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I have a 2002 Nissan Almera 1.8L AT which I've been fixing up. The thermostat needed to be replaced. Unfortunately, I threw out the old thermostat after buying the new one, but I strongly suspect that the one the parts place sold me is too small.

Basically, I cleaned the housing, placed the thermostat with rubber seal on it into the housing and rtv'ed everything letting it set overnight ( because I'd already tried without RTV and had a leak ). When I started filling I immediately noticed a leak. After removing the hose side of the housing, I noticed that the thermostat seemed to look a bit too small for the opening, and measured that the engine side of the housing opening was about 52.5mm, while the thermostat flange with rubber seal measured only 49.25mm. The hose side housing diameter is about 42mm.

I then looked up the thermostat on rockauto, and it gives a flange diameter of 2 & 3/64ths inches which is 52mm.

It seems to me that the source of the leak is the thermostat being too small to seal properly, but I wanted to get other opinions before I go back to the parts place to complain.

Basically, the question is if basic thermostat sealing is provided by the rubber seal around the thermostat flange fitting snuggly in the thermostat housing and the hose side housing pressing against that snug rubber seal? I have other vehicles where it's snug and I don't need any RTV for them to seal properly.

As a bonus, now I've got dried RTV all over the plastic hose side housing. Is that going to cause me a problem now and if so how do I get it off without damaging the plastic housing?

POSTSCRIPT 29/5/2017

So after running around to every parts place in the city, I finally ended up at the Nissan dealership. Apparently, there are two version of the car sold in my country, one made in Japan, which uses a 48mm thermostat, and one made in England, which uses a 52mm thermostat. Even though the English version is more common, the only after market version available is the 48mm Japanese one. Not wanting to pay dealer prices, I just ordered one off of Amazon.

  • Good choice with Amazon. Hopefully it will cure the leaking ills. – Pᴀᴜʟsᴛᴇʀ2 May 29 '17 at 17:09
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The rubber seal around the outer edge of the thermostat should provide the seal without the use of RTV. 3+mm would be enough to cause a leaking issue, no doubt.

As for the RTV which you now have on there, you'll need to clean it up because I believe you'll run into sealing issues with it there. Since the piece is plastic, you'll have to take extra care not to screw it up, but you'll still need to get it cleaned up. Hopefully you didn't use the hardening kind! I wouldn't say it has to be "perfect", but you'll want as much removed as you possibly can.

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