HOME and how to join Forum Open Area General Scott topics Hair line crack in crank case Re: Re: Hair line crack in crank case

Roger Moss

The devil is in the details
I agree that it is much better to get the TPC faces in line, but first you need the block to accurately locate in the case mouth. Check out next assembly you do how much movement there is of the block in the case. Next the faces on the case have to be exactly in the same plane as the barrel location registers. Next the distance has to be spot on. If we are lucky that all that is correct, we next consider the barrel and here exactly the same set of conditions apply, so that you are exceedingly lucky if these faces are at the same height and plane.
I have photos of the fixtures they used to machine the seatings on the case. We should remember that the manufacturing system used was to have a different fixture for each operation, whereas I fix the case once on a 5 axis jigmil and do almost all machining at one clamping. If someone asks about poor running, I always suggest they get the bike running at low engine speed and spray WD40 round the transfer ports as the bottom edge where you cannot see if often the culprit. The last point is that it is helpful if the gasket has some “give” in it, so that it will accommodate slight variations in thickness. Going back many years, I remember these gaskets being made from the same composite cork / rubber as the cork cylinder base rings, only thinner. There was much less trouble with these. However they were changed for the harder “Hallite” type material with very little “give” so they are much less able to compensate for manufacturing errors than the original type. The worst scenario is, of course, where the same current “Hallite” type gaskets have been re used and have already had the little compressibility they once had crushed out of them. You might as well use a piece of steel sheet for all the good they are. We already had a few problems when the cylinder base rings that were originally the same cork / rubber construction were changed for O rings. Thankfully we are back with the original type now. I will suggest to Graham that he investigates reverting to the original material for the transfer port gaskets. No man is perfect, but if you have any pride in your work, the last thing you want is for it to have problems. Without the availability of the original type gaskets, I have two options that I can feel confident with. I either set up the case and barrel together and re machine the block and case TCP face4s as a set (expensive) or use silicone. I do understand that some folks do not like silicone, or Loctite, or Nyloc nuts etc, but I have now had a Scott on the track for more than 40 years and it works for me. Unless a customer gives me strict instructions on such matters, we work on the premise that they have entrusted their rebuild to me on the basis that I will do for them what I would do for myself. Please understand that I use these methods because I have had TPC gasket failures several times and having investigated, sought the most logical, practical and economical method to ensure the sealing. Lastly, I would remind readers that the question was relevant to the anchor area for the TPC screw having been cracked and thus it was important to both fix and seal the TPC without overtightening the center screw. For this objective, the adhesive properties of the silicone are most suitable. We all have our own philosophies on what is correct, but when I consider Alfred Scott’s achievements in relation to the engineering of the day, it is obvious that he was at the leading edge of technology. I feel that if he had had access to the various helpful materials and machinery we now enjoy, he would have used them in a flash. Sorry to go on at length, but unless you examine a problem thoroughly and understand it, you might as well not bother in the first place.