Replacing an old bathroom fan Answered
This is an idea someone may be able to use, but it does not quite rise to what I would want to do as an Instructable.
A widow friend has two bathrooms, each with a very dated bathroom fan in need of replacement. I was able to mount the works (fan motor, fan blade, and mounting plate) for the new fan onto the old mounting plate after modifying the old plate. This saved me hours of work in a very hot confined attic when time and tools available to me were quite limited. Had I torn out the carcass for the old fan and tried to put the new fan carcass in its place, I would have needed to fashion a wooden framework for mounting the carcass that would fit just right between the ceiling joists, all so the fan would be properly centered above the existing hole in the ceiling.
The mounting plate for the new works was just a bit smaller than that for the old works. The height and diameter of the new fan cage were very close to the dimensions of the old fan cage. I began by using a cutting wheel on an angle head grinder to cut through the old mounting plate around the circumference of the old fan cage. Then I positioned the old mounting plate over the new works and its mounting plate.
The glass dish that would cover the light bulbs fastens to a shaft that screws onto a threaded stud centered between the corners of the new mounting plate. I sighted across the corners of the mounting plates so the threaded stud was centered. I clamped the two mounting plates together and drilled four holes for 10-32 screws 1/2 inch long and secured them with nuts and lockwashers. (I did need to cut out part of the new mounting plate so the fitting for the two electrical outlets [fan and light] were accessible to plug in both the light and fan cords.) Had the glass dish mounted to the works differently, I would have used measurements from at least two sides to the center of the fan's shaft to position the new mounting plate on the old.
Once the two mounting plates were held together with four screws it was a simple matter to put the works into the old carcass and plug in both the fan and the light to their respective receptacles. There was no question the works would fit because they were attached to the old mounting plate that had been taken from the old carcass.
The round opening in the ceiling drywall bordered on being too large to be covered by the escutcheon for the new cover/light fixture. I added some drywall spackling around the edges to close any imperfections and gaps.
For me this was an idea that worked and saved me a lot of time.