Alluminium Heads

[mick88]

A question for the learned!
We have all heard of how eventually aluminium cylinder heads become soft or lose there "hardness". I assume it is from the temperature changes that they regularly endure. The question is does this only happen to the heads on water cooled engines, as I have not really heard of it being a problem with alloy head/s on air cooled engines?











Cheers, Mick

[PAT303]

They go soft because they get boiled. Pat

[JDNSW]

My understanding is that ome alloy heads have the mating surfaces hardened (chemically?) so that smaller gaps can be used between block openings (and hence higher clamping pressure) and still ensure sealing without permanent deformation of the sealing surface. The hardened surface is disrupted by temperatures well above the boiling temperature of water, which can never be reached until the coolant is missing in the head.

Aircooled engines, lacking the need for water passages, do not use the narrow sealing surfaces, and lacking the need for multiple liquid tight seals, do not, as far as I know, ever use this sort of hardening. Which has only been used in recent years in liquid cooled engines either.

The basic problem with alloy heads is that their thermal expansion coefficient is a lot different from that of iron and steel, and this has always been a serious problem with using them (together with bimetallic corrosion). Although alloy heads have been used for over a hundred years, their use in cars has been fairly rare until the last few decades.

John

[Tank]

I don't think Alloy heads should be used on Diesel engines, in over 40 years of mechanicing I have never seen the problems common to alloy heads on cast iron heads, Regards Frank.

[Tank]

They go soft because they get boiled. Pat
Pat, I believe that is totally wrong, the 300TDi head has an inherent fault with a weak spot in the head between 2&3 cylinders.
This weak spot warps and allows the gasket to blow out in that area, losing water and pressurising the cooling system, THEN the engine cooling system boils.
What you're saying is that the cooling system boils (WHY) and that causes the head to warp and blow the gasket, what causes the system to boil, I've seen perfectly good cooling systems, working properly and the head gasket blows 99% of the time between 2&3,
With a good operating cooling system the engine will not overheat unless something fails or breaks, water pump, fan belt, thermostats blocked or radiator leak or a head gasket blows.
So the head gasket doesn't blow because the engine gets hot, the head gasket blows (or one of the other faults) and then the engine boils, Regards Frank.

[Blknight.aus]

Alloy heads fail for many reasons. Poor design, poor installation, poor cooling, damage, defect, dud gasket.

In response to previous comments, in my experience ally head failures split about 50/50 over the it failed.because it ran out of coolant got hot, then warped/cracked and it warped/cracked and then blew out the coolant.

You can put so k ch heat into the combustion chamber that the face of the head bet he to melt before the coolant boils off. If your quick enough you can melt th tip to f2f of an aluminium can with an oxy torch while holding it by the bar with your bare fingers, it's the same concept.

The last thing that spells the death knell for ally head is thermal and physical shock as because there is more than one metal and chemical in the block as you continually heat load it and shock it they begin to seperate out creating weak spots that allow porosity to develop. A similar concept can be seen if you disturb a solder joint as it's cooling resulting in a full grainy looking joint.

[Jeff]

I have boiled a few bike engines but never had a head gasket problem as both block, or barrels and head are aluminium, so they probably expand or even warp together as they heat then cool.

Jeff

:rocket: