Im sure this has been beat to a dead horse before but I have searched and read many threads on this topic but I would like your advice one more time. I have a 99 firehawk m6 with ls6 intake, headers, ory, slp lid tick slave and master, monster stage 2 clutch and a custom tune. I bent 3 pushrods doing a 3 to 2 shift and replaced with stock pushrods. I also installed a MGW shifter along with some blue ls6 gm performance parts springs. I am debating installing some Brian Tooley chromoly push rods. I have heard the debate about the weak link pushrod vs. changing a bent valve, but with the ls6 springs I thought it might close the valves in time to prevent an more valve float and I would be ok with the chromoly pushrods. ps. I don't plan on doing a cam any time soon. Thanks for any input.

 

I think push rods are under appreciated. If you're bending them, they're not strong enough.

The problem is that the lifters are pushing up on the push rod, and it requires so much energy to move the valves that the rods can't keep up. That's what's causing the push rods to bend. Any PTV contact strong enough to bend the rod, would certainly bend the valve.

The other issue is valve instability. before the rod bends permanently, it bends a little bit, forming a curve. You get less lift, as the lifter rises but the valve does not. The lifter reaches apex and starts to take pressure off the rod, causing momentary valve lash. Then the rod springs back and "pogo's" back into the cosing valve, sending a shock wave into the valve and springs.

Eventually, bad stuff starts to happen.

A good strong push rod will last forever, and stabilize your valve train. The trade off is slightly more weight. If you're racing and running 8,000+ rpm's, spend the big bux and get titanium.

 

Chromoly push rods, and a trunnion upgrade are a must for all LS motors run hard, so do both at the same. Tooley has them and he provides great service.

 

Those pushrods don't provide more stiffness, you need OD to get a stiffer pushrod.

If you are bending pushrods, this usually is the result of an over rev where the valve train control was lost. You may have already tagged the pistons if you bent the pushrods.

 

Thanks for all the input, I think I will give the chromoly pushrods a try. I am also going to upgrade to the BT trunions. I have a dealer receipt for a puked needle bearing form the first owner when the car has about 10,000 miles on it. Its been fine since but the peace of mind is worth it.

 

Good post Vettenuts. Agreed on all points. Wall thickness and different materials don't help push rod rigidity nearly as much as diameter. Material and wall thickness can contribute to a stronger push rod when used together, but the easiest and fastest way to a much stronger push rod is to simply make it larger in diameter.

When the push rod deflects, it creates lash as Bob mentioned. Push rod acts like a pogo stick when it straightens back out and launches the rocker off the push rod. This causes separation everywhere in the valve train as spring pressure is no longer acting upon the valve train. Even if the push rod doesn't stay bent and only deflects as mentioned above, it still causes valve lash to occur, yet only momentarily. If it bends and stays bent, you will have lash all the time which is even worse than if it deflects.

When lash is created in a running engine while using a hydraulic lifter it causes the lifter to pump up. However much the lifter is pre loaded is how much it will pump up, thus acting like a solid lifter momentarily until it bleeds back down once the lash is taken up and spring pressure pre loads the lifter again.

When it is acting like a solid lifter, it will hang the valve open from the lifter pre load being taken up because there is no spring pressure acting upon it due to valve lash. Obviously a certain length push rod gives us zero lash, but with a hydraulic lifter we use a longer push rod than what is needed for zero lash to pre load the lifter. Thus the push rod allowing the valve to hang open when the lifter pumps up.

If all of this occurs when the piston is at or near TDC with a fairly large cam that doesn't offer an optimal amount of P to V, you will have P to V contact certainly. Things happen incredibly fast at higher RPM, and even if it doesn't occur directly when the piston is at or near TDC, it can still tag a valve just because of how fast things occur in a running engine at high RPM.

 

Awesome post Martin! I probably have this discussion 15 times per day. Yet people still end up going against my recommendation and preload lifters .080"-.100". Of course, they call back wondering why they broke things with 5/16" pushrods after a missed shift