I've had the manifold off and on twice on mine, and each time worried about leaks at that O-ring. Haven't had a problem yet. I did use a special O-ring the last time, which I had been hording. It is from a Catapiller gasket set, was a yellow material and slightly thicker than the original. I had had no trouble with the original, however. That O-ring sticks out on the manifold away from the gasket and does look worrisome. If I remember coorectly, the groove for it is only machined in one side. May be a problem with it slipping out of the groove while one is attempting the installation. Some grease to stick it in place might be helpful. Sorry, I can't be of more help. Good Luck!