Looks like a rebuild is in order...

[E30_Crazy]

I guess I'll have to apologize then, in case I wasted anyone's time with this thread. I just moved 7k miles, work close to 80 hours a week, blah blah, excuses excuses.


While some of my questions may have obvious answers, or easily attainable knowledge from manuals and catalogs, they were mostly a documentation of my thought process. I do appreciate all the quick, helpful replies.

[E30_Crazy]

Yup. Broken camshaft


The cam lobe looks pristine. I'd expect at least a little wearing to show, or some light pitting. This almost looks new to me. Goes to show the durability of these engines.

[Laitch]

Goes to show the durability of these engines.

Is that the cam lobe or the broken shaft that shows the durability? :giggles


Thanks for the photo.

[E30_Crazy]

Yeah, not so much the shaft, haha.


I'll be if I can dig it up, but there should be an entire build thread on here from the PO. Started out as a white RT held together by duct tape, if that rings any bells.

[E30_Crazy]

Here's the original build thread, if it interests anyone. I guess it was close to 100k miles, if that is the original cluster.


http://www.motobrick.com/index.php/topic,7151.25.html

[Martin]

There still has to be a reason for the breakage. Beside the two I have already mentioned bearing failure or head warping, the only other unlikely possibility is metal fatigue. Where the breakage occurred can you see any sign of fatigue, pictures of the breakage end on may give a clue. An inmate who is into metallurgy may spot something. The son in law who was a workshop manager had a problem with a Leyland diesel bus engine breaking camshafts, but that came down to not replacing the bearings when replacing the camshaft. a bearing was nipping up when it reached temperature.
Regards Martin.

[E30_Crazy]

I'll have to recheck everything. But I couldn't find any defects earlier. Maybe I'll start with a cheap(er) used cam if I don't find/correct any problems.

[Martin]

When checking use a straight edge and Plasti- Gauge. Replace the old camshaft and check with Plasti -Gauge on multiple points. If using a second hand camshaft again check with Plasti- Gauge.
Regards Martin.

[billday]

Are there any options for upgraded/better power curve cams for the K100?

According to urban legend, the earliest (1984, '85) K's -- the Finest Year K's -- had "hot" cams and served up more horsepower than later years. Some believe this to be true, some scoff. Something to think about.

[E30_Crazy]

That's what I've found too. Since mine's an '85, I'm now looking just to keep the correct cams in it. BSK has a set of reground performance cams, but they were a few hundred GBP, so nearing $500, even before international shipping.

[The Dude]

Wow!
I hope#4 lobe was on the dwell part leaving the valve up and sealed.In which case you were lucky.Mind you,breaking a camshaft isn't.
 BMW had a voluntary 100 hp limit back then(no legend)so get a standard cam exchanged and concentrate on the new baby...and hope #4 's inlet valve isn't bent!

[Martin]

I'd also be looking very carefully at the last bearing if this nipped, that is where the shaft would snap.
Regards Martin.

[The Mighty Gryphon]

Is there any sign of excessive friction on the bearing surfaces of the last bearing?  Any blueing from excessive heat or visual scuffing?

Have you checked to see if the #4 intake valve moves freely.  I would think the cam lobe trying to open a stuck valve could put enough stress on the cam shaft to snap it.  Can you see a wear spot on the surface of the cam at thee spot where it will first contact the valve shim?  Are there any signs of wear on the valve shim?

It is possible the valve is not opening smoothly because the stem is bent.  If it is bent it's hard to say if the cam caused the bend or something else did before the shaft snapped.  In any event, I think you should check it.