Hello all. I’m sure its been asked before but; any tips on straightening a couple of bent fins on cyclinder head ? Warm it up, don’t warm it up etc ? I’m guessing the alloy is pretty soft, and am hoping so… Thanks Kevin
Don’t do it! If the bike runs fine live with it.
Even when heated the fins, which are cast, will break if the bend looks significant enough to “adjust”. If not significant leave it alone. Doing an adjustment with heat (and plenty of it) “can” be done with care. I have seen it. 3 months later the cracks appeared and the offending bits dropped off.
What we used to do, if the fins were not in a heat-critical area such as the exhaust ports, was restyle the fins and cut/smooth them off.
Or get another head. (I certainly need one or two spare heads 
)
Barrel fins are less critical, but too much off can distort the barrel and affect the liners.
Thanks for that. There’s certainly a bit of conflicting stuff about it out there but I thought much as you suggest. I’m going to get another head, then I can play around (!) with the bent one and see how I get on.
If you are really intent you should consider the following:
Heat. Ideally you should heat, but not anneal the aluminum, as annealing weakens the material. It also runs the risk of valve guides and valve seat inserts dropping out.
Temperature around 300C I believe, but do the best that you can. Below 250C is not much use. Some guff about this at http://www.acl.co.nz/wp-content/uploads/19-Straightening-Aluminium-Cylinder-Heads.pdf
Avoid levering against adjacent fins. You will only end up with more bent or broken fins. For the same reason avoid using a hammer, even with a drift or punch. A slight tap is usually one too many resulting in that horrible cracking noise.
Best is to use a custom made aluminum or copper fork slotted to the fin width and chamfered ends for entry under the narrower bent fin slot. Insert to the right depth and gently ease the fin straight. Alternatively insert at the unbent area and work round until you get to the bent portion.
This is interesting, if a bit of a digression. https://www.youtube.com/watch?v=ZyrHmteTOqc - he uses a g-clamp arrangement, but care is needed to avoid damaging something else.
Stop if you feel or hear cracking. 
Best of luck. 
PS. Is it really so badly bent? Photos?
[Edit: made it clear that you should avoid annealing temperatures and inserted link to better guidance]