Change hole size in the ceiling from 100 mm to 125 mm?

[Martin Lj]

I have a vent hole in the bathroom ceiling that is currently 100 mm. I'm redoing the ventilation and would like to enlarge the hole to 125 mm instead.

I have a hole saw of the right size, the problem is that there's nothing for the drill bit to drill into to keep the hole saw in place.

Does anyone have a good trick?

 

[useless]

Attach a board that is large enough and hole saw through it.

 

[MathiasS]

Place a piece of wood across the top.
Drill a hole in a board and use it as a guide on the bathroom side.

 

[Kalle6666]

Use an adapter so you can use a 100-hole saw to control the 125mm saw.

https://www.motonet.se/produkt/bahco-3834-upsize-utvidgningsadapter-for-halsag?product=70-03399

 

[fsn]

A common trick is to screw a 100mm hole saw into the 125mm hole saw. Then it guides on the outer edge.

I recently drilled up many holes for ceiling spotlights from 73mm to 83mm. I then drew a 73mm adapter with a center hole for the drill.