TonyGXP

It is. great explanation man, thanks.

Mez

iTrader: (1)

All points raised are, in my opinion, very well defined. My thoughts were focused on the geometry shift caused by the loose bearings as intentional design element. But you raised new issues that I had not thought about. Setting aside for the moment the risk of rollers escaping, the shift in geometry because of the loose bearing tolerance was my primary point of this thread.

If the rockers have to deal with a big lift cam, the issues you raised become more important and, of course, more negative.

On my car, I switched to the Yella Terra 1.7 non-adjustable rockers because the GT-11 cam has lift of .631" / .644" lift. The stock rocker tips showed wear that they were going over the edge of the valve tip which can't be good for the valve guides. With the full rollers, the wipe area stays very close to the center.

But the Yella Terras have a bad habit of breaking. I had one break last week after 2 years and 15,000 miles. The cause, I believe, was excessive spring pressure from hitting the fuel cut-off and not lifting off the throttle instantly. This results nasty harmonics in the valve train and valve float on my 26,000 springs which then causes a spike up in open spring pressure and breaking the rocker.

New springs and YT rockers are going back on.

victorf

iTrader: (1)

Thanks for your response Mez and sorry on your YT breaking!

Brian Tooley Racing

iTrader: (13)

Stock rockers were never designed to stay centered on the valve tip, the contact patch moves from one side of the valve tip all the way to the other. The action is much like two gears mesh, or how a rocking chair would rock on a floor. This is called "half arc" geometry and is critical to making a stock rocker function properly. A roller rocker is designed to work in a "full arc" which minimizes the sweep by raising the trunnion height.

As lift is increased over stock, it forces the stock rocker tip to start working in the "other half" of the arc. When this occurs the rocker tip is dragging across the top of the valve, which the stock rocker was never designed to do. The vector force in the drawing is actually backwards, as the dragging occurs towards the intake side of the cylinder head, not the exhaust. Once again this is due to the rocker working in "full arc" which it was never designed to do.

The worst thing you can do to valve tip durability is to shim the stock rocker arms. Although this increases rocker ratio, due to the contact patch moving away from the trunnion, it forces the rocker to move more in a full arc motion, which increases the dragging of the rocker tip across the valve tip, thereby wearing guides and damaging valve tips.

The reason TFS recommends roller rockers is because their heads, like most aftermarket heads, have bronze guides. Stock rockers will wear out the bronze guides in a pair of heads in 10k-20k miles. As the previous owner of TEA, I can tell you that the percentage of heads that came into my shop with excessive wear on bronze guides using stock rocker arms was 100%. That includes AFR heads, Dart heads, and even GM heads retrofitted with bronze guides.

The key to getting stock PM guides and stock rockers/valve tips to live requires valve lift below .630" and open spring pressure less than 400 lbs. As lift increases beyond this threshold, or spring pressure increases beyond 400 lbs, wear increases exponentially. Excessive open spring pressure will damage valve tips quicker than excessive lift will.