snapped camshaft
 

My engine is a 3.2 Short stroke twin plug built on an early large port 3.0 sc motor It has GE60 cams I got on it in 2nd gear and was north of 7,000rpm still pulling great when in an instance, the tone of the engine changed and it felt like it lost 150hp. I didn't ear any knocking, banging or metal on metal.

We did a compression test and leakdown and found all the cylinders were fine except for #6. Dropped the motor and started to tear down the right bank of the motor. No broken rockers. What I found next absolutely surprised me. The cam shaft snapped between the intake and exhaust lobes of cylinder #6. I have never seen this happen before.Does this happen often? Tom

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In 30+ years of working with air cooled 911 engines I have only seen this once.
In that case the last cam journal seized in the cam tower do to a foreign object in the oil passage.
The foreign object was of course high temp silicon.

Henry

That was my first thought. I suspected the last journal seized and the cam snapped, but that isn't it. The broken nub is moving in the journal.

The motor had these cams when I bought the motor. I am wondering if the cam was ground a used billet that had some meat welded on the lobes and if the heat could have cause some flaw in the shaft.

I have backed off the rocker and will have to do a leak down on the cylinder over the weekend. Does anyone think there is a prayer in hell the valve was not tweaked?

I guess the next question is what cams to put in the motor for replacement. Is the GE60 grind still thought of as a quality grind ? I know the valve pockets in the pistons work with the GE60 lift. Are there more modern grinds that would have the same lift but offer an advantage over the GE60 grind available out there. Would switching to a GE80 type grind make a huge difference in HP. (weber 46s and European Racing headers 1 5/8) I do know I would have to check the valve pockets, rod bolts and valve train to see if the motor is capable of spinning fast enough to use the more agreessive grind

Any suggestions

It is possable that the cam tower was installed after the heads were torqued and the tower was / is in a bind. Pull the rockers on both sides and pull both cams. Using the good cam, install it in the tower that the cam broke in and see if it's binding. If it is, loosen all the head nuts and the nuts that secure the tower to the heads and try again. I'll bet you find your problem. I have GE60 cams in a similar motor; hard to beat.

Do you have solid mounts?

Solid mounts
 

the motor mounts are factory 914-6. The suggestion there was binding makes sense, but the engine has been together in its current configuration for over ten years. The cams were installed by Auto Associates in Canton Ct. , a very reputable shop. No recent work that would have cause a binding. I doubt it would last 10 years if it were binding all those years. I am leaning toward a bad billet and may never know.

I've had two cams snap. And that's just on my personal 911 motors, which so far number maybe half a dozen over the years, some race, some street.

The second break was as a result of a rod bolt failure, which tore up a whole lot of stuff, so the broken cam was not a surprise, though unfortunate.

The first break happened while on the track, with no obvious precipitating event.

However, there was something in its past. When I first built the motor (a 66x85 2.3), for some reason I never satisfactorily discoverd, that cam (right side, I think) siezed on my first try at starting the motor. The locating pin ore the teeth off of the inner vernier piece. I couldn't get the cam out of the housing until I pulled the front end plug and applied some careul force. I thought surely I'd forgotten to lube or something, but all journals plus the inside of the plug had nice fresh red Torco assembly lube on them. No obvious signs of foreign objects.

Cleaned up that journal and the cam journal, reassembled, and the motor ran fine.

Until the cam snapped.

Supposition - the earlier incident had stressed the cam. Perhaps caused a tiny fracure to serve as a stress riser.

An engine builder friend said he once had a type IV cam standing on its big gear on his bench while doing something else. Heard a noise, and saw that the cam had broken just standing there. I guess they are somehat brittle.

Poltergeists.

:)

Are cams made from Dilivar....

Seen it happen with welded cams. When you side dress the lobe it can leave a sharp stress riser in the casting. Especially on the right side cam where the lobes are so close together. Another reason I don't weld up 911 cams.

Images
 

The motor is apart and I thought I would post images of the break as well as the Markings. I was told these were GE 60's. Do the markings mean anything to anyone?

Tomhttp://forums.pelicanparts.com/uploa...1464350443.jpg
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Great photo of the failed area.
Looking forward to the experts opinion on this.
Looks as though there was some building up and machining in the area.

Grey Cast Iron cam which looks as if an internal casting defect has propagated due to the alternating torsional stress during operation.

When the peak stress exceeds the strength of the remaining undamaged part of the cam a brittle fracture occurs.

I always worry about parts that are unstressed breaking for no apparent reason other than a poltergeist.:)

Basic fracture mechanics tells us that to grow a brittle crack in a material we need at least enough energy to create two new surfaces.

In a pre-stressed bolt suffering from hydrogen embritlement the energy stored during tightening will provide the driving force to propagate a defect.

The presence of hydrogen bubbles that may commonly be found in grey cast iron will cause a reduction in ductility but don't create any 'hydrogen embrittlement' as the bubbles contain molecular rather than atomic hydrogen and if the part is unloaded there is no stored energy.

That cam appears to have a hard face overlay a.k.a. welded

regards

Yes it is a welded cam. Date code 4.93 is April 1993. Pretty sure it was done for Andial

OMG, what a freakin nightmare :eek:

So glad I am just swapping the cams.

Hey John, Will be sending my rockers and shafts to you shortly for your magic touches :D

Sometimes the weld mitigates accross the lobe as a crak on the surface into the unmachined filet between the lobes it is a very difficult process.

regards

John, what is the tip off it may have been done for Andial? If it were done for Andial, would it make sense for the Grind to be GE 60 as I was told? I should probably send the cams to you so you can tell me what the hell they actually are so we agree on the grind for the new cams.

Tom

Who did the Andial cams, Integral?

This happened in 2012, right? You just got it apart?