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300TDI cilinder head problems


SeriousIIa
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I've had something leaking on my 300TDI '98 disco, because lots of white smoke from the exhaust.

Then I removed the head to check the gasket (2 notch headgasket), which was okay. So I had the cilinder head pressure tested to find out it had cracks in one of the inlet/cooling channels. So I discarded it and went to look for a good secondhand one.

Found one at a breakers. So had it pressure tested just to be sure. And then skimmed so it was perfectly flat. Then mounted it on the block and it ran fine at first (3 notch head gasket because of the skimming). But after about 30 miles We heard strange noises from the engine compartment. So got a tow home to inspect the engine. Then we discovered that the front pushrod had bend. And 4 more had little distortion at the top. So off came the head again to check why this has happened. Nothing to be found in engine and all the valves look okay.

So now my theory is, the head was skimmed (maybe more then once) just a little to much and when the engine was totally warm the cilinders hit the valves making the push rods bend.

So what's the solution? Place another even thicker head gasket?? (is this available?) or go for another head from the breakers with chances it hasn't been skimmed before so I can skim it, and problems won't arise? (is there anyway I can check the minimum dept the valves have to be recessed in the head?

Hope you guys can be of help. Thx for the input.

Cheers Bowy

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The thickness of head gasket to use, is determined by measuring how far the pistons protrude above the deck of the engine block. It is not to do with skimming the head.

The engine overhaul manual specifies the height from the head surface to the face of the valves (I don't know this measurement and would have to look it up).

If you and possibly previous owners have skimmed the head (which rover advises not to do - but I have done myself). The valve seats should be ground to correct the height from the head surface to the face of the valves. Then you may have to change the shims on top of the valve stems.

The squish action between the head and the crown of the piston, in conjunction with the shape of the combustion chamber in the piston have a lot to do with how the engine performs.

IMHO, the different thickness head gaskets are available so that you can achieve the best squish action. Not so that that you can skim the head.

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The difference between one notch and another is only .010mm, so unle4ss the head had been skimmed in the past it's unlikely to have caused the damage you describe. Did you keep the pushrods in order when you took them out and put them back exactly the same? The length of pushrods varies in a used engine - as does the depth a valve sits in it's seat in comparison to the others. If the head is skimmed the tappets should be re-set, and if the pushrods got mixed up during the work the tappets should be reset. As your engine warmed up the metal will expand and this may have been enough to cause one or more of the pistons to hit a partially open valve - resulting in bent pushrods. If the head has been skimmed to such an extent that the piston hits the valves with the correct tappet gap, then the head is no good and needs replacing.

Apart from replacing the pushrods and setting the valve clearances - the only other way to check for excessive skimming is to measure the squish band.

Les.

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Hi John,

I would be interested in the height from the head surface to the face of the valves, if you could look this up I would appreciate this very much!

You're also talking about the shims of the valve stems. What do you mean by this? The adjusting facilitu on top of the valve stems to set the valve clearence or something different (sorry I'm not that familiar with so much technical English <_< )

Les, we took them out and put them back in the same way. Also checked the valve clearances afteward and adjusted them accordingly.

You're also talking about the squish band. What do you mean by this? The same as John or something different? And how can I measure this?

Steve, Probably nog because it was changed about 1000 km back. But will check today just to be sure.

Cheers Bowy

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Hi John,

I would be interested in the height from the head surface to the face of the valves, if you could look this up I would appreciate this very much!

You're also talking about the shims of the valve stems. What do you mean by this? The adjusting facilitu on top of the valve stems to set the valve clearence or something different (sorry I'm not that familiar with so much technical English <_< )

Les, we took them out and put them back in the same way. Also checked the valve clearances afteward and adjusted them accordingly.

You're also talking about the squish band. What do you mean by this? The same as John or something different? And how can I measure this?

Steve, Probably nog because it was changed about 1000 km back. But will check today just to be sure.

Cheers Bowy

Measure the gap between a straight edge held on the surface of the head and the face of the valves.

For inlet valves, it should be 0.81 to 1.09 mm

For exhaust valves, it should be 0.86 to 1.14mm

To determine the thickness of head gasket, measure the height that the pistons protrude above the deck of the block at top dead centre. Check the front and back of all 4 pistons. Use the greatest value.

For 0.50 to 0.60 mm, use 1 hole gasket

For 0.6 to 0.70 mm, use 2 hole gasket

For 0.71 to 0.80 mm, use 3 hole gasket

For 0.81 to 0,90mm, use 0 hole gasket

The shims I mentioned are small caps that fit on top of the valve stems. These sometimes crack and break. The rocker arm bears on these when the valve is open.

The space between the flat crown of the piston and the head surface is squish area. The combustion chamber is in the crown of the piston.

As the piston reaches top dead centre, the volume of gas in the cylinder is squeesed (squish) into the combustion chamber. This flow of the gas being squeesed and the shape of the combustion chamber chamber is critical to the turbulence pattern as the fuel is combusted on the power stroke.

I may be wrong, but I have not seen reliable contrary information, that the thickness of gasket is critical for the proper squish effect.

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The squish band is the distance between the top of the piston at TDC and the valve. To measure it you have to take the head off, put a plastiguage strip on the piston crown, put the head back on, and then rotate the crankshaft. The piston squishes the strip and it is then measured against a scale to determine the gap.

Les.

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If the head has been skimmed too much the injectors will hit the pistons - easy to spot the damage to the centre of the piston.

I would reckon your valve 'stand down' as they call it, is too small. Get the valve seats refaced to get the measurement (as above) up to spec.

Or you have a bent valve which will stick in the guide and the piston will wallop it.

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Measure the gap between a straight edge held on the surface of the head and the face of the valves.

For inlet valves, it should be 0.81 to 1.09 mm

For exhaust valves, it should be 0.86 to 1.14mm

To determine the thickness of head gasket, measure the height that the pistons protrude above the deck of the block at top dead centre. Check the front and back of all 4 pistons. Use the greatest value.

For 0.50 to 0.60 mm, use 1 hole gasket

For 0.6 to 0.70 mm, use 2 hole gasket

For 0.71 to 0.80 mm, use 3 hole gasket

For 0.81 to 0,90mm, use 0 hole gasket

The shims I mentioned are small caps that fit on top of the valve stems. These sometimes crack and break. The rocker arm bears on these when the valve is open.

I've checke'd the gap between de surface of the head and the valves.

Inlet = 0.60

exhaust = 0.85

So probably found the cause of the bent pushrods.

Didn't measure the block (yet). probably also less interesting because it was running fine before and I even changed to a thicker headgasket. Injectors look fine so they weren't hit by the pistons.

So now 2 options left.

- Go to the breakers and get another head to get it pressure tested and skimmed or checked for flatness and fit it.

- Have the valves of reseated to the correct height and after that readjust all the tappets for clearence (or do I have to shim things out then?)

I'm going to call around tomorrow to find out what is the most cost effective option......

Or do you have other suggestions?

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When you strip the heads, keep everything in order so parts from one valve don't get mixed with another on reassembly.

Check valves and guides for wear. Check the caps on the valve stems for wear and cracks - if in doubt replace them.

Get the valves re-seated.

Renew the rubber seals on the valve stems. Even if they were replaced when you fitted the head recently - removing the valves will damage the seals, even though they may look good.

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Checked the timing today. It looks fine, just like the day I fitted it 1000km's back.

So then there is the option of another head or reseating. Checked both costwise today. The reseating of the valves is by far the cheaper one. So will go for that solution.

Will bring the head away so the specialist can do this. Wil also ask him to change the rubbers and everything.

Thx for all the input. I will let you know if all is okay after this. The disco has to go to an event next saturday so a tight schedule!

Cheers Bowy

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