Soft in the head??

[jsmith666]

I keep hearing people with aluminium heads claiming that their heads have gone soft after overheating and gasket failure. Does this phenomenon really occur and if so what temperature causes it and what problems does it cause (I imagine valve seat movement could be an issue).

How can you test if a head has gone soft and if it has is it then scrap....??

Thanks in advance guys....

 

[MaxRaceSoftware]

How can you test if a head has gone soft and if it has is it then scrap....??
------------------------------

seat inserts will move around

headgasket "imprints" much too far into alum surface
also many times the "imprint" depth is uneven
as hardness varies

dull looking finish when resurfacing/milling heads

valve guides move around , loss of valve seal

the exact temperature ?? there are more qualified here in this Forum to answer that question than i am .




Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[Chumley]

Yes, that is possible, but, this post, most likely, will be of no help to you at all. Different aluminum alloys heat treat (increase in hardness) and anneal (soften) at different temperatures using different, very specific methods. Without knowing what specific alloy you are referencing, (there are hundreds of common aluminum casting alloys) its a really tough guess. A metallurgist and a good machinist/welder should be your friends.

Many high performance engine manufacturers keep the exact alloy specifications that they use somewhat "confidential" for obvious reasons.

Remember that there are temperature gradients in all materials under thermal stress. The temperature at the surface of the head in the combustion chamber can and will vary substantially compared to the temperature of the surface at the water jacket. The same for the valve guides, especially the exhaust valve.

Without helping at all, or answering your question, I hope this helps!

Chumley

 

[Azmio]

Jsmith,

The main concern for high performance engine cylinder head is the exhaust valve bridge in case for 4 valves and 5 valves engines. We would like to keep the exhaust temperature below 250 celcius if possible. Above this temperature, the Al may not be back to its normal shape. Once it deforms, it really affects other components.

As for the valve seat, once it is deformed, you will see some compression loss. So make sure you get your compression reading for every cylinders before you torture the engine on dyno. Later, it is a good practice to measure the compression reading using the hydraulic pump.

 

[patprimmer]

A leak down test with a stethoscope in the ports also helps determine leaky valves.

An unexplained change in tappet clearance also indicates that the valve seat might be receding into the head.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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[jsmith666]

Thanks a lot guys.

I think my main point is: if you were given a head for porting or skimming, or even if you wanted a replacement stock head for your car, how would you know whether the head was "soft" or not.

So far I have gleamed that an exaggerated gasket imprint, radically moved seats and loose guides are indicators - as is the mull finish after skimming.

Is there a more fool proof way of knowing whether a used head is worth using?

If a head has gone soft, is it scrap or is there anyway to re-use?

 

[evelrod]

It is SOP to re heat treat an aluminum cylinder head after welding or modification where the material stands the chance of being annealed. I have been out of the al head business for several years, but when I raced a Lotus twincam years back I found a place in Torrence, California that did commercial heat treat. I can't recall the name, but with a Google search or perhaps the Yellow Pages---? In 1980, one of my old heads cost $1200 for a bare casting (~$1800 finished) so spending a couple bucks for heat treat was a no brainer.

Rod

 

[evelrod]

Sorry, I forgot to point out the obvious---all the valve seat and align boring of the cam saddles was done after the heat treat, not before. Also I typically rechecked the line bore with the head torqued to specs on a block. It was usually within specs.

Rod

 

[Majik]

Fool proof way to know if a head is worth reusing?? The fool proof way is to get a factory fresh head. Um.. sounds like you are or thinking about buying a head used from someone and found out they had overheated it at one point or another... or you have done this to an expensive head that you bought.. you can heat treat the head but then you really need to do everything all over again as evelrod mentions.. new guides which usualy require new seats.. cam journals.. Um if it is for a mision critical part then I would replace your head or not buy the one you're looking at. Alot of times if the heads are overheated they will develop small cracks between the seats. Use a dye kit to see. If your asking how can I make this soft overheated aluminum head like new then the answer is. It can't be done because the aluminum is already cycled several times over it's lifespan and espically an overheat cycle. Yes you can get it to where it will probably run fine. No it will not be like new. Many times the cost of the valve work and boring of the saddles or journals etc will cost more than the price of a new head.. If it's a special head that has many hours of port and polish into it them maybe.

 

[LadaTrouble]

Soft heads are very common,and with some engines such as the 12 valve Mazdas you can count on it.Before a pressure test and skim my machinist does a hardness test.

 

[evelrod]

Just a mention of another problem that I have found to be rather widespread among amature engine builders---over torqueing of cylinder head bolts/studs! Indeed, not using a torque wrench at all in some cases.

I most cases with cast iron or properly hardened aluminum cylinder heads, this may not become bothersome or cause failures (at least not right away), but with 'soft' heads it can be disastrous. Overtorquing can distort gaskets to the point of failure and, of course, break bolts/studs---even on cast iron. I bring this up as I had to remove a broken bolt just yesterday!

Rod

 

[jsmith666]

I'd be interested in hearing more about this hardness test...

Obviously in an ideal world we would all use new castings, however there are clearly many occasions where this will not happen (non-availablity, cost etc). What I guess I am really asking is what people look for on 2nd hand castings before they carry out head work. Clearly no one wants to spend time and money installing guides, seats, porting and skimming, only to find the head has been overheated and softened previously. How do you guys avoid this scenario?

 

[RICKYUK]

Take a look at the area of contact where the head bolt/washer meets the aluminium cylinder head,if the head is soft it may have "sunk" around the fastener,this would obviously cause the head to loose its clamping force so it may show up as "loose" head bolts.I think the test you refer to is a rockwell hardness test.

 

[unclesyd]

If there is an inspection company (NDT) near you they should have a hardness tester that will do the job and they shouldn’t charge to much. If you carry a clean head to them they should be able to give you a hardness reading, preferably Brinell (500 Kg load with 10 mm) ball. There are several machines that will give you the equivalent to the Brinell hardness to compare with either a published values or from the OEM.