breaking rocker shafts

[marbill47]

We are having unbelieving problems with a 6.2 hummer engine replacing a 6.5 in a 2500 pickup. We have broken 6 rocker arm shafts for no apparent reason, the truck will run smooth as silk and suddenly randomly break a rocker arm shaft. This will happen on either bank on any cylinder, no bent push rods, no broken rocker arms, the rocker shaft will snap on any random head. The manual pump was relaced with the electric injection pump, all timing is correct. need help Bill

[john8662]

What year model 6.2?

The early 82 6.2's had weaker rocker arm shafts because the clete (surrounding washer) where the bolt holds the rocker arm were a near complete circle, but when compressed caused fatique on the middle of the "c" and caused the clete to break, leading to the rod not being fully supported.

So, which set of rockers do you have?

The early had cast iron arm, the later (85+) have pressed steel rocker arms held in place by a nylon button.

J

[More Power]

Valve contact with the pistons? Broken & loose glow plug tips in the cylinder?

Jim

[marbill47]

I have the later with plastic retainers, they are breaking right next to the clamp, Maybe torqued too tight? Thanks for the responce. No glow plug broken tips, it runs very smooth till a shaft breaks.

[twaddle]

Hi there,
Is there any history to this engine, has it very recently been rebuilt or is this reckoned to be a sudden thing?
If a rocker shaft has been replaced are you replacing with new and are they breaking a second time even with the new/replacement R/shaft?

Are you getting plenty oil to the top of the heads? (just a long shot)

Regards

Jim

[john8662]

New one, the later shafts don't usually break.

How are they being mounted to the heads. Do the set flush on the petistals on the cylinder heads. Remember there is no bracket or anything underneath the shaft, only the small bracket/washer on the top side of the bolt that fastens the shaft down to the head.

The torque requirement is 40 ft. lbs.

Too much and it's possible to cause breakage.

Got some pictures?

J

[marbill47]

Thanks for the info, I will buy new ones and try them.

[More Power]

We are having unbelieving problems with a 6.2 hummer engine replacing a 6.5 in a 2500 pickup. We have broken 6 rocker arm shafts for no apparent reason, the truck will run smooth as silk and suddenly randomly break a rocker arm shaft. This will happen on either bank on any cylinder, no bent push rods, no broken rocker arms, the rocker shaft will snap on any random head. The manual pump was relaced with the electric injection pump, all timing is correct. need help Bill

This is very out of the ordinary. Normally, the pushrod is the weak link when something prevents a valve from opening. I suspect the answer will involve something unusual - like incorrect installation, valve clearance, or perhaps knock-off parts. Those rocker arm shafts are stout pieces...

Jim

[dieselbegreat]

Very important to remember that the shafts must be kept parallel with the head when tightening them down. One bolt may be easy to turn and the other hard, but the shaft must not be at an angle or it can start stress crack. Then when the shaft is completely seated you can torque the bolts down alternatingly.

[Robyn]

I have never seen one of the shafts broken.
I have seen the old cast rocker arms from the 82 engines go away but the later stuff seems to last real well.

About the only thing I have seen fail in the upper end is the plastic guide buttons.
Having a random breakage that follows no particular location is indeed very wierd

Was this engine rebuilt recently?? if so was the block decked??
Valve to piston clearance is critical in these engines.

As has ben mentioned the push rod is usually the fuse and not the rocker shaft.

Good luck

Robyn