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Broken Rocker Arm TR2 - low port head


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Advice from the TR Engine Gurus Needed

Since starting to drive the car to run the motor in I have had a couple of broken rocker arms - exhaust only.
Yesterday I ended up at the local mechanics where we diagnosed the cause as there being insufficient free downward movement of the exhaust valve. i.e on a cold engine when the valve is opened by the rocker fully compressing the valve springs there is no more free travel (as opposed to the inlet valve where it is possible to move it down further) It seems there is no more compression in the auxiliary (3rd spring). Hence when the engine gets hot expansion has the rocker trapped between the valve stem and the pushrod and the rocker finally gives way.
My novice question is it possible to leave out the auxiliary spring and the bottom collar (which is there to centralise the third spring) as this will I think give sufficient free movement?
I presume I can also get a little extra leeway by setting the exhaust tappets a little wider and live with the rattle
The alternative I believe is head off, back to the machine shop that did the unleaded conversion to get him to reset the exhaust insert further into the head. I think this will leave me insufficient time to prepare the car for the trip to Tasmania. I'd like to do a lot more driving than the 300 miles I have managed so far before setting off on such a long trip.
Forever hopeful
Jim
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Jim,

The problem would appear not to be the valve seats, but in the choice of

springs or their setting up, also, of course, a basic lack of checking by the

'engineer' that the valves are going to operate correctly in operation.

If they worked OK before the overhaul then they still should!

I wouldn't think twice.Take the head off and make it 'the man's' problem.

Sorry if it compromises your trip.

Good luck

Roger

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Sounds like Valve Spring Coil binding..

 

Here are Neil Revingtons instructions which I found very helpful

 

When installing a camshaft, to avoid excessive and premature lobe wear it is important to use valve springs which provide the correct seat pressure. We recommend standard spring pressure, certainly no more unless you are consistently revving to 7000RPM!
Only just enough spring pressure is required to ensure the valve stays closed when it should be, without valve bounce. Any more is too much and will put unnecessary load on the camshaft lobes, possibly resulting in premature lobe failure.
When choosing valve springs it is also important to check that the extra lift of our performance camshafts does not result in coil binding on full lift. This should be checked (even with standard camshafts) with the head and the valve gear assembled and valve clearance correctly set. It should be possible to leaver down the valve by an extra 3mm for safety with the valve at full lift.

 

You can at least check this without taking it all apart.

 

Iain

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Forgive me I am no engineer just an avid TR learner.

 

But could this be a symptom of head skimming ? (You say you've had unleaded conversion and often those "doing" a head will skim it)

I am sure I have seen on the forum that valve movement / rocker symmetry can be compromised in conjunction with the push rod length thus the rocker pedestals may need shimming to raise them again.

 

 

Fingers crossed for you.

H

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Hamish

 

you are right, head skimming effectively increases pushrod length, changes the rocker geometry and can lead to excessive valve spring compression and potentially coil binding, hence my suggestion to check for free valve movement at full lift above to ensure the valves are not coil bound..........

 

Iain

Edited by iain
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If you have been getting coil binding you will have heard it. Its a knocking sound from the top of the engine.

 

Rgds Ian

 

PS the recommended minimum clearance is 4 thou between the coils at full valve opening. Check it with a feeler gauge.

 

PPS I have a Piper fast road cam and the recommended valve clearance is 15 thou. it's not too rattly at that figure.

 

Rgds Ian

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The usual culprit for broken rockers or bent push rods is either coil binding or the valve seizing in the valve guide. When you mentioned that a low lead conversion had been recently done to the head, an alarm went off. If bronze guides were fitted, they require a few thousands extra clearance. Otherwise, the exhaust valve will have a tendency to seize in the guide when the engine reaches operating temperature. Been there. Fortunately, I was able to detect it before damage was done.

Berry

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Before posting I did do some research. After about engine no. 21000 the head was only fitted with 2 springs and no collar on the exhaust valve hence my question. So far I haven't found out if the specs for the two springs on the later motor are the same as the earlier springs.

Does anyone know why the factory went to 2 springs?
When the exhaust valve is fitted without the collar and third spring there is 3-4 mm of free movement which I believe is enough to allow for expansion.
Can someone tell me what is likely to happen if the springs are too weak, apart from the obvious valve bounce if the motor is revved hard.
Thanks for all the help so far
Jim
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Jim, perhaps you are referring to TR4 around car CT21000 when the 3rd exhaust valve spring was deleted. Only 2 springs were needed to keep the exhaust valves against their seats. Lowering spring pressure extended cam lobe/lifter surface life.

 

Spring Binding. Check head thickness - new was 3.33". If shaved too much, shimming the pedestals can restore the correct rocker geometry (rear shim needs an oil hole). When exhaust valves are fully compressed cold, there must be another 3-4 thou of clearance to allow for thermal expansion. If hardened seats were installed are they at the same level ?.

 

Valve springs too weak. Where did the springs come from ?. TRF valve springs are consistently to spec. Have found others with pressure 20% under spec.

 

Sticking Valves. Bronze guides can be bad news - depending on the metal mix, bores can tighten 40% more than iron guides. If full stainless valves are fitted, stems can grow above standard valves creating a double whammy. Additional 0.001 - 0.003" inlet 0.003 - 0.005" exhaust valve stem clearance may be needed if going non standard.

 

Viv

Edited by vivdownunder
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Yes correct that engine no. is a TR4 but I presume the two spring sets are made of stronger springs.

New spring set and exhaust valves at the time of head rebuild from TR Register, Australia (so I presume Moss). The free length of these auxiliary springs is 1.67 inches v's my workshop manual specs of 1.54 inches - not sure if this means much.

According to the measurements I wrote down at the time this head has had something like 43-45 thou taken off. Head is still on block so can't recheck this.

How much can be taken off the head without affecting the rockers, spring binding etc?

I had the original cam checked and then changed to a fast road match so shouldn't have any higher lift

My calculated compression ration ratio is about 9.5 to 1.

Back to my original question - can anyone see a problem with removing the auxiliary spring and bottom collar as this certainly gives plenty of free movement (clearance) when the exhaust valves are fully compressed.

 

Jim

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That would depend on the rates of the other two as having been a set of 3 then they would be less poundage. You would be better off with a new paired set and then check the clearances again and if required then shim the pedestals. The amount shaved off is a fair bit already.

Stuart.

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