To all 3 of you who have replied, thanks very much, looks like its a seam then. Ken, my tank is shaped like yours, the line only goes halfway to the rim, not all the way bottom to top on both halves. Silly mistake on my part, apologies. JR, I take your point about the silicon, it wouldnt help, you too feel its a seam/mould line so thats good. AG Falco you have been very clear as you always are and have described exactly what I have seen, the bottom half only, from where the coolant comes in up to the halfway point, I feel relieved. Turning to your question: have I replied to the last thread I started about the oil temperature? I havent yet. I only got back last night from a night away in North Wales last night that was great but everywhere was very busy and I was really tired so hadnt got round to it. Excuses over, very happy to reply to you though, I have read all the info you sent me most has been absorbed and I will go back to check it all out again. I checked the oil temperature 2 or 3 times after travelling for at last half an hour or more. The outside temperatures were 17/18 degrees C and the oil temps showed up as 94 and 95 degrees C. If I had been permitted by the North Wales highways authorities to travel at the right speed for the surroundings, ie 70mph,( long stretches of the A5 with no dwellings at all but still restricted to 50mph for some inexplicable reason and very regular speed warning signs) and the outside temp had been higher I am sure the oil temp would have been high 90s or low 100s.