Hello Peter
Good luck with your quest. You should 1st ascertain which alloy it infact is. Unless you know for sure, you can't carry out the specific hardness treatment. After getting ripped off by a guy with a Metal scanner who could not give me a sure fire answer, I sent a sample off to a company in the UK who broke it down and sent me certificate showing the full analysis.
Have a read of Page 3 and good luck getting it up to temperature at home
http://www.hadleighcastings.com/uploads/LM25%20Alloy%20Detail.pdf
Possibly 4/12 hours at 525 deg c, 250 deg for 2-4 hours and 8-12 hours at 155-175 deg s
an excerpt from the PDF leaflet above
NOTE
Achievement of the specified minimum tensile properties is dependent on maintaining the optimum
magnesium content. Care must therefore be taken during melting and degassing to avoid loss of magnesium
by oxidation resulting from overheating or by excessive chlorination.
HEAT TREATMENT
LM25-TE (Precipitation treated) - Heat for 8-12 hours at 155- 175oC and allow to cool in air
LM25-TB7 (Solution treated and stabilized)-heat for 4-12 hours at 525-545oC and quench in hot water,
followed by a stabilizing treatment at 250oC for 2-4 hours.
LM25-TF (Fully heat treated)- heat for 4-12 hours at 525- 545oC and quench in hot water, followed by a
precipitation treatment of 8-12 hours at 155-175oC
I had a head fully heat treated, and it didn't appear to suffer to much from distortion but I repeat, you will need to know which alloy it is made from 1st and if the company doing the work doesn't say the same thing, they shouldn't be doing the job.
You should be looking for a brinell hardness on a treated item of 90-110 but the likelyhood is, you may not find this on a casting with 30 years of heat cycles or one that is giving problems, ie stripping threads. I believe this translates to around 48-60 Rockwell B.
Remember to test the castings all over as it varies wildly and may come up as hard in one area but soft where welding or other damage has taken place. To give you another example of this, a head that I had some serious chamber welding on, came out between 60.7 and 122 at extremes on a Vickers hardness test HV5kg ASTM E92
HTH
Rob
PS I've resisted saying anything about welded up yokes because I dont know that they are heat treated in the 1st place. my guess is they may well be and one might consider that although they will still work, you should be careful about the press fit around the steering stem and the ability to clamp the stanchions. As is say I dont know that they are fully heat treated !
