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I have been refurbishing a four cylinder TR head that I acquired a while back and the final stage was to get the face cleaned up with a light skim. I was quite surprised when the garage/machine shop carrying out the work advised me that it had a 5 thou bend in the middle.

 

Obviously this is easily skimmed off but my question is "how does a cast iron head get bent."

 

I can understand it happening with an aluminium head because it has a much lower modulus but cast iron is a pretty stiff material, so what causes it to bend?

 

Rgds Ian

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50 years of use where the original stresses imposed by it's machining are countered by hundreds of thousands hot-cold cycles which does a pretty good job of stress relieving the cast head and allowing it to return to original pre machined configuration. That's not counting the occasional accidental overheating caused by a blown gasket/radiator etc which took the head beyond it's original temperature running parametres. Seen the same on almost every 4 cylinder I've built.

 

As for it being a stiff material...ha it bends all over the place, the very act of torqueing it up puts an unsupported load on the overhanging ends of the head which is only countered by the squishy headgasket until it compresses it and halts the bend.

 

Light skim and away it goes for another 100,000 miles.

 

Mick Richards

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I've just rebuilt the engine on my GT6 (okay, different engine but still cast iron block and head). Like yours, my cylinder head needed a light skim to get it flat, so it's a fairly commonplace occurrence.

 

Nigel

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When tuning the head mostly they need skimming afterwards

First for compression increase and second if the combustion

chambers are grinded and polished the volume increases and

should be brought back minimum where it was before.

 

post-13092-0-98379200-1456495264_thumb.jpg

 

So as the others say, no bad idea to look after the gasket surface

and also do not forget the partners at the block.

TR4 needs the proper liner protrusion otherwise coolant pops out

or the gasket will fail......

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Some say ..... That a block should be stressed by being bolted up to a head dummy, a massive piece of steel with holes for the cylinders, before a rebore, else the bores will be uncircular when the head is torqued up.

 

John

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Aha...and so how does your engine react when the "dummy head" is unbolted from the block, ...it warps all over the place, only to be corrected to a degree by the act of torqueing up the head again.

Or even the block is of wet liner design (ha...who would have an engine like that ?) the liners would be machined loose out of the engine or if put in place in the block left unfastened before machining ?

If you took a stress reading of the block after the heads are bolted on again the blasted thing warps something amazing, luckily the amounts out of round of the bores are in the category of "small" ie 1/10s of thous whereas the clearance that the pistons and associated gubbins (engineering term) are measured in amounts to sometimes hundreds of units more, therefore everything is rosy in the land of car Triumph.

If you watch any films of the factories from the 60s you'll see engines being machined in their bare state without any prestressing of blocks with dummy heads, and the last time I went around the Cummins diesel factory their blocks were being machined in the same manner "sans" stressing blocks.

 

Mick Richards

 

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