1983 911 SC Rocker Arms Sheared Off

[vipergreenT]

Hi guys, Looking for some input and advice on this issue. I bought an 83 SC Targa that was vandalized and I am parting out the car here on Pelican. The reason I bought the car was for the motor for my 75 911S coupe. I was told the motor was rebuilt and had no chance to start it or drive it prior to purchase as the fuel and electrical was not hooked up. The car was cheap so either way I am ok. After removing the motor and trans I started to do a leak down test and the numbers were bad. I pulled the valve cover on the driver side and here is what I found. Rockers sheared off as well as the Rocker arm pivot tube mangled on # 1. The other side is fine. The engine has Carrera Chain Tensioners and the chains look good. The Chain drive turns but the driver side cam dose not rotate. Any input or idea what might have caused this. The engine looks fresh from the outside. Oh well.......

[Reiver]

No idea on a Porsche but I've only seen failure like that when a motor blows and the pistons come in contact with the valves during valve opening...they can punch the valves thru piston tops and or break the valve train like this.

[vipergreenT]

Thanks, The engine rotates by hand very smooth and quiet no grinding or noises. I will do leak down on the other side. Pulling the lower valve cover tonight. Thanks

[Flieger]

Maybe a failed tensioner, because that looks like the pistons did more than kiss the valves. No love taps there.

[Tippy]

The fact it is only one side means the cam timing got off. How that happened is to be determined. Flieger probably nailed it with a failed tensioner.

[Peter Zimmermann]

Piston/valve contact breaks rocker arms (and can also damage valves, pistons, and con-rod bearings depending on how many revs the engine was turning at time of failure). Don't waste time finishing the leak down test, just pull the heads off and evaluate. No sense guessing the why, but the left cam somehow got out of time with the rest of the engine, and you see the result.

[RWebb]

BTW - the rockers are cast iron so they WILL break and limit worse destruction

I hope the seller doesn't post here

you can put up pics of the tear down to edify us all

[vipergreenT]

Thanks for the info. I was expecting to see something bad when I pulled the chain cover off but it looks good. Any simple test at this point to know if the tensioner went bad? Now I need to keep selling the rest of the car parts to get some money to do the rebuild. Thanks and I will keep you all updated on what I find.

[john walker's workshop]

looks like the tensioner is all the way extended and the chain is still slack to the right of the idler sprocket.

[cgarr]

Is there even a pin in that cam gear?

[db_cooper]

Originally Posted by cgarr

Is there even a pin in that cam gear?

Looks like no pin for sure. Wonder who did the "rebuild"??

Chain is slack too..cam timing must have jumped..boom!

[Flat6pac]

Pull the bottom and see what is broke there too.
Bruce

[vipergreenT]

Thanks, Same issues on the bottom. There were chucks of rocker arm in the valve cover. The chain is not tight as there is some slack when I lift up the bottom side. Looks like I am in for a busy winter boys. I never thought I would attempt this but I am going to give it my best. I will let you know when the head is off. Thanks for the tips... Any chance the pistons/rods and bottom end survived this? I hope so.

[stormcrow]

It appears the "tensioner" failed. If the chain is loose then you may have "major internal" problems.

The valves probably came in contact with the piston and that's what caused the rocker arms to break.

SteveKJR

"A Porsche does more then just go fast in a straight line"

[Weissach911]

Originally Posted by vipergreenT

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This looks quite like a classic fatigue failure. The fatigue crack grows and increases crack tip stress intensity to a point where the remaining part of the component can no longer support the applied force and brittle fracture occurs.

By the way these rockers are Investment Cast Steel - not Cast Iron.

A Cast Iron Rocker would be very weak in comparison and would probably fail immediately.

[j911brick]

you don't mention checking the passenger side. This is also what happens when you over-rev a 911 motor.

[lindy 911]

Since it's a fresh rebuild, I'll wager the builder didn't time the cams properly during assembly. Upon start up it broke.

[vipergreenT]

The passenger side looks fine no problems that I can see.

[Peter Zimmermann]

At the very least plan on removing the con-rods/bearings, have the rods professionally inspected, and replace the bearings, rod bolts and rod nuts. If your engine was at my shop, and we found that many broken rockers, we would do a complete dis-assembly, and magnaflux/crack test
the crankshaft and intermediate shaft. Something awful happened in there, and we wouldn't take chances at this point.

[Flieger]

Originally Posted by Weissach911

This looks quite like a classic fatigue failure. The fatigue crack grows and increases crack tip stress intensity to a point where the remaining part of the component can no longer support the applied force and brittle fracture occurs.

By the way these rockers are Investment Cast Steel - not Cast Iron.

A Cast Iron Rocker would be very weak in comparison and would probably fail immediately.

I am curious as to why you would say that. To me the surface of the fracture looks quite consistantly rough, which would mean sudden brittle fracture due to overload, no? And the fact that so many are broken must mean they all went at once since even one broken one would make itself known, even if the engine might still run and the chances of all fatiguing cracking and failing at the same time are remote.

Therefore, I would say the valves and pistons crashed together.