Sachs – Wrist Pin Bearing Failure

[Ernie Phillips]

While tuning my bike ('72 125 CMF) last Saturday, bad sounds came out of the engine.  Disassembly revealed a broken wrist pin bearing.  I've never ever had a wrist pin bearing fail!   This motor had approximately 2 hours on fresh piston/bore.
http://s250.photobucket.com/albums/gg276/ernie7711/?action=view¤t=sachs125writpinfailure159.jpg

Looking at the failed part and comparing to a new and used bearing revealed that the cage on this bearing is dinky and it may not have been heat treated.  If you have any of these type bearings (dull metallic finish) beware!
http://s250.photobucket.com/albums/gg276/ernie7711/?action=view¤t=sachs125writpinfailure163.jpg

It is possible that I damaged the bearing upon assembly but I didn't have any difficulty putting it together – but who knows.  To be safe check your replacement parts very carefully.  Since the motor had parts go through the bottom end, I swapped motors with a spare I had (thanks Doug Wilford).  We will be at the races in Ocala.    


Ernie P.
Chattanooga, TN

[454MRW]

Ernie,
Although I can't tell from the picture for sure, the top of the piston looks like detonation damage/pitting. It may have contributed to the shock damaging the wrist pin bearing. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S

[Ernie Phillips]

Mike, Interesting observation.  I didn't notice any signs of detonation while riding under any/all load conditions.  Jetting was purposefully fat (Bing 30mm w/148 main jet), yamaha lube 32:1, and timing was set at 2.3mm BTDC (running digital PVL with 4 degrees of low speed advance),150 psig cranking pressure) Take a closer look at piston.  Damage appears to be caused by broken bearing cage that passed through engine and did "tap dance" on top of piston.  Small dark spot in center of piston is machine tool center.  Challenge my assumptions - I don't want this to happen ever, never!

http://s250.photobucket.com/albums/gg276/ernie7711/?action=view¤t=sachs125writpinfailure168.jpg

Ernie P.
Chattanooga, TN

[thrownchain]

Damage does look to be debris made, I'd definately be trying to score the "older" style bearing if they're available.

[454MRW]

Ernie,
Yes, it's a lot more obvious in the last pic that the piston was damaged due to debris/parts "tap dancing" on the top of the piston. The other pic wasn't clear enough to see that well on my computer. I agree with Dan about using the older pin bearing if you can locate one. I would call Doug and inquire about what type he uses in his engines. I am sure that he would be able to recommend a more durable replacement. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S

[brian kirby]

What is the IDxODxW of the bearing? Wiseco has good bearings and pins available separately. Definitely looks like the cage failed to me, which is very strange. I work at a Motorcycle/PWC dealership and with the jet skis I thought I had seen every type of failure there was, but I've never seen that before.

Brian

[firstturn]

Mike, Dan & Brian,
  I think if you look real close at the failed bearing you will see that is lighter in color as compared to the ones on the right.  A possible cause would be this caged bearing got passed the inspectors without being heat treated.  Brian you are correct this is a failure you hardly ever see.  I have seen this type of failures, but it has been due to wear and not to a problem in manufacturing.  Just my take.

Ron Carbaugh

[brian kirby]

I noticed the different color but I was not sure what it meant, sometimes too much heat can cause color changes, but in this case I dont  think the engine got that hot. It could just be because the cage is apart and has no rollers in it, but it appears to be more lightly made too. I think Ron is on to something, I'd bet on either improper heat treating or no heat treating at all.

Brian

[Ernie Phillips]

Bearing dimensions:  19mm OD x 15mm ID x 19.7mm long.

While measuring the bearings, I discovered a major difference.  Although the good and outlaw bearings have the same overall dimensions, the outlaw bearing has needles (rollers)  that are 2mm shorter.  Big deal?  I think it may have been the killer characteristic.

http://s250.photobucket.com/albums/gg276/ernie7711/?action=view¤t=sachs125writpinfailure172.jpg

Since the wrist pin bearing is free to float between the piston bosses, the bearing can shift to one side.  As photo below shows, this could cause edge loading and overload due to less surface area.  With the correct bearing, the rod is always fully supported on the rollers even at extreme displacements.  Again, I advise everyone to carefully examine 'spare parts'.  There is much opportunity for interbreeding over these last 30+ years ... and many mismatched and bogus parts floating around.   My inattention to detail killed my best motor!

http://s250.photobucket.com/albums/gg276/ernie7711/?action=view¤t=sachs125writpinfailure176.jpg

Ernie P.
Chattanooga, TN

[brian kirby]

I dont think there should be that much float. Even if the bearing floats to one extreme side or the other you should be able to see both ends of the roller. That is easily double the amount of wiggle room I would have expected to see. Are the cages different widths too, or is it just the roller that is different on the outlaw bearing? I think it might a good idea to try to find a slightly wider caged 19x15x20 or 19x15x21 bearing.

Brian

[Rocket]

On the KTMTalk.com website a similar failure occurred with a piston pin bearing.  The following link, if it works, will show you some pictures of the failure.
Rocket

http://ktmtalk.com/index.php?showtopic=267071

[john durrill]

Ernie,
 It looks like the cage on that one is made in 3 or more pieces?
 The ones we have to look at here are made from one piece I think.
That may be the rub. we did have one needle bearing wear out after 15 mins. of use in 2003. We thought it might be the oil we were trying out. So we did not use that brand again.
Sounds like we need to watch which bearings we buy. Sachs Book calls for a 15 x19 x 20.
  Does anyone know which manufacture supplies good wrist pin bearing's?
The old OEM part had much more metal around the needles than the new bearings we have bought the last few years.
John D.

[brian kirby]

Wiseco sells wrist pin bearings separately and they have one in 15x19x20. I think they cost about $10.

Brian

[BrianTaylor]

John Consolidated Bearings show K15x19x20 .... I dont know the manufacture ... you would have to call and ask  and also check lenght of roller if that is important ....BT

Brian Taylor

[john durrill]

Brian,
 The supplier wont be the problem . It will be the bearing manufacturer.
Quality control. Wrist pin bearings we have bought in the past have never come in a box like M20 or 6202's with FAG or SKS or.... on the box.
 I looks like it would be good to know who is making needle bearings now.
 Not a good turn of events.
 John D.