Broken Skirt

[Mike Lenz]

Just put a 77 175 together, rode it a few hours and the piston skirt broke off on the intake side.  New piston in reused, honed bore.  About 4 thousandths clearance.  Why??  I guess with vintage parts the piston could have been dropped and had a hairline crack I didnt see. Anybody ever had this happen to them?
Thanks

[firstturn]

Yes, and mine was probably exactly what you said happen.  The way you can tell is to look at the break and see if some of it is clean and other parts are jagged.  This isn't 100%, but most of the time you can see a distinct two different areas in the broken area if that area wasn't chewed up or crushed.  It is "Failure Analysis".....as in recreating what happened first.  The clearance wasn't ideal, but I have run them that loose in the old days when we didn't have FedEX.  If you want to send me some pictures you have my email address.

Ron Carbaugh

[454MRW]

KTM service bulletin in 4/26/79 recommends a clearance of .06mm-.07mm or .023-.003" for the 175. 004" seems a little loose. Large bore RM400 Suzuki's routinely break the piston skirt when piston clearances get even a little loose, and although the 175 is a much smaller bore, I would think the same tends to apply. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1978-1979 MX-6 175 & 250 KTM's
1976-78 125-400 RM's & 79 PE250 Suzuki & 2012 DR650
2007 CR125R Honda
1977 MC250 Maico

[Mike Lenz]

I dont know.  Ive run some KTM's way too long, alot looser than this, even 400's.  On a 175 I previously would not even considered a rebore until it got to about 6 thousandths. I believe KTM says max clearance is 7 thousandths on 175, 250 and 400 for a used cylinder and piston. Dont know if thats the new 175 or the old one though. The newer style 175 has very short skirts compared to the old one or the 125.  Short skirt and short piston.  Actually shorter than the 125 piston by at least a couple millimeters. could that have anything to do with it?  I dont know if shorter or longer is better in that reguard? Cant see where any of the break looks "old" or anything different. Ill send you some picts Ron. Thanks