I need to make 4 x 40 mm holes in a sheet of 8 mm ali plate for an inlet manifold. I have drill press but my biggest step drill 30 mm - don't know if that would be up to the job anyway. Would some kind of hole saw work , if so what type? Any advice welcome.

A decent metal cutting hole saw, plenty of cutting compound and slow the speed of the drill down should give you as good a job as any, other than laser or waterjet.

Oh and a good centre-pop mark so the drill bit doesn't wander on start up.

I have used 40-70mm hole saws in 8mm steel plate without too much issue just go slow, keep it cool and clamp the thing down tighter than a tight thing!

I made my inlet manifold from 12mm ally ,I used a woodworking coping saw to cut it out including the ports which I cut slightly undersized then blended into the ports.

Replace the drill in hole saws with a length of steel rod, and drill the pilot hole separately.
This stops the drill flutes enlarging the pilot hole

this one says its suitable for steel and ali so should be fine, but for ali you may need to take it slowly
http://www.screwfix.com/p/makita-plumbers-holesaw-kit-6-piece-set/4689r

Mag or Annular hole cutters are better than hole saws way more accurate. Needs a good drill press with low speeds.

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Originally posted by tegwin
I have used 40-70mm hole saws in 8mm steel plate without too much issue just go slow, keep it cool and clamp the thing down tighter than a tight thing!

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Thanks chaps, plenty of good advice there.

I can get them machined for you if its not too urgent.

Thanks for the offer AJC. I am still at the design stage. Also trying to get my runners to fit using standard bends. Very difficult installation.

Sounds complicated! If I can help send me an email.