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Old 01-17-2012, 02:33 AM
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warped heads

Hello everyone .
Just had a pair of alloy rover heads off an overheated motor skimmed and the machinist recorded both the heads were warped front to back 2-3 though and he machined them 4 though total each to get them both true and the same .
My question is , is 2-3 though warpage common or excessive . How much does a head usually warp once its classed as a warped head ?
Mark

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Old 01-17-2012, 06:49 AM
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Quote:
Originally Posted by theg
Hello everyone .
Just had a pair of alloy rover heads off an overheated motor skimmed and the machinist recorded both the heads were warped front to back 2-3 though and he machined them 4 though total each to get them both true and the same .
My question is , is 2-3 though warpage common or excessive . How much does a head usually warp once its classed as a warped head ?
Mark
If "2-3" and "4" are referring to thousandths of an inch, that's not excessive at all. Actually I'm a little surprised the machinist opted to only take 0.004" off (if this is what was actually done and we're not talking metric or something), that's cutting it close to square up a head deck w/0.003" warpage front to back.

The good part is that's going to enable you to- in all likelihood- retain the same length pushrods, providing you use the same head gasket compressed thickness. The lifter preload still needs to be checked.
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Old 01-17-2012, 06:52 AM
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Very common condition for aluminum heads. .003" is max.

I do on the average 2-3 pairs of 3.1-3.4 Gm heads per day. Usual cut is .005-.008" to get them straight.
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Old 01-17-2012, 07:30 AM
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Quote:
Originally Posted by cobalt327
If "2-3" and "4" are referring to thousandths of an inch, that's not excessive at all. Actually I'm a little surprised the machinist opted to only take 0.004" off (if this is what was actually done and we're not talking metric or something), that's cutting it close to square up a head deck w/0.003" warpage front to back.

The good part is that's going to enable you to- in all likelihood- retain the same length pushrods, providing you use the same head gasket compressed thickness. The lifter preload still needs to be checked.
Seconded...
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Old 01-17-2012, 07:31 AM
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Originally Posted by cobalt327
If "2-3" and "4" are referring to thousandths of an inch, that's not excessive at all.
I should prolly clarify. When I said 0.003" is "not excessive at all", I was talking about in general. Heads can be warped far worse than that. However 0.003" warpage IS reason enough to straighten the deck on a V8 aluminum head, as said above.
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Old 01-17-2012, 07:40 AM
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Quote:
Originally Posted by cobalt327
I should prolly clarify. When I said 0.003" is "not excessive at all", I was talking about in general. Heads can be warped far worse than that. However 0.003" warpage IS reason enough to straighten the deck on a V8 aluminum head, as said above.
Depends on the block deck and the choice of head gasket. 0.004" can be driven forever if other circumstances are favorable. No guarantee of that, however.
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Old 01-17-2012, 07:51 AM
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Originally Posted by whyholdback
Depends on the block deck and the choice of head gasket. 0.004" can be driven forever if other circumstances are favorable. No guarantee of that, however.
All I recall about the Rover engine is it's based on the '60's Buick 215 all-aluminum engine. I suspect the shops who do Rover's on a regular basis know the limits, but generally speaking I'd consider 0.003" the high limit. Considering the engine was already apart, just as well machine the heads flat to assure a long service life providing there's no overheating reoccurring. 0.004" will work in some cases, I agree. But to prevent returns, better safe than sorry, you know?

I'm a little curious why such a light cut, though. But there again- if this is a shop well-versed in Rover engines, there may be a good reason.
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Old 01-17-2012, 06:36 PM
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warpage

Thanks guys.
How does the head initially warp ? I know heat is the major factor but how does the head pull on the headbolts allowing the warp to happen . Do the heads warp while still bolted to the engine ? I cant see how this happens when all bolted down tight , or does a head warp when you undo the bolts ?
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Old 01-17-2012, 07:59 PM
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Quote:
Originally Posted by theg
Thanks guys.
How does the head initially warp ? I know heat is the major factor but how does the head pull on the headbolts allowing the warp to happen . Do the heads warp while still bolted to the engine ? I cant see how this happens when all bolted down tight , or does a head warp when you undo the bolts ?
Near as I can tell the warpage is caused by uneven expansion/contraction of the metal due to differences in the temperatures of the coolant passages and combustion chambers and the rest of the head.

A head can warp fully bolted down bad enough that the head gasket seal fails- aka a 'blown head gasket'. I'd guess the head bolts are stretched and/or the head gasket compressed when this happens unless the gasket was a steel shim, unless the warpage was confined to between the bolts in which case it'll warp more when unbolted in all likelihood. I'd also imagine the head can warp more once it's unbolted, even if it wasn't warped enough initially to lose the head gasket seal.
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Old 01-17-2012, 08:43 PM
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I'd also imagine the head can warp more once it's unbolted, even if it wasn't warped enough initially to lose the head gasket seal.
This is exactly how it happens. The metal gets "stressed" with the uneven expansion, and ends up with, for lack of a better term, a twisting force in it. However, the clamp force of the bolts is preventing this force from being released. When the bolts are removed, this "twist" becomes "free", and the metal relaxes into whatever way the stresses dissipate.

EDIT: Now that I re-read this, I hope it makes sense. It did not come out as clear as I had hoped.
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