Teach me about push rods

Eric85629

Eric85629

Member
AKRA jr1, 1/5 mile dusty momentum track
Green plate ducar 196, cl-2 cam, basically stock as of now

I'm finally getting the chance to build up my first engines for my kids & I'm hoping to learn about push rods (and anything else people are willing to share).
I was making a list of all the light weight components I figured would help the engine then on Dynocams I found a 158 head with 3 angle valve job, short valves & all the light weight stuff I was going to buy anyways. With a head like that, would shorter push rods be a good addition? Are shorter push rods for a different scenario? I'm assuming every gram of weight removed from the valve train is going to help a plate engine. Any input you can share would be great.
 
Correct pushrod length is about having correct geometry for the whole valvetrain.

The rocker contact to the valve should start at valve closed off center of valve stem. It should migrate across the valve stem and be the same amount off center the opposite side at valve max open.
This keeps the thrust sideways against the valve guides somewhat equal.

Experience will teach you which side to be off if you cannot obtain perfect geometry.

You can sacrifice valve guide wear for maybe some lift, or profile gains.

Longevity may suffer.

The amount of weight difference will be near negligible for the range of usable pushrods.
Especially in the grand scheme of things.

Jmho
 
95 shaw said:
Correct pushrod length is about having correct geometry for the whole valvetrain.

The rocker contact to the valve should start at valve closed off center of valve stem. It should migrate across the valve stem and be the same amount off center the opposite side at valve max open.
This keeps the thrust sideways against the valve guides somewhat equal.

Experience will teach you which side to be off if you cannot obtain perfect geometry.

You can sacrifice valve guide wear for maybe some lift, or profile gains.

Longevity may suffer.

The amount of weight difference will be near negligible for the range of usable pushrods.
Especially in the grand scheme of things.

Jmho
Click to expand...
Thank you, that's some great knowledge to share and a detail I would guess a lot of people over look. I will have to look how my current engines move and pay attention to the new build.
 
95 shaw said:
Correct pushrod length is about having correct geometry for the whole valvetrain.
Click to expand...
I like the logic behind your statement. Is there a way to judge if I need a different length without buying numbers pushrods & trying them?

I was told that shorter push rods are only used when valve clearance is an issue. Is that true? Maybe I'm missing something but I would assume a metal rod transferring cam movement to a valve would move the valve the same distance wether the push rod is stock or shorter.
 
The geometry as described above is the way to determine.

The pushrods can be measured with calipers to determine the length you have.

Shorter pushrods are needed if the head has been planed a substantial amount.

Pushrod length does not affect valve lift or duration.

At least not in a substantial amount.
 
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