Vette @ 71

Issue #6, improperly located rocker pedestal (also called geometry problem) IIRC was found on about 50% of the heads that poster John-g had either personally measured or others had using his tool which was designed to measure it.

Unfortunately, with all the available data, we have, most of it is either on # 4 or # 6 not both on the same head.

Need to go back and read Hib's experience with a new set of heads he received from GM where he found numerous indications of poor machining..

dcamick

Excellent!

Mark2009

Not really. The results of those test were inconclusive (it is possible the bad geometry was the result of worn guides, not vice versa).

383

Is that tool (The "John-g block" for measuring the pedestals.) available to the public?

Z06Ronald

Excellent Michael! If even I as a layman can understand your explanation everybody else can.

Michael_D

Thanks to everyone for keeping things friendly.

If you were directing this to me, I'd prefer to not give my "opinion", as I'm trying to keep my comments to facts, personal experience and theory.

So in having said that, we have seen brand new heads where the seats are not concentric to the guide (within recommended tolerances), and it has been on both the exhaust and intake.

We have also seen, through owner experiences and pictures, rocker scrub to not be optimum, in some circumstances. - most often with aftermarket, higher lift cams.

The first data point strongly suggests that seat to guide concentricity is not within specs, in some cylinder heads leaving Linanar. Why? I cannot say, as I do not have first hand knowledge of Linamar's process. Maybe Hib, with his contacts could find that information, but I do not have any contacts within Linamar. Mike Chapman would also be a good person to reach out to.

As to the second data point, I have another thread describing exactly what a person should do. It's a really simple and cheap test. All it takes is a sharpie, and a couple washers stacking it an OEM lifter to find out if you need to correct a Geo problem.

Michael_D

Great, thanks for the feedback. I have trouble finding the happy medium between too much / not enough info on this site.

juanvaldez

Can you point me to that other thread?

Vette @ 71

I would agree that the cause of the worn guides based on his work is inconclusive but the geometry results were what they were. The chicken and egg analogy. But are you suggesting that worn guides, however they wore , could result in bad geometry? How likely is that?

Vette @ 71

Shoot him a pm , likely he is still interested in gathering more data..

Mark2009

Yes. Say the guide wears oblong perpendicular to the cam, the valve stem lays over towards the exhaust port (perhaps adapting to its originally crookedly-machined position), and presto! - bad geometry and developing oblong wear pattern on rocker.

Vette @ 71

Sounds like the failure scenario you are describing begins with guides in a crookedly-machined position, add valve cycling motion which initially initiates some guide wear and which if allowed to continue will eventually result in bad geometry and (oblong wear pattern on rocker) and additional guide wear. 1 separate initial condition resulting in an additional compounding condition accelerating guide wear and eventual valve failure if not corrected. Likelihood?

Mark2009

As likely as any of perhaps half a dozen scenarios (not all of which are routinely discussed).

383

I searched for "john-g" and got nothing, whats his user name?

ctsv510

john_g_46

Michael_D

Thread on high lift cams:

https://www.corvetteforum.com/forums...-max-lift.html

If you want to check rocker scrub, the most accurate way to do that will require the heads to be removed (or done during head install). They will need to come off so you can install solid lifters, or modify a set of hydraulic lifters. - Install the solid (or modified) lifters. Reinstall the head, with gasket, snug it down and drop in your pushrods. Then coat the valve stem tips of both int/exh of one cylinder. Install the rockers, taking care to not screw up your sharpie paint job. Then roll the engine over a few times. Remove rocker, inspect pattern. Pattern should remain within the middle 1/3 of the tip. Once it moves outside that 1/3 window, you will start to have problems.

Normally, this is checked by a competent engine builder when the engine is being put back together. I will check rocker GEO at the same time I check PTV-contact.

Some folks advocate "mid-lift" rocker contact. That works too, but I'll still check the entire movement of the rocker by rolling the engine as described above. Mid-lift checks do not tell the whole story.

MTIRC6Z

On the topic of mid-lift geometry...I have a sample size of one which indicates that even when mid-lift geometry is very bad (geometry was initially set-up for a .480 lift cam and guide wear test was done after running 50,000 miles with a .610" lift cam) there was no truly measurable guide wear which resulted from being so 'out-of-spec'

Now with that said, the guides in my sample size of ONE were OEM PM and obviously my sample was not a LS7 head (ie. it was a LSA/LS3 head).

This experience might lead one to conclude that bad rocker geometry is likely to be a far more significant contributor to guide wear than bad mid-lift geometry. Personally I'd be far more concerned about getting good rocker geometry and would not consider a pair of heads 'fixed' without verifying the rocker geometry was good.

I know of two examples where any concentricity issue there might have been was taken care of with proper machining and yet the guides again wore out prematurely thus revealing the heads also had a rocker geometry issue. Of course I am making an assumption here in believing that bad rocker geometry was the cause of the second set of worn out guides...and for all we know even the first set of worn out guides might have had nothing to do with bad concentricity.

Cheers, Paul.

Undy

Couldn't you measure the contact pattern with an adjustable pushrod checker?

1) Just measure the amount of additional compression required to bottom out the lifter with your standard pushrod installed.

2) Add that amount of compression to the length of your pushrod.

3) Adjust your pushrod length checker to the new total length.

4) Install your adjustable pushrod checker.

5) Check your contact patch.

I would think that your geometry would remain the same as with your regular pushrod. The only 'possible' problem I can see would be deflection of the pushrod checker under load.

Michael_D

Hu.... I never thought to do it that way Undy. Good idea! I think it would work, as long as you are patient, and give the lifter time to bleed down. Would it be as accurate....maybe. You could also pull the springs, install some light weight checker springs.

Undy

If you're running dual springs, just install the inner spring.

I've got Comp Cam's pushrod length checker and I'm running dual springs. So... the next time I do a wiggle test I'll give it a go.

I'll mount a dial indicator on top of the rocker, above the pushrod and lift the rocker up, off the valve stem. I've got a bronze (non-sparking) screwdriver that I can file a sharp edge on the blade. I can wedge it between the rocker and the valve stem end then lift up. That should give me the amount of remaining lifter plunger travel.