'68 Corvette - cause of push rod damage?

[Oliver S.]

A Corvette buddy bought a '68 L36 Corvette last year. The car has got a tri power installed and often stalls at idle. After the leaking tri power (don't know if original type or aftermarket) had been sealed - to no avail with respect to the stalling at idle - he removed the valve covers and found the push rods to be damaged.
He removed the camshaft, which appears to be a BB solid lifter cam based on the part #. The lifters are solid ones. So the initial hydraulic lifter engine had apparently been changed somewhere in the past.
What could be the cause of the damaged push rods?

Oliver

OH3.jpgOH2.jpgOH1.jpg

[Leif A.]

Re: '68 Corvette - cause of push rod damage?

Incorrect valve lash adjustment...over tightened.

[Gene M.]

Re: '68 Corvette - cause of push rod damage?

Oliver,
Rocker arms must be in bad shape too. Check for any broken valve springs.

[Steve G.]

Re: '68 Corvette - cause of push rod damage?

Wear. Once worn through the case hardening on the tip the rod will wear down pretty rapidly and severely. Re-assembling rockers and pushrods without ensuring that the pushrods are assembled to the rocker they were originally running with will accelerate the wear fairly dramatically. Just as l lifters not matched to their original camshaft at re-assembly will wear out the cam and lifters. Hopefully the lifters were marked when that camshaft was removed.

Steve

[Duke W.]

Re: '68 Corvette - cause of push rod damage?

You really haven't given much to go on. I can't read the data on the end of the cam, but it appears to be aftermarket. Tell use what is says and also what if any data is in the casting.

Tip wear can occur with excess valve spring force. Tell us about the valve springs.

Bent pushrods are usually due to overrevving or interference.

As stated, what is the condition of the rocker arms, and studs? High lift cams may need longer rocker arm slots to avoid binding.

There were several different big block push rods supplied by GM depending on engine configuration. What is the diameter on the pushrods and is there any ID information visible?

Duke

[Oliver S.]

Re: '68 Corvette - cause of push rod damage?

Initially, my buddy wanted to just install new push rods. I offered my help by asking a question regarding the cause in order to prevent the same thing happening again. For that I was provided with some pictures - but no additional information.
The number on the camshaft is 340282, which I deciphered based on this page:

http://www.nastyz28.com/bbcmenu.php

The cylinder heads could be the original ones - at least they bear the respective #.
During disassembly the parts were most probably not marked - hence no relationship/dependency maintained.

OH4.jpgOH5.jpgOH6.jpgOH7.jpg

Oliver

[Patrick B.]

Re: '68 Corvette - cause of push rod damage?

The heads look like original dated oval port heads for a 68 427 -390. The rocker arms are worn out and it looks like the cam is a GM solid lifter over-the-counter racing cam somewhat similar to an L-88 cam but not a production piece. What a horrible combination. What pistons are in it?

Now is the time to put a stock hydralic cam and stock spec valve springs, new rocker arms and push rods in it. Since it has closed chamber heads with small 100cc volume, it probably does not hot rod pistons, but you should check what it has.

[Duke W.]

Re: '68 Corvette - cause of push rod damage?

My recommendation for engine restorations is to remove the pushrods/rockers/balls, carefully inspect them, then bag them so they can be reinstalled as matched sets. If no wear is indicated, they will likely last for the life of the restoration. Production cam lobe dynamics are mild as are the OE valve springs, and especially on small blocks, the production valve train is essentially bullet proof. With careful attention to valve spring installed height, hydraulic lifter lobes will rev to 6500+ and mechanical lifter lobes to 7000+, and if the heads are properly massaged, the engine will usually make useable power to these valve train limiting speeds.

Wear is evidenced by galling, which can be seen and felt with the finger tips. If the wear surfaces are merely "burnished" the parts are perfectly acceptable for reuse as long as they are installed as matched sets, so the same wear surfaces mate as before the engine was disassembled.

It's tough to tell for sure from the photos, but it looks like the rockers have galling on the wear surfaces.

The fix is to install all new pushrods/rockers/balls and use assembly lube.

The camshaft is listed in the Chevrolet Power Manual as "short track competition". It's slightly milder than the L-88 cam, but it's still totally unsuitable for a road engine. It's designed with very high overlap for headers and open exhaust, and even modest exhaust back pressure from a road legal exhaust system, even with headers, will absolutely kill low end torque and fuel economy. Install the proper L-36 cam or nothing more aggressive than the production L-72 cam along with valve springs that meet OE spec if you want it to last and have broad torque bandwidth for responsive operation in normal road use.

This is another example of an "engine rebuild disaster" - improper disassembly/parts management and poor new parts selection. Hopefully it will serve as a lesson for others when they embark on an engine restoration.

Duke