11/32 .120 with restrictors in my road race engine (manton). Cost me 220 for the set. Well worth the investment if you plan to spend any time at high rpm, especially sustained.

Pictured next to a 5/16 regular magnum pr for reference.

 

tick sells a set that are 5/16 but that have a .105" wall thickness instead of the regular .080" bit more expensive but worth the $ IMO, they are made by trend

 

What length accuracy do they specify on their website? I took a look but couldn't find it.

 

Another vote for manton here

 

Heads are off? Why not go 3/8" and be done with it.

Im for going overboard on valvetrain. Stability is one thing I dont have to worry about.

 

3/8 on stock heads and some aftermarket requires cutting the pr hole normally. 11/32 does not.

 

True, but it took me less than a hour to setup my AI 243's to accept 3/8" rods.

 

Most people doing swaps dont have that capability or know how though.

 

7/16 in my engine

 

Whoa Hammer! What did you do, just order out some power poles and turn them down a bit?

 

I'd heard it was baseball bats

 

LOL. My bad! I knew there was a more sensible approach....

 

Im guessing hammers heads only screamed a little when he put em in....

 

Only the first time, though....

 

since I am not an engineer....how does thicker wall compare to bigger O.D on a push rod....equal...better...worse ???

 

That is really a whole other argument. It doesnt just depend on the pushrod though It is valvetrain stability as a whole.
There are a few tests showing 3/8 .080 have less flex than 5/16 .120 wall then there are tests showing the opposite It also depends on length as well as material used, and any taper, as well as valvespring pressures/control.
It gets pretty deep not in just what is learned but what is argued.
For 99% of builds 5/16 080 chrome moly will work. for the rest 11/32 080 or 120. A small percentage would need anything over that. Even so they have to have a small ball end on them for both rockers and pushrod cups.

 

Last time I checked, all pushrods were of steel alloy. Therefore, material has no bearing on the stiffness of the pushrod since all steel pushrods will have the same modulus of elasticity.

Any tests showing that a 5/16 0.120 wall is stiffer than a 3/8 0.080 wall pushrod of the same length is incorrect. The 3/8" pushrod will be 85% stiffer. Length will have an effect on stiffness, however assuming that one is comparing pushrods for the same motor then the same length would be required.

With aftermarket springs, the 5/16" will flex at high RPM and I would recommend the largest OD pushrod that will fit. Double taper pushrods can fit where a straight pushrod may not, however they tend to cost more.

 

The only way to gain a substantial increase in bending stiffness is to increase the OD. Increase the wall thickness will only change the stiffness by a few percent, i.e., the gain is marginal.
Using hardened pushrods makes no difference at all with regards to stiffness.

Read this >>>>>>>> An Investigation Into Pushrod Stiffness

Note that one of the members from Sweden did some testing and his results and my analytical results were within 1%. Also, I have had several engine builders contact me after I posted that thread and tell me they find the same type of results on the dyno.

 

What orifice size did you use on the restrictors?

 

Pushrods are tricky, because they are a compression load, and steel is strongest in tensile loads. Increasing the wall thickness from .080 to .105 on a 5/16 pushrod only changes the stiffness by approx. 17%. This is based on mass and area. Going to 11/32 x .105 increases stiffness by 110%. So when you go to higher spring rates and higher ratio rockers, the bigger pushrods make complete sense

Very true. Plenty of people overspring their valvetrain instead of providing the amount of spring needed and nothing more. But, having oversprung your valvetrain, I see no other option than to make the pushrods stiffer to compensate.
There are some aluminum pushrods out there, but I'd never use them. But, yeah, you'd assume in most testing the material was constant. Typically, they are 4140, which is a good all around steel alloy. And the modulus is why you can use cross sectional area to approximate strength quite well. Modulus is unitless.

Length does affect stiffness, but most of ours are ~7.4". Certainly as you go longer, increasing OD makes even more sense.

Yup, spring rate, spring rate, spring rate. Not only that, but multiply your spring rate by the rocker ratio. Oh, and on the power stroke when you're opening the exhaust valve, add the cylinder pressure times exhaust valve area. Easy to get well past 2000 lbs being supported by a pushrod.

YES! Because when you incrementally increase wall thickness, you are adding to the INSIDE diameter, not the OD. The percent area change is rather small. However, making the cylinder bigger adds a LOT of material, because you're adding to the OUTSIDE diameter. That's geometry.

And hardened rods do NOT change the modulus. They only make the rods stay on the modulus longer. In other words, the yield strength is higher, but the modulus of elasticity is exactly the same.