Re-hardening Aluminium heads?

HTR

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Location
Southern New Zealand
Car
LR Freelander 1
OK over time I've acquired some spare heads for my '98 LR Freelander which has the K series 1.8 mph engine. Yes they are all K series heads and are complete. One is a VVC head. The engine it was on had been overheated. I stripped it except for the valve seats. There are fire ring indentation marks on the exhaust side of the head.

My question is can aluminium heads be rehardened? Just what is the process?

I could carefully peen around the fire ring area of each cylinder and have the head skimmed. Would that do it?
 
Taking more material off is unlikely to harden it, but cryo treating it will increase the hardness. It would be good to have it stress tested for cracks deep inside the metal etc...

As long as it is true and flat, you'll be effectively increasing the engine volume by machining it and taking off some of the head surface.
 
Please let us know what you find and discover, I'm sure many others are in a similar situation.
 
Update - I've been asking around far and wide. Most people say no. However it can be done. It is a heating / cooling process which I think is referred to as Precipitation hardening. It is quite expensive so it would no be viable in the ordinary run of things. Only really useful for very rare or precious collectable cars... My MG Rover K series heads are hardly that and the VVC heads are still available on the 2nd hand market still. I won't be having a go at that.
 
Thanks so much for the update. That sounds very much like Cryo or Cryogenic engine treatment?

There are lots of heads around as you say. Is it time to fit larger valves and get the head gas flowed whilst you have the head off?
 
Cryo is a cold process, liquid nitrogen... and mainly used in very high performance vehicles, adds strength and hardness to the alloy.

I will be doing some work on the head in prep' for swap over. I'll keep the stock valves [ex = 24mm, inl't - 27.5mm] I'll do the following: enlarge the exhaust valve seat openings by about 0.5mm to 21mm. Similar with the inlet side as well. A small amount of reshaping the actual valve throats, cutting away the valve guide protrusion and the associated cast 'boss' behind each, opening up the port by lowering the hump behind the valve guide to the exits and a general tidying of the casting... increasing the exhaust port size from about 19.5mm to 21mm. Again the inlet ports will get similar treatment.

Valve seats will be re cut - triple angle, and the valves will get a 30º back cut too and a radius put on the edge of the valves. Combustion chamber: un-shroud the valves by 1.5mm [or maybe 2mm] and blended into the combustion chamber. The valves are recessed slightly in the head so I'll remove that 'lip'. Give the chambers a good polish up and the exhaust ports too. I'm going to fit a VVC 135 exhaust cam as my inlet cam. Fit the VVC intake set-up - yes I'll alter the inlet port/entry to remove that step, or casting mismatch, between the stock K series MPi head and the VVC inlet manifold. My only wondering is will I need to swap the fuel injectors over?

It's no a race engine so for me a 'flow' test isn't really necessary.

Fun times :)
 
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As long as it is true and flat, you'll be effectively increasing the engine volume by machining it and taking off some of the head surface.

Obi when any head is "skimmed" the volume of the combustion chamber is reduced which results in a slight increase of the compression ratio is BUT the engine capacity always remains the same.
 
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