Aluminum Cylinder head "cold flow" in service 2

[thruthefence]

Could someone comment on this, and the 'real world' aspects of it?

http://autospeed.com/cms/A_108653/article.html

By real world, I mean problems associated with aluminum heads losing their hardness in service to the extent that you're failing head gaskets, ect.

 

[larrylcoyle]

One of the things that I dislike about articles such as this is that they create more confusion than they try to resolve. When I read this article the very first thing that I noticed were the following words:

"Typical alloying elements:"
"Generally"
"Brinnell hardness"

And other very vague words that could pertain to aluminum head X but not XX or YY.

And then at the very end of the article he mentions the following procedures for you to follow:

"So some simple reminders:

Always check the hardness of the head
Always make sure surfaces are clean, straight and flat.
Dispose of your old gaskets sensibly - they may contain asbestos.
Always use correct bolt torques and torque sequence. It's always a good idea to have your torque wrench properly calibrated.
Always lubricate bolt threads and under bolt heads; avoid using stretched bolts.
When attempting to straighten or weld an alloy head, be careful when applying heat, you may be softening it."

Well, other than the first one regarding checking the hardness of the head, the remainder are just common sense procedures that anyone with any quality training or experience would follow every time they install either a new, used or reconditioned cylinder head.

I would suspect that this article is directed more toward OEM aluminum cylinder heads and the progress that has been made in their production over the years. The manufacturer of aftermarket aluminum cylinder heads has for years been far ahead of most OEM's. Of course you also have certain aftermarket manufacturers that provide special products for the OEM's simply because it is more cost effective for them to do so.

I hope this helps answer some of your questions, if you have other specific questions, please post them.

Larry

 

[RossABQ]

No mention of time scales for softening. Ford used aluminum heads on flathead V8's back in the 30's and 40's, and many more aftermarket manufacturers made them back then and even now. I am not aware of any significant loss of hardness in those heads over a 60+ yr period that affects their ability to hold a torque. I tend to believe this is the answer to a question no one asked.

 

[MikeHalloran]

Over the past 40 years, I've owned five cars with aluminum heads and blocks, and put a total of more than half a million miles on just those five, with zero head gasket problems.

I've had more head gasket problems with iron/iron engines.

Aluminum does lose strength above ~400F, so, theoretically, there is some danger of permanent distortion in the event of a serious overheating incident... but most people don't keep driving when the car is obviously overheating.

If some exigency forces you to drive an overheating car for an extended time, you basically have to assume the engine is then scrap, no matter what it's made from.


[ The first of those cars, a Corvair, had a cylinder head temp gage that briefly reached 600F under load after I replaced all the jugs and pistons because of a piston failure. Once it was re-broken in, which took ~3000 miles, the gage never went above 350F, even climbing mountains at speed. (There was a serious hill near my house that the car could take at 55mph from a standing start, but if you hit the bottom at ~90, it could hold that all the way up. God, I loved that little car. ]



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

thruthefence,

The aluminum alloys used for cast cylinder heads is normally a heat-treatable alloy. It develops its mechanical properties through a solution heat-treatment, quench and age hardening process. The heating and cooling cycles the head undergoes during normal operation is very similar to the artificial age hardening heat-treatment process that gives it its initial strength. So, in effect, it gets a heat treatment every time it is thermally cycled.

Aluminum alloys exposed to high temperatures and sustained loads can experience a phenomenon referred to as "creep". But this is not something a cylinder head would normally experience.

The excessive strains that the joint between an aluminum cylinder head with ferrous fasteners might experience is mostly due to the CTE mismatch between the fasteners and the clamped aluminum head material. The relative stiffness of the two mating parts is a function of the two mating materials MoE and their section properties, and has absolutely nothing to do with tensile strength as the article suggests. However, the compressive spring rate of the head material is normally much greater than the tension spring rate of the fastener, so the fastener is more likely to fail (plastically yield) first. The tensile strength of the cylinder head bolts on most (cost sensitive) production engines is marginal under the best of conditions, so it is quite easy to plastically yield them. And the head bolt is threaded into the block, not the head. Even if the block is aluminum, the head bolt thread engagement produces a thread root shear strength far in excess of the tensile strength of the bolt body.

Any plastic yield of the fastener relieves the preload on the head gasket joint, the gasket loses its seal, and the leaking, high-temp exhaust gases cause a local overheating of the surrounding structures. That's the typical failure mode.

Even if an aluminum cylinder head somehow experiences localized annealing, it is no small task to re-heat-treat it as the article would suggest. To do so would require a full solution heat treat, quench, and age hardening cycle. This would mean removing the ferrous valve seats and bronze (or iron) valve guides. The heat treat cycle would also likely stress relieve the head structure initially, and then produce further distortions when it is quenched. Thus it would likely require re-machining of all close tolerance surfaces, such as cam journals, deck surface, valve guide bores and valve seat c'bores.

The article you linked had some accurate information, but it also had some very misleading information. That's just my opinion.

Good luck,
Terry

 

[larrylcoyle]

Something to watch for is some of today's re-manufacturers utilize ovens in their cleaning process. There have been instances where for whatever reason the ovens were allowed to get too hot whether this be a calibration problem or something as simple as a bad gauge problem. Whenever this happens whether it is an aluminum or a cast iron head it will create several problems. The problem is that unless you are working with a reputable shop that truly understands the consequences, you will not notice any problems until the cylinder head is back on the engine and miles have been driven.

One of the most common symptoms is that the valve seats tend to move or even in severe cases almost fall out. This usually causes (in the severe cases) a valve to break due to the interference or (in minor cases) the seats will just move around in their normally press fit pocket and engine performance will suffer because the valves will not seat causing the engine to miss and performance to suffer.

One diagnostic that you can do yourself if you are the one removing the cylinder head is to examine the deck surface of the head and see if it appears as though the head gasket has "imprinted" itself into the surface several thousands. You will be able to notice it and feel it. This generally indicates that the engine has overheated considerable and the heat treat at that surface has been impacted. Generally you can have a reputable machine shop resurface the cylinder head with the proper tool to achieve the proper surface FMS. In some cases for overhead cam heads, after you remove a certain amount of material, you will be required to purchase a metal shim to add back/combine with the head gasket when you reassemble the engine. This is to keep everything in the correct alignment.

If you get into a situation where the cylinder head has just lost too much of the heat treat process, buy a used head from a re-builder and be done with it. Make sure you get a warranty in writing though.

 

[RossABQ]

Larry, You reminded me of a related point. There are specialists who "flatten" warped OHC heads by heating them in an oven while clamped in a fixture. While this apparently is successful is restoring high-dollar heads to an undistorted gasket surface, I wonder what it does to the alloys? They do not quench at completion of the cycle. I believe they go higher than 350 F., too.