Has anyone here with an engineering/metallurgical bent considered stress relieving the head prior to boring? I've not read anything about it, but my brother, a 20yr metallurgist made a convincing case for it today.
Given the age of these bikes, and seeing as how mine sat outside for 10yrs, he felt an oven heat of 250-275 for 8 hrs, then cool to room temp would insure that the boring remained stable. Otherwise, once bored, the risk of stress remaining within the head might very well throw the bore off a few thousandths of an inch. And since we're dealing with a few thousandths of an inch...
I was also wondering what grade of aluminum these were made of? (356, 357, T-6...)
Stress Relieving Prior to Boring Cylinders & What Gradei'm not sure how stress relieving the head will have any effect on the cylinders..they are two seperate pieces and are not attached to each other while boring. unless you are talking about bolting on a torque plate to simulate the stress the head puts on the cylinders? high performance engine builders do that to insure the bores are perfectly round
Stress relievingAs a tool maker who deals with metals every day, I thought I'd throw in my 2 cents weorth.
I'm not convinced a stress relieving process at this stage is necessary. Stress relieving is a thermal process performed to negate induced stress from a previous process, I.e., welding, heat treatment, agressive machining, grinding, etc. Seeing as how this bike is 40+ years old at this time, the iron sleeves have had countless cycles of running hot and then cooling off. Also, if the bike has sat outsied in Pennsylvania, it has gone through ten seasons of cold winters and hot summers I was once told that engine builders once sought engine blocks that had sat in salvage yards for years just for that reason. After a period of time, the cast iron "Seasons" and the chance of warpage after performing a rebore was minimized due to this hot/cold/hot/cold cycle over several years. Since the aluminum is not what you're actually boring. I see no reason now to stress relieve the entire head. It would seen to me it's about as relieved as it will get. For what it's worth. Gary '65 CL77 Owner
As an engineering patternmaker who worked in foundries for many years, I honestly don't think stress relieving would accomplish anything some 40 years after the castings were poured.
In a perfect world, in the day these items were cast, ideally they would have been rough machined, then stress relived, then finnish machined, but that never happens on a production line. These castings would have stress relived in service, through the various cycles of heating & cooling. I can add some weight to Ggray's story - in the days when time wasn't such a factor & things were done slowly, large iron castings (especially for machines, etc) were often poured, fettled, rough machined & left to rust in the yard for a year or so before being machined - they gradually stress-relieved themselves & were stable when they were machined.
Something else to consider is how tight(or not) the liners fit in the cylinder casting, head stud torque, and their elasticity. If the liners are marginally tight already, they only stand to loosen up as the aluminum expands more than the steel. Also, the head studs are quite long. This allows them to stretch more than a normal bolt and "give" somewhat. These are also relatively small castings, not V8 heads that are 20" long and held down with 4" long bolts at 60+lb-ft.
If it helps anyone sleep better at night, buy all means do it. It certainly won't hurt the head; it gets a lot hotter in operation than 275*F.
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