KTM 125

[Bill Campbell]

I am starting to get a KTM 125 engine together but need some help in getting the right parts. I read the post last year about 125 MC5 and hope to get an engine that will be reliable. I have 89 engine cases but I think the cylinder and head I have are earlier. I heard that KTM changed the location of the wrist pin on the 125 piston like the 175 to improve reliability.Was this correct and did KTM alter the length of the barrel to accommodate the different position of the piston crown.If this was the case can someone supply me with the dimensions etc so I can idetify the year my cylinder was manufactured. If as I suspect I have mismatched parts can an earlier cylinder be matched to a later engine configuration.

Cheers
Bill

[G Ellis]

If you have 1989 cases, I believe that they would be water cooled. What year are you trying to build? Later Gary

[Bill Campbell]

My cases are type 51 cases, definitely 1st generation KTM 125 cases. Believe 1st water cooled was 80 late 79? and is type 501. Bill

[Mike Lenz]

The 76 and 77 were the first design.  They have 125 KTM down the side of the cylinder. 78 was changed.  I think even the crank was changed.  I can look up just what was and was not changed for you if you need it.  I just built a 76 125 MC-5. Any KTM engined 125 should prove very reliable if set up right. Im not sure exactly what the problem was with the first 76's, but from what Ive seen it was not any major design flaws. Everything is pretty much the same as the 74-76 175 engine. And other than the smaller cranks breaking on the early 175's, they are all VERY reliaable. I believe the shorter piston was just a weight savng move. Im not sure of the differances of the 77 verses 78 piston in the 125.  In the 175 both the piston and the cylinder were shortened. But not sure about the 125. On my 125 I made alot of modifications to make it competive for MX. In addition to running 1979 wheels and pipe, and the OEM all alum clutch plates and inner clutch hub, I lightened a couple of gears, lightened and stuffed the crank, ported the cylinder, fitted a 36 Bing and fitted a late 80's Motoplat internal rotor with a CDI and advance/retard mapping. Im still in the process of sorting it out, but it seems to have potiential. Its pulling a 36 Bing with a 200 main jet (same main jet as my ported 40mm Binged 400!) without any blubber on top, so it is moving some air, thats for sure! I get an explosive hit and then it signs off right away. Now I have to mess around with the timing and how high to set the cylinder to try and get some over rev to pull those rather large shifting gaps for a 125 (same tranny as the 250 and 400!)...if it ever stops raining long enough to dry up!

Something else I wanted to throw out there.  Would restricting the pipe a little move the supercharging effect further down in the powerband? Like an insert of smaller perforated tubing into the existing silencer. I didnt raise the exhaust ports much at all, or even any of the port heights or timings, so I know my high hit and immiediate sign off is not from overdoing it there.  I just widened and opened up the exhaust (alot) and opened up the intake (alot), and put a third boost port in the middle, same as I have done to 250's and 400's that have worked very well. One other question Ive always had but never asked is why did KTM make those fancy intake finger ports and then have them half blocked by the corners of the piston skirts? 125-400 are all like that. Well this weekend I had my 125 and 400 apart and trimmed the skirt corners out of the way.  It will in effect close off the intake ports a very small amount later (the bottom outside corners of the ports), but it makes no sense to me to have half the finger blocked by the skirt on the intake stroke. We'll see! Wow this is long. Wow it is late.

[Bill Campbell]

Thanks for the input Mike. I'm not experienced in porting and like the idea of retaining the reliabilty. I reread my first post and saw the typo. I have a 79 bottom end so it should have the better crank if that is the case. It has the aluminium clutch which I'm looking forward to using. I guess I will try and find a later style piston and see how the cylinder fits. To complete the motor all I need is a 55mm piston. Any suggestions as to where I can get one?
Go the Pies! Bill

[Mike Lenz]

You have the 76 style cylinder?  Either way have you tried Alan Buehner? Ebay has them off and on, but how do you know which 125 piston your looking at? I have never had a 78 style 125 KTM piston to compare. However I have a Wiseco in my bike, so who knows which bike it was for.  All I know is that it was the right height and the ring gaps were in the right places. What I can also tell you for a fact is the 76 cylinder and piston will work on the 79 bottom end. My bottom end is a 78, my top end 76. If there is a wrist pin location change with the 78 piston it wont work with the 76 top end. However I am not so sure it was changed on the 125.  If it is the same and the ring end gaps are in the right places the 78 piston may very well work in the 76 top end also. Do you have a piston?  If you do and you know it is for your top end simply measure from the top or bottom to the center of the wrist pin.  Make sure you buy a piston that is the same, and then check the ring gap locations when you buy a piston...and you should be good to go. I know the 175 was shortened, but Im not so sure the 125 was.

[Bill Campbell]

Thanks Mike. All I have to get is a piston that fits. I have tried Al with no luck on the piston side. I'm not in a hurry to get it together so patience may help in the end.I have a mate with a few 125 pistons that we can check against. I will probably get the bottom end sorted and see which piston I need before I put the cases together. Will get there in the end.
Bill

[Bill Campbell]

I have convinced myself that there is no change in the type of piston for a KTM 125 from 1976 to 1982 since the Wiseco catalogue only shows the one piston. So I am now looking for a 55mm piston , rings and wrist pin. The Wiseco part number is 462 P4. Anybody out there have one they are willing to part with? I can pay by paypal or Visa.

Cheers Bill

[Doug Bridges]

Hi Bill
I also have a 76 MC5 125 and I just talked to Al Buhner yesterday. He says they have the piston kits in standerd and .20 over.

Doug Bridges
76 MC5 125

[Bill Campbell]

I need 40 thou over and Al doesn't have any. Hopefully somebody can help me out over there.

Cheers Bill