KTM 125/175 interchangability

[rickf22]

Re circa 1976 / 1977:  Any chance that a KTM 175 top end will fit a 125 lower end (e.g. like early 70's Yamaha 125/175 interchanged) ?

I presume the overall engine mounting would be the same at least, so one could swap an entire engine. True?



 

[DKWRACER]

Use extreme caution here, what is of concern is the total volume of air transfered from the bottom of the case assly.

 

[rickf22]

More directly, my concerns are:
Is the stroke of a 125 and 175 the same?
Is the cylinder-to-case bolt pattern the same?
Is the top-end rod bearing the same?

E.g. In the case of Yamaha AT1 and CT1, all those things are the same. One could change engine size by changing only the top end, and doing so was identical to what the factory did.

I doubt that was a common practice, but maybe it is more common than one thinks.
As another example, a Maico 400 and 440 are the same bottom end, just different bore on the top-end. One can swap top-ends and it will work.

Anyway, what's behind all this?  I've wanted an MC5-175 fora trail bike for a long time. And I might (just maybe) be getting a 125, and I'm wondering about making it into a 175.


 

[john durrill]

rick,
the stroke is the same as the 125.
 get a base gasket  for both and you will se if they line up. my guess is they will.
 Larry Perkins would know what the differences in primary and transmission gears are. once you have the cylinder , head and a piston you can split the cases and se if the piston will clear the crank when its all the way down. same with the liner to crank and case clearances.
 dont forget to set the deck height so you take that into account when you are checking for clearance of all moveing parts.
The pipes are a bit different and you would need the right size carb.
hope this was some help. thats the top end we are running on my 125 six-days. it works well.
John & Peter

 

[Larry Perkins]

The KTM 125 and 175 share many components but not all.  The rod and crank are the same as is stroke of course.  The center cases are different because the 175 has more transfer area and I believe volume in the crank case.  This could probably be duplicated on a 125 case set if you were going up to the 175.  It would take either a good machinist with a pattern or a top-notch engine builder that understood two-stroke dynamics.  I know someone who raced a BIG 125 KTM(175 barrel and head) back in the "Post-Day" but I think all that was done was to match the 125 base area to a 175 base gasket via dremel tool.  The ports were also matched and polished.  It was a winner and it was a time when a decent amount of fast 125 bikes were in actuality 175's.  I raced for a shop that had several 125 100's and 175 125's.  I am not condoning this today just merely telling how they got to ROME.  By the way I am not sure on the trannies but could look it up.  I do know the 125 and the 175 have different pinion gears.  There is five teeth difference I believe.

 

[rickf22]

Thanks very much guys! Looks like such a swap is beyond what I'd call easy.

 

[dirtbike]

Forgive me if I'm totally wrong out but I have always thought that the crank volume should be as small as possible or at least didn't really had anything to do with the bore and stroke. When moving to a larger cylinder diameter the volume under the piston increases just the same as above it.
A larger displacement will mean a higher compression unless lowered som other way. This is true for the area below the piston as well and a large container in the crank would lower the effective crank compression.

Of course the ports and channels between the crank and cylinder have to match. One has to prepare for some grinding there I guess.

It would be interesting to hear because I havn't give it a thought actually. I just presumed things to be this way. Am I wrong ?

 

[john durrill]

dirtbike,
 i think thats what DKW said. volume of air transferd from the bottom to the top.

rick,
 its not a hard conversion if you can use a dremel with a little skill.
 the real question is will it pay off.
 would you save enough weight to make it worth the money and time spent.
Larry might know the difference in engine weight between a 77 125 and a 77 175. if the primary gears and clutch hub are lighter and the tranny lighter , you would gain some power lost from the recipracating mass of the power train.
 that in it self might help a lot.
I have a 125 head and a 175 head for the 77 engine, bolt pattern is the same. in fact the 125 head came off a 175.
 The reason we put the 77 175 cylinder on the little Sachs 125 engine was to make a light 175 motor. It must be 20 lbs lighter than a 175 KTM engine.
 we had to detune it to give it a chance at some reliability. But for what its used for the loss in HP was OK. The little bike weighs in at just 200 lbs with a half a tank of fuel and has great torque. makes a little more hp power than a hot B motor but pulls from about 2000 to max rpm cleanly with a lot more torque than either the A or B Sachs 125. its easy to start , dosent load up. All in all its a joy to ride in the woods.
 So for us it was worth the time and money spent.
JOhn & Peter




 

[paavojaelvis]

Crankcase volume is a compromise between proper primary compression ratio and (desired) max. fuel/air mixture volume, which could be sucked into the crankcase.
Bengt,  you are partially right, the _uneffective_ crankcase volume must be kept in a minimum. There are some areas in the crankcase, especially in older engines, that gas/air mixture gers "trapped" and will never reach the transfers when piston comes
down, like the very bottom of the case, space between main bearing and seal, etc. The volume of those areas has to be minimized. When you look at modern reed valve engines and the volume they  have behind the reeds in their crankcase, it's HUGE! And it's all located very close to transfer area. OK, primary compression ratio is lower than in the 70's, but with transfers having good flow characteristics and a pipe giving supercharging effect, there's no such need. Here's a link to a website, this guy has been tinkering with two stroke quads ans jet skis. http://www.macdizzy.com/2stroke.htm
Lots of theory, porting and crankcase stuff.  Have a look!

Paavo  

[dirtbike]

Hmm, ehh, well I see!
But don't you think that the huge volume behind the reeds just has to be there from a design point of view. Also the reeds varies their opening time depending on how much gas mixture that flows through. I still believe that in a piston controlled intake the crank volume should be kept as small as possible.
Maybe not if the engine was turbo charged though when something else then vaccum and pressure is supposed to draw gas from the carb and press it up through the ports. I know that the pipe has some of that effect but only in a very narrow RPM range. I´d say that large waste crank area mostly affects torque at lower RPM in a negative way.

IMPORTANT MESSAGE though, I don't know but I'm about to find out now.