Review on UL-Certified AXIAL 1751 High Speed Muffin Fan for DIY Cooling, Ventilation, and Exhaust Projects – AC Infinity, 120V AC, 172mm x 150mm x 51mm by Chad Jeffries

It fits perfectly without wasting space in my 185mm OD. It's a little noisy, but it lets a lot of air through. The only downside is that it only comes with two bolts and three nuts. The fan has four openings. It can't be firmly installed with just two screws in my opinion. Screw in four nuts and bolts and please save us a trip to the hardware store. It's hooked up to a thermostat set at around 30 degrees to ventilate a small outbuilding and runs daily. Addendum: AC Infinity contacted me and sent a package of screws even after I explained that I received some of the hardware magazines. Excellent customer service.