Pushrods and the wood inserts

[David Jones]

Nowhere have I found a definitive reason as to why Porsche used a wood insert in the pushrods and I have been thinking about it and I have a theory. Contrary to opinions I have seen on past posts I believe they actually improve oil flow to the rockers and valves but as this is my theory you are at liberty to disagree.
As the pushrod moves back and forth in the oil in the pushrod it acts as a pump as it moves contrary to pushrod motion by virtue of it's inertia. When the pushrod reverses direction at the top of it's motion the pushrod continues to move in the direction it was moving and forces oil out of the tip of the pushrod at a greater pressure than would otherwise be available from the now closed port in the cam follower. Even as the wood wears a point on it's end, it is the shoulder of the wood rod that exerts the pumping action and it does it before the rod blocks the oil hole as it travels to the end of the stroke.
Comments are welcome.

[Mike Wilson]

It makes sense to me but I have absolutely no background as an engineer of any type.

Mike

[John Hearn]

It is my belief they used the wood to reduce the volume of oil in the pushrod.
This would then reduce the weight (mass) of the moving pushrod.
My two pfennig.....
KTF
John

[Martin Benade]

Is this a Porsche-only detail, or was that a common practice at one time?

[Mark Sabbann]

I began my response to David's questions before I read John Hearn's response. I offer the following scenarios for evaluation and further input:
Scenario #1
The wood plug and the metal push rod are fixed together such that there is no relative movement between the two, but there is space for oil around the wood plug. So the wood plug has no ability to move the oil around it and the oil flow through the push rod is due to pressure differentials between the crankcase and the rocker box.
Scenario #2
The oil bath inside the push rod is thick enough that the wood plug never touches the metal push rod. The oil is the only thing in contact with the wood plug and the oil’s viscosity is what drives the movement of the wood plug via shear forces imparted by the inside wall of the push rod. So the oil moves the wood plug, not the wood plug moving the oil.
Scenario #3
The oil bath inside the push rod is thin enough that the wood plug end will contact the metal push rod end and the push rod movement will drive the wood plug movement. But when the cam is driving the push rod towards valve opening, the wood plug has to be jammed into the push rod at the cam end and that would close off the oil port at that end. When the valve spring is driving the push rod at the rocker end, the wood plug is jammed at the rocker end and that port is closed off. So it seems that the wood plug is always jammed against one end or the other of the push rod and one port or another is always closed. Without a place to go, through one port or the other, the oil won’t flow. However, if there is a minute amount of time between the cam power stroke and the valve spring power stroke where the wood plug is floating between the two ends of the push rod, both ports become open to allow oil movement.
Scenario #4
With an oil port at either end of the push rod, it seems pretty obvious that it is there to allow the rod to become a conduit for oil movement and the rod needs to be hollow to achieve that. In an effort to lighten the push rod to minimize inertial forces, keep it hollow to allow oil flow and keep it rigid, the wood plug is added. The wood plug is lighter than its corresponding volume of steel, even a hollow steel tube needed as a conduit and lowers the inertial required. It is lighter than its corresponding volume of oil which lowers the inertia of moving it. It is more rigid than its corresponding volume of oil which helps maintain rod rigidity. The wood is not stronger than the steel but it is a trade-off between strength and control of inertia. But Scenario #3 and plugged ports could still occur.

The unknown reason for the wood plug could be a combination of several things. The inertial issue is resolved with a wood plug. There could be positive displacement (fluid transfer with minimal pressure differential) of oil due to the mechanics of the shoulder of the wood plug. There could be pressurized displacement of oil due to the differences between crankcase pressure and less pressure at the rocker tip (approaching atmospheric pressure in a vented valve cover).

[Mike Ruddy]

The wooden pushrod insert for a 36hp VW engine had notches at each end. Maybe these cast some light on the insert's purpose.

[Harlan Halsey]

356 Carreras also have wooden inserts in the oil passages in the rotating parts. Most likely this was done to reduce the volume so as to get the oil where it needs to go more rapidly on start up.

[Jay Darlington]

what I would do is to remove the wood. I just did mine and what I found was one push rod plugged at one end with a piece of the wood. after many hours on an engine these wood pieces form a long tapered point that can break off and plug the hole in the end of the rod. I also found that using a gun cleaning wire brush that the inside had quite a lot of sediment inside.
Jay D.

[David Jones]

I do not buy the idea that the wood insert is there to reduce the time for oil to fill the tubes on start up as that assumes the oil runs out after shut down. It is not as though it is an inline four with vertical pushrods. Some oil will run out but most will stay in the horizontal pushrod as I can attest to when dismantling an engine that has been running days or even weeks before, that is the time when it runs out on one's leg as one removes it from the engine.

[Dick Weiss]

The wood plugs were always in the straight steel pushrods--never in the short aluminum-long steel pushrods in the Supers and some later SCs.
I don't recall if the wood was ever in the long-straight aluminum pushrods.

Dick

[James Davies]

If the wood is moving back and forth inside the pushrod, that means the ends of the wood are worn off and are not as originally manufactured. They should fit tight and have a notch in the center so that the hole at either end of the pushrod does not get plugged. See Mike Ruddy's photo above.

[Mike Ruddy]

Looking through a dusty bookcase in my workshop earlier today and I came across my copy of Upfixin, volumes one thru five, '56 to '60.

I've just perched some spectacles on my nose and sat down for the evening to re acquaint myself with a book I haven't looked through for a long time.

The prose has a certain style, enthusiastic and very much of it's time but then at the same time incredibly technical. These guys were were working on and writing articles about what were pretty new cars in those days and they knew them inside out.

On page 37, Dick Ferguson of Lincoln, Nebraska disassembles his pushrods and finds there to be wood inside! The Editor remarks "The wood provides a pumping action and helps cool the oil"

As near a contemporary explanation as you could ask for ?

[David Jones]

Thanks Mike, I feel a little vindicated as I surmised that the wood would provide a pumping action in my original post where I was looking for confirmation or contradiction on the intent of the pushrods. Nice to know the logic and engineering part of my brain still functions to some extent.

[Wes Bender]

From a purely physics point, I would think that the "pumping action" would have to come from inertia and I'm not so certain that the weight of the oil wouldn't provide as much "push" as the wood does. It would be interesting to know what the Porsche engineers were thinking when they came up with the wood insert idea.

[David Jones]

Wes my contention was that the end of the wood insert would act as a primitive check valve. In my first post I spoke of the wood rod blocking the oil from running back out of the bottom of the pushrod because the wood rod has by inertia stayed seated as the pushrod travelled away from the cam lobe in the lifter. Likewise as it reverses direction it now stops the oil from flowing out of the tip as is forced back down toward the cam lobe.