I have a home project that requires a hot item to be bolted to an electric motor. I want to insert a thermal insulating washer between the hot item and the bolt to limit the heat transfer to the motor. Can anybody suggest what material I should look for in a washer? Even better would be a suggestion on where to get them.
I’ve done some googling but “washer” is too generic a term (I get a lot of appliance hits). Thanks!
Cork or fiberglass washers are typically used, but you are really going to have to worry about the heat transferred through your bolt. You might consider using teflon hardware if the working temperature is less than 400 °F or so.
A heat sink may be a better solution. Use a tap to thread a hole through a block of aluminum. Then attach your part and the aluminum block to the motor. The block should absorb and radiate away the heat.
How hot will the hot item get?
When I have needed such, I made them from linen filled phenolic sheet, cut with a hole saw.
You might mount the hot item between two nuts on a threaded rod, rather than directly against the motor.
The heat conducted will be proportional to the square of the rod diameter and inversely proportional to the distance along the rod.
As far as metal nuts and bolts and fasteners go, stainless steel and titanium have conductivities around 15 W/(m K), whereas steel is around 60 and brass around 150. So, stainless is probably your best bet.
Nonmetal ones are much weaker. Depending on the forces involved it might be better to use smaller metal than bigger plastic. I think plastics are often around 1 W/(m K) if they are just the polymer, but often more if they are filled. The cheapest and commonest polymer, polyethylene, is not very strong, does not handle very high temperatures, and has one of the highest conductivities of all the polymers. I would guess that polycarbonate or polyacetal or nylon would do better on all of these.
You might try “garolite” or asbestos or an asbestos substitute, for a washer. You might also try leather washers. I know McMaster-Carr sells garolite and leather. They are great for web credit card sales.
You might also try a lockwasher, not tightened very far. This will act like a spring and have only a very long conduction path, and so be a poor conductor. In particular you might try an external tooth “shakerproof” lockwasher. Also on the spring theme, you could try a Bellville washer, which is thin and slightly cone shaped, to be a short stiff spring.
They use fiberglass as an insulator in seal bars that seal plastic wrap.
You can make washers outta that.
Stainless steel nuts n bolts don’t like heat. You might have trouble removing them.
I’d use regular steel bolts, like from an auto store. They might even have some of that there gasket stuff you can make washers out of. 
MCMASTER-CARR is a good supplier for all sorts of hardware, industrial supplies, etc. Great website (easily searchable and lots of technical info), fast shipping, and all you need is a credit card.
I have made standoffs out of G7 garolite to prevent heat tranfer. I used threaded inserts in each end without the inserts touching. In your application, a setcrew could be used as a stud to attach the standoff to the motor and then attach your hot item to the standoff. Garolite is pretty fragile however.
There are also ceramic fasteners but they are pretty pricey.
Thanks for the suggestions all. The project is a home-made cotton-candy machine I’m making with my nieces. The hot item is a cup made out of some tin cans we cut up that is fastened to the electric motor. The cup is heated for about 5 minutes so we don’t need anything heavy-duty. We ended up making two washers out of 1/4" thick cork and placed them between the bolts and the hot item. It worked well enough and it was cheap.
We fired it up yesterday and got some cotton-candy. Now we just need to work on our technique. 