Broken case

[jcsmith]

Can someone explain to me why the KTM engines rear motor mount breaks the left case? Chris:(

1974.5  Mint 400
Frame 54 410 78640
engine 4 5500708

[454MRW]

Vibration loosens engine mount bolts, requiring frequent re-torqing, especially 400's. I use locktite. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R 79 CR250R

[jcsmith]

Thanks Mike. So the threaded bushing on the left side, rear mount, is it meant to take up excess space ( possibly with shims ?)instead of squeezing frame with bolt to tighten frame to engine ? Also ,do you think it's best to repair with cases assembled or separated?  I appreciate your continued info.
                                  Chris

1974.5  Mint 400
Frame 54 410 78640
engine 4 5500708

[454MRW]

The width of the swing arm is adjustable with the nuts using thin 24mm wrenches on each side of the swingarm. It should have approximately .010 inches of clearance to the frame with the rear engine mount bolt holding the frame tightly against the engine cases. Your swingarm may be too tight allowing the engine cases to be loose thus causing movement and damage.I turn the swing arm nuts in slightly then tighten the rear engine mount, then back the swing arm nuts out until you reach the proper clearance, only then tightening the swing arm bolt cinch lock bolt. Mike
Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R 79 CR250R

[Cal Alexander]

I'm currently restoring a 72 Six Day. When I was tearing down the bike I noticed that the swing arm was excessively tight. It was so tight it was limiting the movement of the swing arm so the suspension was not working effectively. When I pulled the engine out the lower left motor mount was cracked. The nut on the motor mount had also vibrated loose and the top of the mount had cracked through. Wondering if this was all caused by an overly tight swing arm?

75 250 MC
73 Six Day
72 Six Day

[jcsmith]

Mike, I appreciate that info, but I'm still not understanding the purpose of the threaded bushing on the left side that the rear engine mount bolt goes thru.

Cal,This seems like a common spot for breakage from loose fasteners from what I've been reading here.Was your swingarm bolt rusted to the tube and bearings?


                        Chris

1974.5  Mint 400
Frame 54 410 78640
engine 4 5500708