Skimming cyl heads.

You get it line bored afterwards if you're worried.

Wouldn't the cam rattle around somewhat?

Which is why you don't just skim a head that has above a certain amount of distortion - you straighten it first.

You're welcome to try that and get back to us with the results. Don't plan any long trips though. What you need to appreciate is that when a head distorts, usually because of overheating, it doesn't always just distort in a nice even linear fashion that will straighten back out under head bolt pressure. There are often localised pockets of more extreme distortion, often around the exhaust valve area. I see this very frequently when I'm running dust cuts along a head to get it straight with as little metal removal as possible. After each cut you see little areas that still haven't cleaned up because they've sunk with the heat generated.

Furthermore it isn't just distortion along the long axis - there can be considerable distortion across the width too which the head bolts can do nothing about as they are on the sides of this.

You also miss the point about gasket clamping forces. If half of the force on certain head bolts is being used to straighten the head, say the inside bolts on a head that has bent concave, there isn't enough left to generate the required gasket seal.

Finally, the bit you wouldn't have known about to be able to consider it. When a bent head is put back into service and heats up again the stresses inside it alter and the clamping forces on the bolts go to pot. The ones that were doing most of the work trying to straighten it can end up loose if it straightens itself after several heat cycles.

There is. We're not talking about an exhaust manifold flange here that's 8mm thick. It can takes hundreds or even thousands of pounds to straighten a head casting that's 4 or more inches thick.

The straightness requirements of cylinder heads for proper gasket sealing are commonly known tolerances in the engine world. A couple of thou at most along the long axis and three fifths of FA across the short axis but ideally as flat as you can possibly get everywhere.

Don't be silly.

Dave Baker presented the following explanation :

So, can a head be brought back to true by the use of heat treatment?

The reason I'm asking, is that (don't know if I mentioned it)...

Some while ago I had some fairly bad judder on my front brakes, it was getting worse and I assumed distortion of the disks. I fixed it by several cycles of very hard braking, to get the disks good and hot. Result - judder fixed plus I assume distortion straightened out.

That was not distortion, but deposits on the disk, there are several sites with a full explanation on the web.

Yes. That's the primary method. You then skim out the remaining low spots.

Whether this was analagous I couldn't say.

All I'm really saying is why skim it at all if it's only slightly bowed, and if it's more than slightly bowed, it might be better find a s/h one that's in better cond'n if poss.

I've seen that as well, and I can appreciate that local 'sinking' can be more of a concern than a simple bow over the complete length.

Another point which I would agree with.

Which is why I'm not suggesting that more than a few thou can be reduced by the head bolts. I know for a fact that a head can be bent a few thou quite easily. On occasion when I've clamped slightly bowed cyl heads on a mill, it doesn't take a lot of clamping pressure to ensure the head is flat and in contact with the milling m/c bed along it's complete length.

To straighten a head yes, but it takes far more force to straighten something bent so it remains straight when the force is removed, than the force required just to hold it straight.

Permanent straightening usually requires a considerable degree of over bending, even on something simple like a solid bar. I can understand that on a mass produced cylinder head with it's 'hollow' structure, straightening is not really an option.

Certainly that is the ideal. No argument.

IIRC it was a maximum of 6 thou along the length or diagonals of a 1.8L Mitsubishi Galant head that I helped refit last year after the cyl head gasket went. Using a straight edge and feelers I found it was bowed 4 thou along it's length. A couple of guys said it should be skimmed. Anyway it was fitted unmachined and has since done nearly 10k miles without any problem. I'm just thinking that maybe a little more tolerance can be pinched without causing problems.

There seems little concrete evidence to suggest that it's a definite no no. If a head blows after being refitted without skimming, unless one knows how badly distorted it was beforehand, it doesn't really prove anything.

Has anyone actually done any research into how bad a cyl head has to be before failure is a certainty? Mike.

Our US cousins call it 'pad poop'.

Mrcheerful wrote on 07/12/2007 :

I've read theories on it being both distortion and pad deposits. I tend to favour the distortion theory because of my own experience. I would expect basic brake use to clean up any pad deposits in time, whereas my judder was becoming worse. I got them good and hot which worked - therefore my assumption was that they were suffering hot spot induced distortion.

Sorry. I assumed when you mentioned "thousands of pounds" you were talking about force not money.

It surprises me that simple stress relieving can be all that's necessary in many cases. Surprising, because IME experience, new, or 'green' castings, are full of stresses which become apparent as soon as you start to machine them.

Are production cylinder heads and blocks stress relieved before final machining? Which is what I've often had to have done to ensure the accuracy of finished cast components.

I know many years ago AEC at Southall used to stress relieve their cylinder block castings by ageing them. You could see large stacks of them in their yard from the Grand Union Canal towpath.. Open to all weathers for as long as 2 years I was told, before they were finally used for a new engine. Mike.

Interesting story snipped.

Cast aluminium has a pretty long heat treatment cycle anyway to bring it to the required hardness for machining. I believe that usually takes care of the stress relief side of things.

Iron castings were often treated this way when time didn't mean money quite so much as it does now.