Bent/Broken Push Rods

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whitebetsie
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Bent/Broken Push Rods

Post by whitebetsie » Mon Sep 11, 2017 10:08 pm

Hello everyone, we have recently overhauled the engine of a 1594, including boring out cylinder & inserting oversized pistons etc, completly rebuilding the heads with new valves, springs and guides etc. The engine was reassembled and started. It ran perfectly for a while but when it was brought out on the road and reved to about 2300rpm, disaster struck. For some reason 3 push rods broke and 3 more bent ( all on the inlet valves). We are sure the valve clearances were set right but cannot think of anything else that may be out of place. The head was rebuilt by an engine specialist who has worked on these engines before. Has anybody come across this before? Any suggestions would be appreciated.... thanks in advance

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ollek
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Re: Bent/Broken Push Rods

Post by ollek » Tue Sep 12, 2017 6:16 am

The push rods must have bent and broken because the valves did not move (too tight in the guides) or the timing gears are timed incorrectly. One more possibility is that the pistons are turned 180 degrees to the correct position. The valves will touch the pistons if the dents in the pistons are not right below the valves.

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whitebetsie
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Re: Bent/Broken Push Rods

Post by whitebetsie » Tue Sep 12, 2017 7:21 am

They are the possible problems we thought it could be but we timed the gears using the factory markings and also the tractor started and ran for a while after the rebuild before the valves broke. We checked the valves and they move quite freely (unless they tighten up when the engine warms up?). Brass guides were used after the holes were bored out. We are gonna take off the heads again and check the pistons but we took photographs with a phone when we put in new pistons and they look to be positioned correctly. Thanks for the suggestions!! Much appreciated.

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ollek
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Re: Bent/Broken Push Rods

Post by ollek » Tue Sep 12, 2017 7:39 am

Brass is expanding almost 50 per cent more than steel, when the temperature is rising. The original guides are therefor made of steel, not brass.

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whitebetsie
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Re: Bent/Broken Push Rods

Post by whitebetsie » Tue Sep 12, 2017 8:14 am

I was thinking the brass might be the culprit,, there was no guides in the head initially just a reamed hole for them. The company we got to build the head bored an oversize hole and fit the brass guides.. I guess they will have to be replaced. Have you come across that issue before ollek?

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ollek
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Re: Bent/Broken Push Rods

Post by ollek » Tue Sep 12, 2017 8:33 am

I have never heard about anyone who have fitted brass guides, but I do know that brass expands more than steel.

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skyrydr2
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Re: Bent/Broken Push Rods

Post by skyrydr2 » Tue Sep 12, 2017 9:13 am

It is standard practice to use bronze bushings for the valve giudes.
Were the pistons aftermarket? And piston pin hieght correct? Rocker arms on correctly? There are 6 lefts and 6 rights? Bushings in rockers good?
Intake valves incorrect diameter thus protruding ever so slightly into combustion chamber and then getting whacked at rpm from pistons?

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ollek
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Re: Bent/Broken Push Rods

Post by ollek » Tue Sep 12, 2017 10:04 am

Steel bushings are used where I live as bronze wears faster than steel, but US is different to Europe in many other aspects as well.

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db2d
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Re: Bent/Broken Push Rods

Post by db2d » Tue Sep 12, 2017 11:09 am

Bronze is not the standard material for valve guides in this country and certainly not in David Brown engines. The inlet valves run dry without any lubrication.
Brass has a tendency to ' pickup ' if not lubricated

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whitebetsie
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Re: Bent/Broken Push Rods

Post by whitebetsie » Tue Sep 12, 2017 2:17 pm

Thanks for all the replies,, we bought all the replacement parts from Enmark and we double checked all dimensions against the old parts before fitting them. Everything seemed to match up perfectly. The only other part that was fitted was the brass guides that were supplied by the company that rebuilt the head . I know that brass expands easily with heat and that the valve could pick up very easily. Apart from the guides there is slight wear on the underside of the rocker arm where it is in contact with the valves but it's only very slight and only on the exhaust valves (the push rods for the exhaust valves didn't bend at all). As I said initially the engine started and ran fine for some time on low revs. It was only until it was driven on the road at higher revs that the damage happened,, I think the brass guides need to be switched for steel ones even if it's not the cause of the problem here..

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ollek
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Re: Bent/Broken Push Rods

Post by ollek » Wed Sep 13, 2017 7:21 am

The push rods will get bent or brake it the valves do not move because the valve stems are seizing in the guides or the valves hit the pistons.

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rid54
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Re: Bent/Broken Push Rods

Post by rid54 » Wed Sep 13, 2017 8:14 am

This is intriguing. I'd place my bet on the valve stem guides being the culprits. If timing were off and caused interference between pistons and valves, that should manifest itself irrespective of engine speed, I think. Are there any scars/marks on piston crowns or valves that suggest that there has been contact?

I was thinking, that if the valves move sluggishly (perhaps because of the clearance in the hot brass bushes being too little), there will be an unnatural looseness in the valve train if the valve doesn't follow the mechanism as the cam lobe recedes during the closing phase (valve spring normally keeps the mechanism tight). Some engines I have worked on will allow a pushrod to move sideways under those loose conditions and "unseat" the joint between rocker arm and pushrod (ball/seat). If the joint will not reseat itself, during the next opening phase the valve will open far too much and might hit the piston. I am by no means sure about this engine having the mechanical layout to allow this to happen, it's just a thought. The remedy would be to replace the bushes for steel ones, to ensure smooth valve movement at all temperatures.

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GeoffDEAL
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Re: Bent/Broken Push Rods

Post by GeoffDEAL » Mon Sep 25, 2017 12:22 am

Hi if its the guides it may be the type of brass bronze guide material used, bronze guides are commonly used on race motors to reduce wear and improve life of the guides. these motors run 7 to 8000 + rpm have run them myself and are better than cast iron guides. So there is some problem with the material used or clearance if its the guides.

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GeoffDEAL
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Re: Bent/Broken Push Rods

Post by GeoffDEAL » Mon Sep 25, 2017 2:52 am

Hi again phosphorous bronze is the material to use will stand small clearances and wear well, are you sure enough oil is getting to guides and not brass should be bronze as described. cheers Geoff

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ollek
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Re: Bent/Broken Push Rods

Post by ollek » Mon Sep 25, 2017 6:56 am

I don't know about race engines, running on petrol, nitro methane or methanol, but David Brown diesel engines are a different story. The valves in a DB engine are originally designed to run dry. Older DB engines even had seals on the top of the inlet valve guides to stop oil from getting into the combustion chambers. And furthermore, David Brown never used anything else but steel or cast steel for valve guides.

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