KTM 125 pistons

[Mike Lenz]

I have a wiseco 404 piston i bought off ebay I believe that has quite a bit shorter skirt than the Wossner pistons Al had made.  The KTM books have three different part numbers for 125 pistons dating from 76-83.  Does anyone know the difference of the first two ktm pistons, the 50-30 piston(76-77)and the 51-30 piston (78-79)?  Wiseco also has three numbers for this era. 397, 404 and 462.  Anyone know the difference of those? If skirt lengths were this different the pistons Al had made should be trimmed for some bikes?? Or was I sold a mismarked piston off ebay?? All I have to compare is this ebay piston and one of al's. Help!

[gooddirt]

I have what looks to a Al B. 54.5mm nos wiseco listed on his page, and a  OEM longer skirted with larger tranfer cuts on the sides piston one ring 54.00 but no ring for it, do  not know what year on that one. I am going to use a sachs piston on my 1976 and just cut a slot in it below the rings.

[gooddirt]

I have 397 that was in my 1976... has a relocated top piston pin from center to the side looks the same as the 404 on ebay...

[Mike Lenz]

From Your and Al's help it looks as though we solved one question. The 1980-83 125 motor takes a longer skirted piston than the earlier motors.  This is apparently the single ring OEM and wiseco 462 Piston.  The 462 piston is also single ring.  So if you use a Wiseco 462 or one of Al's Wossner pistons for a pre 1980 engine you need to cut about 4.5 mm off the intake skirt, at least for a 78-79 engine.  I dont think the exhaust side would matter, would it.
FYI I measured a Sachs Elko 125 piston and it is about 2.5mm shorter than the 462/Wossner pistons, or about 2 mm longer than my 404 Wiseco. All the domes measure up pretty close from the pin.

What we still dont know is what was the difference between the 50-30/397 pistons and the 51-30/404 pistons. My 404 piston measures about 22.2mm from the bottom of the top ring groove to the top of the pin hole and 60.5 to the bottom of the intake skirt from the top ring groove.  How does a 397 measure out?  That should give us the final answer! If the transfer cut aways are off center more on one to another, thats easy enough to change.

[Mike Lenz]

Oh yea the skirt widths all seem pretty close 42.5 to 43.5mm wide.

[gooddirt]

27.70-75 ish mm  and  62.70-75 ish mm both from the bottom of the top ring groove

transfer cuts in piston look center of the pin hole  

skirt widths 42.70 intake and 39.70

[Mike Lenz]

I read the dial wrong on the pin to top ring measurement it is 27, not 22.  But I get 27.1-.2 on my 404 piston, so we may have a slight difference there to the 397?  However, unless one of us could get them side by side were still guessing that we are measuring the same exact way. It is close enough that they are still interchangeable, so I dont know if it matters that much.  If there is a difference it could be adjusted with the different thicknesses of base gaskets.  

I also found out that the 83 125 had a different piston number.  I dont know when that changed, 81, 82, or 83?  And I dont have a 81 or 82 book, so we cant be sure those were still the same as the 1980.  I have notes from back in the day, when I was a dealer, that they were the same in 81 and 82, but without the books I cant be sure.  If any of you have a 81 or 82 and are wondering, maybe see if Al has an engine manual for those.  The 80 part no is 500 30 007 001. the 83 number is 500 30 607 001.

Heres what seems clear to me so far. The 404 has a shorter skirt than the 397, by a couple millimeters. The 397 seems very similar to the Sachs piston. The 462 and Al's Wossner pistons have the longest skirts (4-4.5mm longer than the 404). The 462 and Al's Wossner pistons seem to be interchangeable with any bike 76-83 I believe, with a skirt trim required for some. More to come.

[Mike Lenz]

Anybody have a Wiseco 397 and 404 piston to compare side by side?

[Mike Lenz]

With Al's help I also found out that the 79 piston was a single ring.  Same liner part number, so I think it was purely a performance/ring drag change.
So here's what we learned the way I see it.
The 78-79 pistons were the shortest skirted pistons(404 Wiseco).  The 76-77 pistons were next, about 2mm longer(397 Wiseco). And the 80-83? pistons even longer, about another 2mm(462 Wiseco). So the Wossner pistons Al has will work in any of these 125's with a intake skirt trim of 2 or 4mm for some. This will get us darn close to OEM specs.
Thanks for the help.