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Help! Broken Rod Bearing

 
joelswecker
I love my Chevy Chevette!
Posts: 3
Joined: 08/13
Posted: 08/18/13 04:40 PM

I have an 86 chevy k10 truck with a top-end rebuilt 350. Engine came from a running Camaro (1975) with only 35,000 miles. Rebuild consisted of an RV cam, lifters, rods, and 64 cc heads; nothing special but all NEW parts. The engine ran great for approximately 100 miles, then we parked the truck for approximately 2 years to paint, lift, etc the truck. When complete we drained the gas, oil, and removed spark plugs and put all new in the engine. Engine ran great, but developed a knock about 30 miles into running it after the truck was out of the shop. Popped the valve cover and found the #7 cylinder Exhaust rod bent and almost broken. Replaced the rod, ensured it was seated right (we originally thought this was the problem) and adjusted the rod correctly like all the others. Truck ran fine with no problems until about 30 miles down the road again! Just popped the valve cover, same problem! What's causing this? Oil appears to be getting the valve correctly and turning over the engine by hand doesn't prove any abnormal resistance. I'm fairly new to engine rebuilding and need some help!  

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waynep712222
I have an SS396 tatoo
Posts: 251
Joined: 03/13
Posted: 08/18/13 08:24 PM

check the sides of the rocker arm stud for a groove down in the smooth area... that would allow the rocker arm to fall over .

how is the tip of the valve... is it square.. or rounded over... allowing the rocker arm to work its way off the stem and bend the pushrod..

once in a great while.. the valve stem to guide clearance will be too tight.. or has seized up.. locking the stem when it gets hot and then the weak link will cause the push rod to bend..

and a rare one.. a thin wall valve guide liner that slipped down allowing the top of the valve stem to rock.. sometimes holding the valve open... either the push rod comes loose.. or the rocker arm rotates slightly because of the dancing valve stem.. this bends the pushrod..

you will also want to lay a ruler across the top of the rocker arm studs.. to make sure you have not had one thats pulling up..  

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joelswecker
I love my Chevy Chevette!
Posts: 3
Joined: 08/13
Posted: 08/29/13 10:15 AM

Update.....took the head into a local performance automotive machine shop. They found the rocker arm stud had a small chip in the backside near the valve spring which was causing the rocker arm to slightly catch at high rpms thus breaking the rod, twice. The break also damaged the side of the rod port into the head which gave it more 'play' which would allow the rod to dance around following the first break. New rocker arm studs along with a corrected guide plate and the fix is as good as new. Thanks Wayne for your reply.....  

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