Broken pushrods

Saddletramp

Saddletramp

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The pushrods broke on my Volvo MD2B.

It has been suggested that water in the bores was the culprit. I could see no evidence of head gaskets gone. I fitted a water trap and high swan neck when the engine was fitted. Neither of which were present on the old engine. Could it be that if the swan neck is too high that it is easier for water to flow backwards. Though the baffles in the trap should stop this.

That is assuming it is water in the bores. Could it have been lack of oil to the top end? Want to identify the cause before I put it all back together again.

Also does anyone have the size of the large nut on the end of the crankshaft.
 
If it's water in the bores, can come in two ways, through the valves or from the water jacket. If it's come in via the valves, then it must have come from the exhaust or through the inlet (unlikely) then you need to look to your exhaust, if it's from the waterjacket, then it has to be either cracks or gasket. Make your choice.
 
Could be water in the bores or a sticking valve allowing the collet under the rocker to move out of location and so cause the valve and piston to collide.

The water in the cylinder can come from a damaged head gasket and sometimes this can be confirmed by removing the injectors and cranking the engine over and seeing if any water comes out of the cylinder and or traces of moisture on the injector nipple.
 
No piston damage which is what I was expecting as they only have .5mm clearance. It could have come in through the exhaust. Hence my question about swan necks.
 
I cannot see how water in the bores will lead to broken push rods. Bent con rods maybe but not push rods.

It can only have happened if a valve was stuck closed (or partly so) or if it was prevented from opening or if a rocker was seized on its shaft. A valve stem could have suffered corrosion I suppose if water got in to it via the exhaust system. A rocker is only likely to have seized if the oil supply failed (and I have known and had to repair a broken oil feed pipe) That could also lead to a valve stem seizing.

You say push rods, plural, which ones. Both on the same cylinder or both exhaust or which?

What other things are obviously wrong.

I do not know the size of the nut on the crank shaft maybe it is in the w/s manual
(That's on the other computer I'll switch it on) But you can measure it.

Frankly it is so tight (I'll tell you how tight in a moment) that unless you have a couple of angry gorillas and a scaffold pole you don't stand much chance of undoing it. You wont need to unless you have to remove the crank shaft. Big ends can be done accessed via the side covers if you need to get the pistons and con rods out.


Cannot see a size for the nut but it is done up to 500 Nm or 369 lbs ft. There is also a special puller to get the flywheel off.
That's an MD 11 but I doubt if the MD 2 is much different.
 
I assume "swan neck" means a loop up just before the transom exit. i.e between the water trap and the outside air.

This only stops water from waves from getting into the engine. For practical purposes it does not matter how high. If it is too low, it is a problem.

With water in cylinders is more normal for head gaskets to go in this condition. Last time I saw a push rod go was because of a bolt that had fallen through the injector port (during maintenance). The engine started but then the bolt moved. I assume everything still wiggles and is not seized with respect to the valves.

If you did have water in the cylinders then you should be able to tell from the engine oil. Drain it and get it in a glass jar. The trap would have also been overfull when you disassembled the engine.

The most likely cause of water is via the water injection point just after the manifold. This small pipe must be sited and vented properly or siphoning happens. Once the siphoning starts it normally can not stop until the exhaust and cylinders are filled.

In my experience water due to siphoning is due to:
1. Fitting a dripless stern seal to some manufacturers recommendations
2. Not having a siphon break but overloading a yacht for a long passage, or with a large crew in cockpit, to the point that the siphoning occurs. Water injection point must be above the sea level at all possible yacht positions/loadings
3. Not maintaining the siphoned break non return valve if fitted. (Best to remove them and use a small high vent)

In your case I can not tell without a picture or more information. There is always bad camshaft timing. Was the engine being started or was it running for some time when the problem occurred? Was it idling or at high revs?
 
Our old MD2b died when the head went. Water had etched a gap between the waterjacket and the cylinder.

We tried - as a get you home measure - to fill the gap with red Hermetite but it didn't last very long before water started seeping in again.

It happened on the front cylinder and when the cylinder head gasket was removed and you ran a finger round the cylinder you could feel a slight 'dip' in the metal and evidence of rust.

Engine was nearly 30 years old right enough.....

Donald
 
carbon buildup in the valve stem(s) can cause the valves to sieze so bending /breaking pushrods and cam followers ,had it happen to me on a lister,also had it happen on a generater.
can't see how water in the bores would do it as has been said that would lock the engine up , did it happen while the engine was running or after you had tried/started it.
 
I agree with Vic - water in the bores can lead to a number of things but I couldnt see it leading to broken pushrods. That has to be the result of some valve train problem - like the timing slipped and a valve tried to open with the piston at tdc. Are there any witness marks on the pistons? Are the rockers seized?

Sounds like a strip down job top me, unless there is something fairly obvious.
 
Both inlet and exhaust pushrods are bent or broken on the front cylinder. The engine had running at about 1500rpm for about 10-15 mins. I had previously changed the oil and though I filled it to the recommended amount. It was near the end of the dipstick. I thought there may have been oil starvation due to the angle of the engine. This was the first oil change after fitting the engine. It had done about 20 hours. I can not see any evidence of damage on the piston or the valve which you you would expect from a collision. Oil pressure was 40psi. All appeared to be free. Valves where not stuck in stems though a bit black.

The reason I need the nut size is to buy a socket to turn it. One of the pistons has seized where air has got in through the exhaust manifold. I have now got both heads off and filled the cylinder with diesel and oil.
 
This "seized " piston, is it no. 1 and is it at tha top or bottom of the bore?
I,ve never seen broken push rods, rockers yes but never broken rods, very odd.

Ted.
 
The siezed piston is no2. I took no1 head off. Oiled it, that is okay.
Because the exhaust manifold was off no2 is rusty, at the bottom of its stroke. Whole bore has light rust.
 
If the oil pressure was 40 psi then it is not likely to have been oil starvation (impressed though that you have a gauge). Assuming the dipstick is the same as the one on the MD 11C it must be screwed in properly when running with good seal at the top as it is part of the oil pump suction. Also the coarse strainer. The level is checked without screwing it down. Always fill to the level on the dipstick rather than by volume added and check/top up again after a short run to fill the oil filter

Picking up on Orizaba's point could the rods have become disengaged from the rockers due to being incorrectly set or other reason? Would that even have led to the rods becoming bent or broken. The oil feed to the front rocker shaft OK as far as you can tell?

Surprised about the no 2 piston and the rusting.

I deduce that this is a new engine to you. Has it been rebuilt following a previous incident with push rods that were already bent slightly?

Sorry not much in the way of ideas. If you intend rebuilding it then double check everything as you go that may have had some bearing on the matter.

Pistons hitting valves do a lot of damage. We dropped a valve on the MD11C. Although we stopped it conditions dictated that we started it up again and ran it until were in a safe enough situation to lash the dinghy alongside and motor into port with just a small Seagull.

Sorry I misunderstood you reason for wanting a socket to fit the crankshaft nut. You'll just have to measure it I guess.

Thanks to garvellachs ... its 55mm
 
some thoughts

were you motor sailing this can cause oil starvation in some cases

if the piston is seized in the bore losen the cylinder nuts and the block will rise with the piston till tdc then pull the piston down from the block you can also take the side doors of and split the con rod

watch out as there are shims between the crankcase and block they look like gaskets but are critical as they determine compression hights

check for broken valve springs and leaking seals on the water pump shaft
 
I've removed the nut and flywheel from an MD11D. The nut was 55mm and I tried to turn it with a 60cm bar - and couldn't. In the end I hit the nut with a lump hammer and chisel and that loosened it - a previous mechanic had done the same thing to judge by the marks on the nut. I put a puller on the flywheel and cranked that as tight as I could and then hit the end of the crankshaft firmly with a drift. The flywheel came off its taper with a bang (don't take the nut right off!). So I expect, if yours is similar, it will be very tight indeed. The oil to the rockers etc of an MD11D is fed via a copper pipe up thru each head - that pipe can be broken when removing the head. That was the cause of trouble in the rockers of mine - someone had put it back broken, or not realised it was broken. I scapped the engine.
 
I agree. I have never known a marine diesel to break push rods. Are you sure it's really the push rods (the things that push the valve rockers up and down) that are broken? Both of them? Where are they broken? At the top, the bottom, or midway? Certainly can't see how water in the cylinders can cause broken push rods. Bent con rods maybe....
 
Thanks for all the comments. I have spoken to Volspec and the engineer in my yard. Everyone has a different theory. I have not checked the oil to see if it has water in it yet. I can confirm tha it is the pushrods that are broken in the middle. Not only that they damaged the cam followers which have had to be replaced. Luckily these lift out with a magnet or it would be a block off job.

Bit wary of putting it back together without conclusive reason for failure. I will keep an eye on the oil. As Vic says 40 psi should be good, though if a pipe was blocked I would still get that.

Thanks for the nut size. I can now buy a socket before I return to the boat.
 
Just a thought in passing. I think I would try not to disturb the blocks if possible. that may mean improvising some means of clamping them down while you are freeing the seized piston and turning the engine over. The shims under them have already been mentioned. Don't get 'em muddled up if you do eventually have to take the blocks off In theory they have been selected to give each cylinder the correct compression (in practice they are probably the "default" sizes) IIRC there are two under each block but of different thicknesses.

Hard to believe that the oil feed pipes could be blocked, check them by blowing down them perhaps (not that that will be easy) If one of those is blocked then heaven knows what else might be. If it proves to be necessary you can remove them by just lifting the block by about a cm or so and unscrewing them.

Definitely check the rocker gear on the affected cylinder and also the other two push rods for straightness if you are reusing them.

I hope you have a w/s manual It does not seem to be available for down load any more.
 
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