Broken Rocker Arm

[Ripley/Fla]

What would cause a rocker arm to snap in two - just before the valve stem? Is there anything else that could be damaged in the head that I should be aware of? As usual, thanks for any advice!

[steve_wood]

Dumb question, but is the valve stuck? That would cause it. Any sign of foreign object damage in the cylinder or on the piston? Something getting stuck in there and lodging between the valve and the piston could break the rocker arm. Anything wonky with the tappets and push rods? They would have to be way way tight to cause the valve to bottom out and break the rocker but it could be possible.

Anybody else have any thoughts?

steve

[Cotten]

Michael!

The real wonder is why it isn't more common of a catastrophe.

Steel is a mixture of metals and carbon, etc., locked into a very strong disarray by the heat treatment given to it. The copper coating is part of that process.
All those molecules are still moving however, as does all matter above absolute zero. When you place the steel under extreme load in one direction, and then reverse it quickly, literally millions of times over a half-century or more... the mixture can separate to where the metal molecules will line up in their natural order, or crystallize.
This allows for micro 'cleavage planes' where micro cracks can start.
Eventually they become a "stress riser" where the metal molecules oxidize and will not heal themselves back together by their own affinity, and the oxide literally becomes a wedge to spread the micro crack into a serious one.
Then a 'castastrophic failure' occurs.

When you bend a piece of wire back and forth until you can break it, you are basically doing the same thing.

One thing that can accelerate the problem with rocker arms is an excessively loose valve adjustment with solid lifters. But that usually takes out the cam lobes and tappet rollers first.

....Cotten

[Ripley/Fla]

This is an Ebay deal, so I have only a picture of the heads. The valve is out of the stem, tho. Seller bought heads a long time ago and has no info on why it happened. So to quote him - "who knows, why."

[duoglide58]

Many years ago I had a guy rebuild my engine, and he did not know what he was doing. I am to blame for choosing to give the guy my bike to work on. From the start, the engine made a lot of valve train noise. I told him that it was too loud and I thought something was wrong. He said it would get quieter when the engine broke in. Turns out a rocker arm ended up breaking. I thought we might as well replace all the rockers at that point. Well with about 50 miles on the engine, an exhaust valve stuck and chewed up everything. The valve train noise I heard was the valves sticking until one eventually seized up. The guy was going to warranty the work, but I got to thinking if he did not know what he was doing the first time why risk letting him do it again. I gathered the pieces up and took them to a reputable buider who found all kinds problems with the prior rebuild.
So given my experience, I would suspect a sticky valve.
Doug

[steinauge]

Another thing that will do that is inadequate valve spring installed height usually caused by installation of an oversize head valve and NO checking of installed height or stem protrusion afterward.

[rrhawg]

as steinauge implied not enough clearance to coil bind.