Thermostat help

[jmc330i]

I've been having issues with my M44 converted E30 running too cool, so this weekend was going to pull the thermostat and swap it for a spare (unknown age) genuine BMW stat from my spare M44.
This morning a mate has given me a new Circoli thermostat and as I was testing the spare stat I had, just for the hell of it thought I'd bung the new Circoli stat in the pan of boiling water as well. I wasn't overly happy with how far it opened up after being in boiling water for 20+ mins compared to the genuine BMW one.
The genuine stat had opened up to pretty much twice what the Circoli stat had in a much quicker time. Both are 95 degree stats.

So which would you use, new Circoli or unknown aged genuine stat?

[mcspike]

I'd go with genuine. I had major hassle with a cheap one which came on the car i'd just bought. It was very new but I was forced to drop the money on a BEHR one after some overheating...

Flawless now, even in this heat romping on it just to make sure.

[jmc330i]

Well it turns out my M44 already had an almost new genuine thermostat fitted so I doubt it's at fault. But I've fitted the unknown BMW stat just to see if there's any change.

[jmc330i]

I've never looked closely at a thermostat before but assume it should be fully closed when cold so no water gets through, other than the small hole??