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Any thoughts on how to straighten out this dint.

Assuming it isn't a part of the design, the left hand air-box outlet on my R75/5 looks like it has received a bump. I was thinking of using an electric heat gun and ?something (maybe gal pipe), to tap it out from the edges in. Ive straightened out body dints of my daughter's VW beetle using this method but cast metal, - I'm thinking it might crack. What do you think?

Thanks
Ian

Heat will help but not really necessary. Round pipe or similar (largest as will fit) into the hole as you mentioned, tapping the side of the pipe to push the dent outwards.

If (big if) it can be pushed out, heat and going very slowly might help you. If you hit it, then plan on many small hits. I would prefer to use a mandrel on the inside and clamp slow pressure on it, increasing after a few minutes.

Somebody dropped it.

If, by some very strange chance you know a brass instrument repairer ... he will have a set off very large steel balls and they are the ideal tool to very gently ease that bend back into shape.

Sibbo wrote: he will have a set off very large steel balls

I remember having a set of those.

There was a time

You are safe to tap that dent out with some sort of bucking bar to back it up.
That is an extruded tube insert and not part of the casting.

My first method to expand it back would be to use a muffler flange expansion tool. Designed to go into the opening of a pipe, expands to remove grooves caused from the clamps. I would take it to a muffler shop to see if they have one that will expand to the correct size. Mine goes to about a 1 1/2--2 1/2 opening, has 2 steps.
As far as bending cast aluminum, I can say from experience, it only bends once, when you try to straighten I have never had many happy endings.

I've made it a practice, whenever bending aluminum, to anneal it first. If the aluminum has ever been worked, such as in extruding, it is virtually guaranteed to be hard.

Annealing aluminum is easier said than done, since aluminum doesn't change color as it is heated. I've heard that the old timer "panel beaters" when forming aluminum, will deposit a layer of soot where the bend will take place. Then they heat the area until the soot goes away. At that point it is annealed. (The devil in that detail is that the transition from solid aluminum to molten aluminum is rather abrupt.)

Most everybody has scrap pieces of aluminum lying around. If you have some pieces of extruded aluminum all the better. Bend the aluminum (say with one end held by a vise and vise grips on the other end) then heat and bend while observing. With a bit of practice you might develop the confidence to go after the real thing.


Ken

Another method would be to use a length of pipe in a diameter slightly smaller than the opening, insert and "pry" it back into shape.