Team Chevelle banner
1 - 7 of 7 Posts

·
Registered
Joined
·
1,007 Posts
Discussion Starter · #1 ·
i posted this in performance a couple days ago and someone suggested i post it here for all you engine builders. mine is puzzled over this problem. since this original post i've also noticed that when the valves are being opened the rocker arm looks like it wants to get sideways. kind of twist to one side or the other. the rocker arms are 1.6 comp cams roller tips which worked fine on my factory heads.

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 two 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.
 

·
Banned
Joined
·
10,053 Posts
If they are at all like the darts it could be the guideplates, Anytime you install heads i suggest installing them on the block as soon as you get them with checking springs, adjust two valves then run the locknuts down, Now remove the whole nut and do not move the lock nut then install it without the pushrod and take a look at the ARC the rocker travels through - Now install the pushrods, this will give you an idea if you need to cut and weld the guide plates or go buy the Isky adjustable guide plates
 

·
Registered
Joined
·
361 Posts
I hate to say this ,
But,
Proper Valve Geometry is a "Science" not a Given !
Anytime aftermarket stuff is used , everything needs to be checked !
Rick
 

·
Banned
Joined
·
10,053 Posts
I hate to say this ,
But,
Proper Valve Geometry is a "Science" not a Given !
Anytime aftermarket stuff is used , everything needs to be checked !
Rick
:yes: The process i listed above to see if the angle of the pushrod throws the ARC off we go through on each and every engine. I bet we spend around an hour setting up valve trains, Cam timing and Geometry along with spring pressures and heights
 

·
Registered
Joined
·
1,477 Posts
All heads have the guideplates and studs installed and torqued from the factory with no particular positioning. Once the heads are installed, the studs have to be loosened and the guideplate shifted side to side to get the best average rocker tip positioning for the rocker pair for that cylinder, then the studs torqued. (and sealed) I'm seeing a trend where the larger runner heads really should have shaft rockers with offset pushrod seats to move the intake pushrods away from the port walls. Using the supplied guideplates can put the roller tip off to one side of the valve stem, which imparts a load on the valve and guide which leads to accellerated wear. Sometimes 2 piece guideplates can fix this issue, but more often than not the intake pushrods need to be farther in the intake port than what has been machined out so you are forced to offset the rockers for the pushrod to have clearance. A full roller rocker deals with the "sweeping motion" on the valve tip much better than a roller tip due to the fulcrum and roller bearings forcing the roller tip to be at the same angle side to side as the valve tip. If the roller tip is offset on your current rocker arms, it wouldn't take much for the rocker to twist sideways, ultimately wearing your pushrods and guideplates, even to the point of failure. Take a very close look at where the roller tip is in relation to valve, and it's a good idea to turn the engine over (disable the ignition) and watch how much side to side movement the rockers have compared to the valves.

I have also heard of poor machining on some of the latest heads to come from Pro Topline before they went into recievership and consequently bought by Comp, so you could be dealing with the rocker studs threads being machined offset compared to the valve tips. (Hopefully not)
 

·
Registered
Joined
·
1,007 Posts
Discussion Starter · #6 ·
thanks for the replies. my guideplates are not adjustable, the holes are the same size as the studs. they looks right. i'm more thinking it's the rocker arms now after hearing replies. i was planning on going to full roller arms anyway. i'll keep you posted.
 
1 - 7 of 7 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top