Going to 10mm Studs for Exhaust Manifold
 

My T25 flange has 8mm I believe. My BEGI manifold is 10mm.

What size drill bit are you guys using? What material? Thanks.

I heard google has the answer

You need to drill with the 'M' size less the thread pitch.

So, a M10x1.5 must be drilled with 10-1.5=8.5mm

This method is not an estimate, but the correct way to determine drill bit size.

That would be the method of calculating tap drill size. OD minus thread pitch. But he's asking what size to drill the new flange holes so that his 100 mm studs will slide through them and he can put nuts on them.

Such a hard question.

Open Google
Type "metric to standard calculator"
Click first link
Enter 10mm
It will tell give you a decimal approximation of inches. Find a drill bit this size or bigger.

Or, using your brain (gasp) and a calculator: 10mm = 1cm. 2.54 cm in an inch so using a calculator, that's 1/2.54 = .394 inches. A 3/8 drill bit is .3875 inches, so that's close and a common size. You could just drill them 3/8 and then woller the holes a bit to get the last few thousandths.

I asked this b/c I wasn't sure about how much expansion/contraction I would need to account for w/ all the heating/cooling cycles.

I thought I remember reading somewhere (maybe Abe?) where the holes were drilled too small and b/c of expansion/contraction caused it to crack. Hence, my question.

However, this might have been a figment of my imagination.

^Anyone know?