Thanks, I ordered it there too! It looks much more identical to the correct one than the one offered on Ebay: https://www.ebay.de/itm/185792693221
Well, that got a little heated (pun intended). Waiting for George to step in.....
We agree and the important dimension is therefore the diameter of the flanged copper sleeve which measures 11/16'' on the Robertshaw thermostat.
That's why I measured the flanged sleeve from the one sold by SUMMIT and it also measures 11/16'' (17.46 mm):
So I don't see what law of physics could prevent it from shut off the bypass properly.
It would be all the more surprising since I have just noticed that the packaging bears a label with the SUMMIT reference on one side and on the other side:
The only reason I chose SUMMIT over Flowkooler was because I had something else to buy from them and was saving Flowkooler postage.
So there is no reason to be upset.
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Yes, today I got my T-stat from Summit as well - I do not think this won't work! I tested it in hot water (180F), it really opens up wide. It will close up the bypass and it will allow a good flow rate for sure. I cannot image this is in any way different then the original one!