Re: broken rocker arm hold down...why?
Do you have individual rockers or are they on a rail all attached together?
If you have singles,
By "Hold Down" do you mean the bridge that connects one to the other and keeps them from twisting, or the fulcrum (mount, tower, pivot, ect) in the center of the rocker arm?
The most common causes for broken rocker arms (and in your case the mount) are...
1. Flaw in the material.
If there is a flaw in the base material, it's a place for a crack to get started.
If there is a flaw in the heat treating, it may have become too brittle and just snap off. (usually one quick break)
If the material is too soft, you will usually see where the crack has started, stopped, moved again, stopped ect...
There are literally billions of these things manufactured every year, and bad ones often ship to customers.
2. Lifter pressure.
Some times the valves in the lifter will malfunction.
(usually do to crap in the oil, but sometimes it's a mechanical defect)
The piston in the lifter that's supposed to pump oil up through the push rod is jammed in place and doesn't move.
This will cause the lifter, in effect, to be taller than normal, and will break rocker arms and push rods.
3. Sticking Valves.
If you had a valve stick in the guide, that will break off parts in a hurry!
4. Slop in the valve train.
Clearances that have worn and opened way up, compressed lifters (often called 'Stuck' lifters) where the internal piston is at the bottom of the lifter (Broken spring, wedged down by crap in the bore, ect.), broken valve springs, valve stem lash caps came loose, ect....
When excess clearance gets into the process, gravity pulls the push rod down, and the lifter will propel the push rod back up with quite a bit of force... But now there is slack in the system, it lets the push rod hammer on the rocker arm assembly.
This is REAL hard on things. Hydraulic valve trains are a zero lash system, and should never be allowed to hammer...
So many cats, so few recipes...