cooker hood ducting sealing

Hi all, we have a second hand Britannia cooker hood which comes with an outlet which is 150mm diameter. The external diameter of the duct is 160/162mm and we are struggling to find someone to core it for us, as hire places usually go only up to 6" in size (150mm). Industrial corers can do it...

I recently installed a new integrated cooker hood in my kitchen and a handyman ducted it to the outside. Previously the air was recycled into the kitchen. The diameter of the exhaust on the cooker hood is 120mm. The ducting kit we bought is 100mm. We needed a reducer to go from 120mm to 100m...