Engine problem

[1940Chevy]

While starting my 350 SB after changing the thermostat and sender. I heard a POP like a back fire. Then heard a rattle under the driver side valve cover. I shut down the engine and pulled the valve cover. I found both rockers on number eight cylinder were off. I thought the lifters had collapsed. So I pulled the intake and checked. The lifters look OK. I am confused and don't know if I should replace the lifters or what. I don't know what could have caused this to happen. Can anyone help or know what happened and how can I fix this? Thanks!

[raceman6135]

I have no idea why two rocker arms would come off when you changed the t-stat and temperature sender. Sounds like a coincidence.

With regard to the rocker problem, since you have the intake off already, I would set up a dial indicator to measure the lobes of both the intake and exhaust. Check these values against a factory technical manual (if using a stock camshaft) or against the camshaft manufacturer's "cam card" that came with the cam.

It is possible (although extremely rare) that both lobes are worn to the point where the camshaft must be replaced. In all my years of working on engines, I have never heard of two lobes wearing to the point where the rockers came off, unless there was a major engine catastrophe or unless a brand new camshaft was not properly broken-in.

Are the pushrods bent? If so, this would indicate a mechanical interference of some sort -- perhaps a bolt or nut got dropped down the carb when changing your parts?

If the pushrods are okay, and the camshaft is okay, and the lifters are okay, I would reinstall the lifters (and they MUST go back into their original positions! If they got mixed up, you MUST buy new lifters for that cylinder, lube them up with proper camshaft break-in lube and then reinstall.), adjust them, and try restarting the engine.

It is truly weird that both rocker arms would come off, though. Hopefully someone else has some better ideas.

DJS

[1940Chevy]

Thanks for the help. I found the problem. I had replaced the heads back in March. Well, I kept looking and looking for a problem. The lifters were OK and the rods,rockers,etc. Well, while racking my brain I noticed the studs in the head that held the rockers and so forth. They were sticking up really higher than the others. The studs are pressed in the head and for some reason pulled out. Now, I have to see if I can replace them with out taking the head off. I have never seen that happen before and of course the warranty ran out on the heads a month ago. Thanks again for the help. I guess we can add this to future possibilities to mabye help some one!

[River Bum]

A month ago, I would still see if they would pickup the warranty, a good place probably would work something out with you. Its a free call, I would try. A cheap fix

Ron

[Saltmine]

Pulling rocker arm studs used to be a common problem on small-blocks. It was usually caused by improper valve adjustment, or over-reving the engine.
No, contrary to popular belief, running the valves too loose is usually the culprit
on pulled studs. You see, excessive lash in the valvetrain can cause an "impact" type force on the studs, which will pull them out much faster than a steady pull.

If you have to run the valve lash loose, consider screw-in studs or pinned studs (even though the rocker arm studs on a Chevy V-8 are pretty hard, and nearly impossible to drill)

Normal valve lash on small-block Chevy V-8's is one full turn past zero lash.
Yeah, I know, everybody thinks anywhere between 1/4 turn and 3/4 turns is right, but on the rocker arm nut, turning the nut one turn will move the lifter plunger .050" which is exactly half of the lifter's travel (.100"). Set looser, you lose duration and lift from the excessive lash.

Screw in studs can be installed, but you have to devise a way to keep the metal chips out of the engine when you drill out and tap the heads for the threaded studs. It's a lot of work, especially if you plan to do it on the car.
But, why were the studs pressed in instead of screw-in in the first place? Any hot-rod application, especially involving high rate valve springs or hot camshafts should have threaded studs and stamped guide plates, in the first place.

[Scribs]

...and i got a lesson in valvetrain on a SBC

which is helpful as i am looking for a good SBC to rebuild just as a project and to learn more about engines and how to rebuild/build engines.

[olddog]

If my memory serves me, Chevy used screw in rocker studs on their high performance stuff (1960- 1970). I wouldn't use press in for any high performance parts (high lift, heavy springs, solid lifters, or roller lifters). For that matter, I would go to roller rockers for anything above a mild street engine.

[Daniel Wood]

...and i got a lesson in valvetrain on a SBC

which is helpful as i am looking for a good SBC to rebuild just as a project and to learn more about engines and how to rebuild/build engines.

Scribs, there is a tape that you need to buy. It's about rebuilding small block Chevrolet engines, but it pretty much replies to all.

I can't find the number right now, but I know someone who has it. I will post it later on. Sorry

[Daniel Wood]

If my memory serves me, Chevy used screw in rocker studs on their high performance stuff (1960- 1970). I wouldn't use press in for any high performance parts (high lift, heavy springs, solid lifters, or roller lifters). For that matter, I would go to roller rockers for anything above a mild street engine.

olddog, they also drilled a hole in the side of the casting that the stud (whether it was pressed or screwed in) so that they could drive a small pin from one side through the stud and out the other side of the casting that holds the stud in. It was pretty good insurance.

I forgot to mention not that it matters. But this is a 8 year old thread. :D

[Daniel Wood]

...and i got a lesson in valvetrain on a SBC

which is helpful as i am looking for a good SBC to rebuild just as a project and to learn more about engines and how to rebuild/build engines.

Scribs, click on “ COMP CAMS ” (http://www.compcams.com/) and then select Shop and put this part #191-1 in the search box. I highly recommend that you watch it, you could learn a lot. It applies to other engines as well. It's only $22.


http://www.compperformancegroupstores.com/store/graphics/00000001/How_to_Build_Small_Block_Chevy_191-1_600x600.jpg

[Scribs]

thats going to be a very big help!! thanks very much!!!