Replacing Water Outlet On Block (for Heater)

[cwp_audi]

I have installed a new plastic water outlet and "O" ring on the back side of my engine. (The old one appears to have deteriorated from engine heat.) Unfortunately, when the engine gets to "operating temperature" water begins to leak around the new outlet at the flange. I have tightened the bolts holding the outlet in place as much as I dare. I am afraid over tightening because the outlet is made of plastic. Has anyone else experienced this problem? If so, how did you resolve it? Do I simply need to tighten the bolts more or should is try some silicon sealer in addition to "O" ring? I was also thinking of try to try down a slightly thicker "O" ring. Any assistance would be greatly appreciated. By the way the car is a 1990 model 80 with the 2.0L 4 cylinder engine.