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Discussion Starter · #1 ·
so i was driving home from denver tonight in my '70 corvette conv with a very recently rebuilt, original 350/350. only now it has pro-topline alum heads. the engine has less than 300 miles. i'm almost home when i hear a 'clang, clang, clang' and then it goes away. i know something is wrong, but i have great oil pressure, it's not hot, so i keep going. then i notice it's running rough. so i gently coax it home and take off the valve covers and notice it's got 2 rocker arms off and flopping around. they are to the same cylinder, an intake and an exhaust. the push rods are completely toasted, one isn't even showing, the other is broke. the other rockers are fine. then i notice that on the 'push-rod guideplates' there is a groove worn in most of them that would allow the rocker to start getting kind of sideways. the cylinder that failed is the worst of all when it comes to this wear. i am not a engine guru, but somewhat handy. i put the push rods, lifters, rockers on, and adjusted them as i've done many times before without problems. this engine ran great, no noises, great power. the cam is mild for aftermarket cams. i drove it nice, didn't do any high rpms yet.
what caused this? something i did? the valves seemed to be adjusted properly, no noises other than usual valve train noise when hot. i'm really bummed, but i'll be more bummed if i did something to cause this. any help is appreciated.

thanks,glenn
 

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is it running ok now? worse case sen. the lobes went flat(cam wasted)
too many causes to really guess, did whoever build it ck for valve/piston clearance, coil bind, retainer clearance-were the rockers getting oil? do the rocker nuts have some resistance, they should be locking nuts-were the rockers adjusted correctly to begin with
its all just a guess w/o being there
my guess its some kind of binding problem, maybe bending the p/r's, which could cause them to wear on the guide plates-roll them around, see if they are bent-maybe the guide plates need adjusting-are the pushrods heat treated? need to be if used on guide plates
a 350/350 had pop up pistons, high comp., so adding diff heads would def need some checking-hopefully you can find the problem so it doesnt happen again
 

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My 1st thought is valvetrain geometry/aln issues due to new pro-topline heads and or possibly piston to valve clearnace but thats less likely at this point.

I sat tha becasue since you also stated most all the guide plates are showing p/rod wear that smacks of the valvetrain geometry issue thing to me.

You need to check where the rocker meets/rides on the valvestems etc to check the v/train geometry.

Do a search here on it and you should be able to find the proceedure to do this ofstart another post in the engine forum asking for the proceedure to do it or asking for experieced anfine builders help like/MIKE-WOLFPLACE/BILK/GARY-GOFAST/CARL-CNC/ETC WHO ARE ALL GREAT EXPERIENCED ENGINE MACHINEST/BUILDERS THAT HELPUS OUT HERE IN T/CHEVELLE.

GOOD LUCK.

SCOTT
 

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I've found that you'll often need to use adjustable guideplates with the aftermarket SBC heads to assist in centering the rocker more directly over the valve tip. Isky makes a decent set.
Are the other rockers crooked?

 

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I've never seen a push rod wear a groove in the guide plates. Sounds like some serious geometry problems. Can you post some pics of the broken ones and some of the others?
 

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I don't doubt the geometry is off--but--how do you wear out guideplates in three hundred miles unless the folks who made the guideplates forgot to HARDEN them?

I screwed up and bought guideplates for an Olds that had 5/16 bolt holes in them--standard Oldsmobile size. Problem is, I had converted the heads to use BBC rocker studs. It was HELL trying to bore the 5/16 holes out to 7/16. Couldn't do it with a drill bit, ended up with a carbide rotary file.

MAYBE the pushrods weren't hardened, as the pushrods wore they tore up the guideplates. It's the only thing I can think of that doesn't involve the guideplates being defective. Are all the pushrods torn up, too?
 

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Check and see if the screw in studs are tight. I wonder if one of the screw in studs came loose and you lost alignment of your guide plate and rocker arms. Its strange it was only one cylinder.

Dave
 

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There's no way the pushrods wear the plates in 300 miles. The plates are (supposed to be) harder than the pushrods. A lot harder, see Shurkeys post. The ones I've had couldn't even be filed, had to use stones to clean off a rough edge or two.

Did you get the right pushrods? Pushrods used with guideplates are harder than normal pushrods. There was a deal awhile back where some cruddy vendor was selling "Heat Treated" pushrods. Normally that would taken to mean hardened, but technically that's not necessarily true. Certain types of heat-treating can be used to soften metals.

Are the pushrods hitting the holes in the head? The holes in the head should be much larger diameter to prevent binding. If the pushrods touch both the plates and the holes in the heads there will be trouble.

Got any pics?
 

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Discussion Starter · #10 ·
thanks for the replies, i've been out for a couple days since. let me answer a couple questions; no the rods aren't hitting anything other than the guide plates, i.e. they clear the heads sufficiently. the push rods are completely undamaged, i mean not a bit. the alignment looks right if you eyeball it. i'm using comp cams roller tip rocker arms. can i email pics to someone who can post them here? i don't think i have that capability.
thanks,
glenn
the engine builder is stupified as well. says they have'nt seen this problem before, just the reverse where the rod gets eaten by the guide. if it can happen, it'll happen to me...
 

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Discussion Starter · #12 ·
paul, all of the guides are worn, some more than others. some are hard to see, others have a noticeable notch worn.
the best theory so far is that the guides were mistakenly made out of too soft of metal. even then you wouldn't think 300 miles would do one in. one thing the engine guy and i saw was that the valves in the open position, the rocker arm was crooked like it was in a sideways twist. not dramatic, but i thought this might be a clue. i know that the rockers can move a lot, yet nothing should be making them want to go sideways right?
 

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You wouldn't happen to have got a hold of a set of off-set rockers would ya?Or maybe the heads need an off set rocker and you don't have em?I don't know.Just a thought.
 

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Discussion Starter · #15 ·
okay, here's an update; took off a guideplate for the engine shop. they took a file and ran it accross and it easily grooved it. i did the same thing and it was easy to file. soooo, we're going to put on comp cams guides, and comp cams full roller rockers. i'll be keeping a close eye on them too.
the theory is that the arms were causing a little sideways pressure on the guide that probably would have not been a problem, but when combined with soft guides, was in fact a problem.
 

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what guideplates were they? are they ones that came with the pro-topline heads?
 
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