanks!
I have a dyno availible at my work.
I'm using the flat top cylinder 90-91 with a flat type piston and a milled/cut head. Got a new cylinder also from a later year 93-99 so i will try that also.

93 cylinder have exhaust powervalve and transfers lower than 90-92. Check them, you might be surprised. 92 porting is very close to same year RS125 road racer. Since you started porting better check all options :D .

125mx.fi wrote:

93 cylinder have exhaust powervalve and transfers lower than 90-92. Check them, you might be surprised. 92 porting is very close to same year RS125 road racer. Since you started porting better check all options :D .

Yes thats why i have the 90-92 cylinder in the first place!

With the crankcase modifications done, it was time to assemble the engine. I replaced every single bearing in the engine, all of them.
The ones people sometimes forget are the clutchcable lifter/lever bearing, the powervalve govener bearing and clutch housing needle bearing on the primary shaft. (Just a little reminder)

Some glass beaded engine parts. They look nice and clean again after 25 years of use.


The bearing inside the powervalve governer is a little more difficult to service, use heat carefully together with the right bearing tool (thanks for the tip Nino). I modified one govener with spacers at the end of the spring, this to add load on the spring. Creating a retarded exhaust valve opening rpm, just to test it. In combination with a Honda CR250 air boot and a 38mm carb instead of the stock 36mm this might mod might work.





The engine is getting more and more complete. Using oem Honda gaskets, i like the quality from these gaskets!

At the time i was working on the engine i saw a ad that i couldn't resist to go to and check it out. New oem Honda CR125 hpp pistons are NLA over this side of the ocean. The guy selling them had 13 pieces for sale together with a new 93-99 cylinder. This was a all or nothing deal so i went for it


So if annyone needs a oem Honda piston

With the lower engine being assembled i started piston height mesurement. Together with the info from the head volume i could do some calculating. The stock Honda head is quite small in total volume. I think this was the way to go in the 90's when leaded fuel and compression ratios could be higher?

6.3 cc head volume in a stock cylinder head.
1.37 cc deck volume with piston in tdc
0.80cchead gasket volume (thickness)
8.47 total combution chamer volume

I ended up calculating 15.47:1 static compression ratio (uncorrected ratio)

nice thorough work looking forward to see it progress Are you going to run the standard ignition ?

Great post, thanks for sharing your knowledge and tips/tricks.,,,, keep the updates coming!

For now i leave the head in stock shape, on the dyno the higher compression ratio is not going to cause trouble. But on the sand tracks here i'm not sure for now. I will pick that up later, there are some other ideas that came across my mind regarding the cylinder head. So i got a good friend modify the head from stock gasket to o-ring sealing. O-rings are a lot cheaper than the stock Honda gasket, and i think over all the sealing should be a little better.



I found some 98-99 type powervalve guides and valves, these seal a lot better than pre 98/99 type. They also have a more stable construction.

I took some time to get to know how the hpp valve system operates and how to give it a good spring tension with left and right powervalve working exact at the same time. Got that ready, assembled the cylinder head and that got me a ready to fit engine






Happy with the result!

Drooling (: nice work

Looks nice! It will be fast rewwing engine, helped by lighter (80g) new model crank. However small head volume might limit overrew. Did you make it 1,0mm squish?
My experience in adding preload to governor is that 3mm is too much on stock cyl. Btw theres three different wire thicnesses on governors so they are not all the same.
Interesting to see dyno charts since you can test what difference every chance makes.
One thing you should test is what difference digital ignition makes compared to stock analog one.
Endless list to test, reeds, carb, exhaust, etc etc

Good build to follow

Looking good!

Here is a ‘91 I restored a few years ago:

Very nice work. Also interested to see some dyno runs.

billyslad wrote:

nice thorough work looking forward to see it progress Are you going to run the standard ignition ?

Yes for now i am. Maybe later on i will buy a digital ignition.

125mx.fi wrote:

Looks nice! It will be fast rewwing engine, helped by lighter (80g) new model crank. However small head volume might limit overrew. Did you make it 1,0mm squish?
My experience in adding preload to governor is that 3mm is too much on stock cyl. Btw theres three different wire thicnesses on governors so they are not all the same.
Interesting to see dyno charts since you can test what difference every chance makes.
One thing you should test is what difference digital ignition makes compared to stock analog one.
Endless list to test, reeds, carb, exhaust, etc etc

Good build to follow

The squish with the stock head ended up a little lower than 1,0mm. Thanks for the pre load shim thickness tip!

Yes i knew that there are 3 different springs wire used. A friend of mine was also changing his bearings from the powervalve govener and noticed different wire thickness. The list of testing material is indeed endless!

Brad460 wrote:

Looking good!

Here is a ‘91 I restored a few years ago:

Nice build, clean bike!!

A tip of the hat to you Sir, Beautiful work!!! Hard to beat a Honda for a builder/racer The quality was second to none. Can't wait to see the chassis build!!

Rocky739 wrote:

A tip of the hat to you Sir, Beautiful work!!! Hard to beat a Honda for a builder/racer The quality was second to none. Can't wait to see the chassis build!!

Thank you!