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Do pushrods generally bend in the middle or near the lifter end?

General engine tech -- Drag Racing to Circle Track

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BradH
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Do pushrods generally bend in the middle or near the lifter end?

Post by BradH » Tue Oct 08, 2019 12:55 pm

The papers I've read on column theory indicate that a pushrod should yield in the middle when it reaches its limit. However, I've also seen failed pushrods where the bending occurs much closer to the lifter end where the load initiates.

Which is more typical from what you've seen? My working assumption is this scenario is where the pushrod material's capacity is exceeded, not where there's an interference issue that skews the loading, FWIW. Thanks - Brad

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Re: Do pushrods generally bend in the middle or near the lifter end?

Post by PRH » Tue Oct 08, 2019 12:59 pm

Looking at witness marks left on the heads where pushrods that clearly weren’t up to the task had been used......... it seems they bend the most near the middle.

I’ve seen where there was hard contact from the pushrod banging into the head in an area that had about 1/4” observed clearance when running the pushrod through its cycle by turning the motor over.
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Re: Do pushrods generally bend in the middle or near the lifter end?

Post by PackardV8 » Tue Oct 08, 2019 2:34 pm

If no interference prevented it, pushrods would normally bend in the middle. However, there are guide plates, head slots, lifter and rocker operating angles, cam lobe profiles, spring loads, adding up to a multivariable regression analysis. They bend where they can when the greatest forces are applied.
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Re: Do pushrods generally bend in the middle or near the lifter end?

Post by Rick! » Tue Oct 08, 2019 3:51 pm

Get several pop cans.
Step on one and then doink it with your finger.
Repeats steps one and two by poking the can at a different height on the can.
It will buckle pretty close to where the eccentric load is applied.
A pushrod that doesn't see eccentric loading "usually" buckles in or near the middle - Euler buckling estimates the load but snap-over is where the formula breaks down. Pure buckling can be induced but a lot of times an eccentric force helps things along.
Pushrods that intermittently contact head gaskets, heads, intakes or whatever may tend to buckle near those points.
A good piece of glass will help sort out pushrods pre-disposed to buckle.

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Re: Do pushrods generally bend in the middle or near the lifter end?

Post by Dave Koehler » Tue Oct 08, 2019 5:41 pm

Can doinking. Gotta remember that one.
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Re: Do pushrods generally bend in the middle or near the lifter end?

Post by strokersix » Tue Oct 08, 2019 6:25 pm

Pushrod distributed mass means that pushrod stress at the lifter end will be greater than pushrod stress at the rocker end at valve opening acceleration. In other words, the lifter end has to accelerate the pushrod but the rocker end does not. Significant or not, I have no idea.

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Re: Do pushrods generally bend in the middle or near the lifter end?

Post by cjperformance » Tue Oct 08, 2019 7:07 pm

Most of the bent pushrods i have seen on canted valve engines where the pushrods are not inline with the lifter bore are bent near the lifter, maybe 1/4 - 1/3 the way up the pushrod from the lifter end.
Yet on engines where the pushrod is inline with lifter bore its usually bent half way up or closer to the rocker.
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Re: Do pushrods generally bend in the middle or near the lifter end?

Post by Kurshuk » Tue Oct 08, 2019 8:25 pm

The middle third of the pushrod is where all of the bent or broken pushrods have failed on both sbc and bbc that I have experienced. Assuming as others have said... with no interference issues.

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Re: Do pushrods generally bend in the middle or near the lifter end?

Post by engineguyBill » Wed Oct 09, 2019 1:51 am

Generally speaking, most pushrods tend to flex in the middle of their length and will bend in this area unless they are influenced by some sort of interference in the top or bottom of said length.
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Re: Do pushrods generally bend in the middle or near the lifter end?

Post by BradH » Wed Oct 09, 2019 8:44 am

Good replies; thanks - Brad

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Re: Do pushrods generally bend in the middle or near the lifter end?

Post by Truckedup » Wed Oct 09, 2019 9:18 am

Back in 1972 I had a 69 Chevelle with the 350 HP 396...It would on occasion bend a pushrod right under the rocker arm, I believe these engines had guide plates ? Seemed to be no consitant reason the the problem .I never found the cause because I was dumber than I am now...
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