I have a 1970 L46 engine rebuilt to stock. About a year ago I replaced the cam with a correct GM spec cam. The engine was returned to original 11.1 compression. Since the rebuild I have started and run the engine from time to time, perhaps in total I've run it 1000/1500 miles. Ran well, idled well, today after a few hundred yards there was a noise. After checking around I found a bent pushrod on # 6 cylinder. The result of this was the stock rocker had slid from the valve top causing the noise and backfiring through the carb.
I have several questions, should I ...
Replace the single bent pushrod then readjust.
Replace the complete engine set of push rods then readjust.
Try and understand why the rod bent in the first place. Suggestions?
During the engine rebuild the hydraulic lifters were changed but the pushrod were not. To the best of my knowledge the pushrods are original and have about 80K miles on them.

 

If it were me I'd change out just the one rod and rocker but try to find out why it broke in the first place. It feels a bit extreme to rebuild the whole engine

 

Check if the rocker stud has worn at the side ,due to no guides . I had one do it on my 71 and bent a push rod .Had to replace the stud .

 

When you say it was rebuilt, what was actually done to the top end?
New valve job with new valves, springs, retainers, etc?
We really need detailed info on what was done.

If it was me and I had the valve covers off anyway......
I'd disassemble all the rockers and remove all the push rods.
I'd look for shiny wear on the sides of the push rods where they go through the heads.
If the rockers were original, I'd inspect the ball that the rocker pivots on to see any excess wear, and THEN I'd chuck them. If you are just replacing them with the original style rockers which is not much money, it is way better than using 40yr old rockers.
I'd also inspect the tops of each of the valves to see if they are flat.

I'd check the studs.
If they are screw in now, I'd install push rod guides.
I'd put in roller rockers instead of the original style.
I'd install new push rods after checking to see if the pushrods are the correct length now that work has been done.
There is the possibility that the rockers do not center on the valves if the push rods are not long enough.
They make a simple measuring push rod tool.
They make longer push rods for just this need.
The proper way would be to mark the top of each valve with blueprint ink. Install the new rod and rocker setup. Adjust them the old manual method (you will find several papers here in the tech section...just search hydraulic valve adjustment).
Make sure distributor center wire is disconnected so engine will not fire.
Crank the engine manually and check to see what the witness mark on top of each valve looks like.
It should only erase the blue ink from the center straight across in a small line. That is the only way to know that the rocker is centered on the valve and pushing it straight down.
Do that on #6 first.
But do them all.
If no problem shows up after this, recheck your valve lash settings.
Plug the main spark line back in and fire it up.

Warning....it could be messy without the valve covers or you can use either the little deflector clips they sell or a cut out set of valve covers that some guys use to just adjust their valves with.
We used to set them hot by loosening them up until they started to clack.
Turn it back down slowly to stop the clacking and then 1/2 more turn.

Or you can put the covers back on and tighten them just snug. Start it and see if you hear anything.

Personally I like to see the pushrods rotating and oil coming up through the rockers.

I would NOT just replace the one rod and rocker.
You will be doing this whole thing again sooner or later.

Good luck.
Keep us posted.

And check the Advanced Search function in the drop down menu for more info. Look in C3 Tech subgroup.

Bman

 

sounds like one of 2 things happened.
1/ The rocker stud,if pressed in from factory, pulled out, measure another stud height.
2/ the hyd. lifter failed and stuck "up" and the valve hit the piston.
better check the lifter if you didn't find that the stud pulled out.
Pushrods need a rock solid hit to bend, so something that is suppose to move like the spring or valve, didn't move so the weakest link is the pushrod.

 

Usually not the push rod at fault .what I have seen is on an engine that not driven much is a lifter will bleed down and you will have to let engine idle for a while for lifter to pump up and return to normal .On race engines they have push rod guide plates to stop this from happening
Good Luck
Wes

 

Thank you I'll check.

 

Bman,
I don't use the forum often so if this is not correct ... sorry!

Several things were done during the rebuild, Block rebored, new pistons and rings. new bearings and camshaft, new hydraulic lifters and valve springs. The machine shop said the valves, seats and guides were okay. The rockers and rods were also reused. the main parts used which might impact this problem were.
Camshaft 3896962 ... Jegs
Lifters 812-16 ... Comp/Cams
Valve springs 981-16 ... Comp/Cams.

You and others have provided some valuable input. I think what I'll do for now is to inspect the rocker stud for wear and replace the bent rod. This should get the engine running again until the cooler weather comes around. Then I think I'll remove the heads again and redo the valves, guides, seats, rockers, rods etc. I have a question, is it a good idea to have the rod guides which were stock on later 350 engines? If so, do the rocker studs need to be upgraded?

 

To do it right...you're going to machine the stud bosses and install guideplates with good studs. Then you're talking about redoing valves, guides, seats, rockers etc.

For similar $$$, I think you'd be much better off to get a nice set of aftermarket aluminum heads with all the good stuff...pick up a ton of power and be a happy camper. That way rockers and pushrods etc can be selected to fit the new stuff.

If trying to keep things stock looking, orange paint goes a long way towards disguising things.

JIM

 

If they are the original L-46 heads from 1969 they use press in rocker studs without guides unless they have been modified. As others have said check to see if the stud is pulling out. There was a stud available that was .003 larger or have the heads modified for screw in studs with guide plates.

 

The L-46 was the predicessor to the [later] LT-1 engine and it wound up to 5600 rpm (L-48 only to 4800 rpm). I am not personally familiar with the L-46's internals, but would be very surprised if it did not have similar valve guide system to the LT-1 engine. Wrapping up to 5600 rpm with the 'loosy-goosey' L-48 valve system would be 'iffy', at best.

In any event, you should upgrade yours to a sturdier configuration, if you want reliability of the valve train. Good lifters, springs, pushrods...which should all be compatible with the cam you installed...are required, IMO. You might also change out the cam drive gear and chain, if you have more than 75K miles on the engine and they haven't already been replaced. Don't want to get that thing working like it should, then throw 'craps' because of an old chain or timing gear.

 

with Jim

Last set of iron heads that I had refinished after I ported and polished them ended up costing almost $400 in machine shop labor and parts from the shop. That was after I supplied the spring, keepers, and the new guides that I wanted installed.

But it will be interesting to still find out what was the original cause.
You know us guys...we just have to know the answer.

 

Take your time.
I'd still like to know what caused the bent rod. Did a lifter get stuck in the up position? Like others have pointed out....the geometry of the lifter, push rod and valve spring was compromised resulting in the bent rod which is the weakest link in that chain.
A couple of pics of your heads with the valve cover off and the bent rod would go a long ways to solving this mystery. All you have to do is use the Manage Attachments window in the Additional Options window below these posting windows.

Good luck and let us know what you find.

Bman (Dennis)

 

I'd want to figure out why. Do you have a bore scope that you can look through the spark plug hole at the top of the piston, maybe the guides are tight and slightly siezed? Just guessing. Maybe the rocker goofed and caused the lifter to push the push rod against the head, are there any marks a on the push rod about a third the way down? There has to be some sort of evidence as to what happened.