The Stovebolt Site Forums The Shop Area Engine Shop 2 broken rocker shafts in 2 weeks

I have been posting on another site about this engine work. to bring everyone here up to now! I have a 1950 gmc 3/4 ton with a 228 that has been bored out to 248 with standard dome pistons a year and ahalf ago the fiber timeing gear went and bent all the exhaust valves. So I had the motor rebuilt. The cylinders were in great shape so I had new rings installed the crank was ground and the head was completely rebuilt with new valves, guides and head resurfaced. put the engine back together and set valve lash and timing started it up ran great drove around town no problem took it oup to around 45 mpg and the rocker arm shaft broke. I replaced the shaft I thought that maybe it had been stressed or crack when the timing gear went Set vavle lash and it was runing agin tried to go to a car show and about 4 miles outside of town the rocker shaft broke again. I got it home and pulled the head and took it back to the machine shop to get it check out maybe the guides were to tight causing the shaft to break.
The machine shop called this morning and said he didn't find anything wrong the guides and valves were all fine. He did say he reground the valves because maybe the vavles were setting deeper in the head maybe causing a coil binde in the vavle spring. I am not sure if this is a stock cam so I am going to check this when I get the engine put back together. Is there anything else that maybe I should check before I start to put it back together?

Hy 501long150GMC5w, did the shaft break in the same place and in the same way both times? I have heard of the wrong valve guides being installed, which are too tall above the head and cause the keeper/retainer to contact the top of the guide before the valve is fully open. This usually causes broken rocker arms, or bent pushrods, please keep us informed on what you find.

Thank You for the post!
The machine shop check to make sure the guides weren't to tall or if the retainer was hitting the guide. He said it allchecked out.
Yes it broke in the same spot both time, the no. 6 exhaust valve.

Hy 50long150GMC5w, did the shaft break in the support/stand, or did it break immediately outside of it? I can't think of anything that would cause that much stress on the shaft, that wouldn't show up as a bent pushrod or a broken rocker arm. If you put all the supports/stands in place, will the shaft pass through them easily? Is there any discolouration on the shaft once it breaks? Could you post some pictures of the broken shafts?

Are we talking about the rocker itself or the shaft that the rocker sit on?

"4 miles out of town the rocker shaft broke again."

The rocker shaft broke at the oil hole for the stand. So right inside the stand. All the push rods and rockers are fine nothing bent or broken.
This conplete setup is the one that was in the motor when the timing gear went. The only thing that has changed is the head.
I'll get some pic's posted as soon as I pick up the parts from the machins shop. There was no discoloration in the shaft were it broke.
3B your post makes me think that maybe the head isn't flat on top were the valve train bolts down. I will check this out!

Thank You everyone for your posts.

I am hoping to get this figured out soon so I can drive it. Going to miss another show this weekend.

New pic's posted at http://community.webshots.com/user/Tonywp24?vhost=community

Hy 50long150GMC5w, I'm no metalurgist (spleling) but those look like clean breaks to me, not a constant wearing type thing, but rather a shock type break, As I said in my post yesturday, I can't think of any kind of load that could cause that break that wouldn't also show other damage. It's a puzzler all right, breaks can begin from holes, but I see no evidence of a fatique break. Please keep us informed.

I would check the rocker shaft stand I'll bet the stand has to much up and down play in it and the shaft has a chance to flex up and down in the stand maybe the stands hole were the shaft fits is tappered "larger on the end that the shaft sticks out or long end" so as the rockers open and close the valves the shaft is being bent up and down against the bolt going thru the stand.Thats what it looks like to Me.


Pete

I had no idea that rocker shafts were that hard and brittle. That is amazing. What does it feel like if you bolt it down with no pushrods in place? Also, I know on the 216, all the tower bolts have a special shank that locates the support with a very snug fit. It looks to me like it is under a big strain as soon as it is installed. It breaks at the weakest spot, I would think.

Check the heigth of the stand against the others for distance to the center of the shaft opening. IMO you have some small stress going on there and in the photos it appears that the shaft has a visable wear ridge. It is extremely difficult to determine from a photo but but I'd think of an alignment problem. Also is it normal for the oil hole to be larger on one side than the other as indicated (again) by the photo. Is it being lubed correctly?

The holes in the shaft are not for oil they are for the mounting bolt to go thru to hold the stand down and keep the shaft from turning if this happened in the shaft stand than I still think it's because the hole in the stand is tapered and the shaft is flexing there is an exhaust valve on one side and an intake on the other so they are pushing 180 degrees apart.


Pete

Hey Pete52,
Your insight is appreciated but throw a period(.) in there once in a while, I can't read that much without takin' a mental breath.

"The only thing that has changed is the head."

What head casting was in before, and what are you using now?

I want to Thank You All for your posts!
Something that I just came across this morning is the pistons are in wrong! The machine shop that put the block together. said I can pull the motor , bring it in and they will correct the it. I think it might be easier to pull the pan in the truck then to pull the motor.
I am not sure if this would cause the rocker shaft to break!
The head casting number is a 2136477 and it is the same as the one that came off the motor in the begining of all of this!

I can't get past the alignment idea. Are you sure there were no shims under the stands when it was taken down?

I made acouple of relies about overheating and noisy valve train. I totally forgot about the broken rocker arms and shafts. I broke several rocker arms and 2 rocker shaft. The shafts broke right at the rocker towers. What the problem was, was the valve stem keepers were in the wrong (lower) slot. Once in the proper(upper) slots the breakage stopped and the noise stopped. The head just had a valve job. It took ME several weeks to finnally find it.

GEC52 Thank you for the post.
I will have the machine shop check this as I am not sure were they are now.
53moneypit Thank you for that post. I am sure there were no shims under the stands when I took the head apart the first time!

Now how did you come across the fact that the pistons are in wrong? I take it you have the head off. Don't some of the GMC motors have pistons that are not flat on top but fit around the valves? If so, if a piston were in backwards, this could cause a lot of desctruction. Was the motor really noisy?

truckernix: The first clue should have been that the pistons were hitting the spark plug when I first started the motor up and it was missing. I pulled the plugs and found that the gap had closed up on a couple of them. Being new to these old inlines it didn't click. Someone had a post on anouther site that there was no way the piston can hit the plug, that started me looking into this a little deep and with breaking rocker arm shafts. The piston is domed and the dome was put together on the wrong side because now that is the way a domed piston is away from the valves and closer to the plug.
No the motor wasn't really noisy. It gave me no sign that I had any problem. It started up and ran fine. I do think it had something to do with a higher rpm because I had drove it around town for a few days both times before the rocker shaft broke. They didn't break until I took it out of town and running it at a higher rpm.
The motor is at the machine shop.

Wow, this is one for the history books. They are a super tough engine to withstand that. That must of had you scratching your head to find plugs changing like that. Well, soon it will be right and you will be able to enjoy it.

It has been a long season to see the truck just setting in the shop every day! Can't wait to get it going and some drive time.
I will keep updates as this goes!

Got the enginge back from the machine shop last night. What he found was it was coil binde. there were 0.060 shims under the valve springs, intake and exhaust. He turned the pistons around made sure it had a stock cam and went through the valve train. The shims he pulled out look more like a spring seat. But he checked the spec on the springs and the hieght was wrong, so he removed the valves removed the shim and rechecked the hieght and it is 0.005 over spec. Has anyone run into this before?

The installed height can vary depending on how much the seat has been ground which makes the need for more shims under the spring to maintain the correct height. If the seats have been replaced that could also change the installed height. Another possibility could be the valve spring retainer hitting the top of the valve guide. I had a BB Chevy that I did 30 years ago and didn't check clearance between retainer and top of guide. I broke a number of roller rocker arms and bent pushrods before I figured it out. In my case the camshaft I installed had a significant amount of lift over the OE cam and that caused the clearance issue. It could be the wrong guide installed or the guide not installed properly. Just a couple ideas. Don