I am looking at my bellows for my thermostat that operate the flaps on my fan. Going from cold to hot how much should they expand? Placed them outside 26°F then brought them in and put them in a baggie and placed them in boiling water they expand it about a quarter of an inch. Question is is that sufficient?

Fully expanded the bellows will occupy almost the entire bracket.

Always use the bracket to check. If they over expand, they can crack and then they are trash. Awesomepowdercoat sells new ones.

I spoke with the gentleman from advanced powder coat who also makes reproduction bellows for the 914. He mentioned the space inside the bracket should be about 13 mm or roughly a half inch for expansion. I placed the bellows inside a plastic bag and stuck in boiling water. Of course at 210° or 100°C I was only getting about a quarter of an inch expansion. I took a heat gun and heated the bracket up roughly 300F° as measured by infrared temp gun and got closer to a half inch expansion which would occupy the full area of the support bracket. So two things, my bellows appear to be I working order and at about 300F expand about a half inch

they can go bad and be totally expanded
top pic hot-expanded... bottom cold-retracted

see here some info and pic:
http://lajalousie.net/spip?article32

212F is barely warmed up for an air cooled motor. My bad for not catching that at the initial post. The hot air test is way more appropriate and is the way I bench test them.

I hate to say it, but Harbor freight has a nice heat gun. Careful where you aim it. It can melt things if you're not watching.

in action. And with a HF heat gun, which you definitely need to watch carefully



https://www.youtube.com/watch?v=WFrcKSB-oUM

http://www.914world.com/bbs2/index.php?showtopic=138500

here's some add'l info

no need to head them much above the rated temp,if they only expand with higher temp the bellows is most like faulty.
I test with a head gun it takes little to get them expanded.
Only expand them in the bracket as mentioned,also never set the cable too tight when cold this will cause them to fail.
I also have tested with the stock cable and a shimano bike brake cable,the bike cable is a larger diameter and runs better on the roller...real cheap to buy around $6 for the one I used.