- Tue Aug 21, 2012 11:56 am
#16459
Hi m1ks, Like you I have no desire to fall out with anyone here, first thank you for your comprehensive explanation, as I said earlier I have to a degree lost track of this thread due to multiple similar posts, the comment regarding 350 seizures was a follow on to MadMikes post I wrongfully assumed that another poster had said "they all do that" to which I do not agree. --------- again I am not trying to be clever but I have been repairing., tuning for racing and latterly restoring old British singles and twins for more than 50 years now, At the risk of teaching my Granny to suck eggs, modern engines can run much closer tolerances due to better matching of materials and water cooling ETC, modern engineers also are more used to working to these closer tolerances and consequently I find that a lot of rebores on these old style engines are too tight, you point out that the bike was rebored prior to you buying it. -------- from your original post and the pictures shown it is evident that the piston has been excessively hot which has resulted in the seizure, also from the pictures there is no lack of oil circulating, the seize it appears happened after a run at a higher consistent speed than previous rides, what happens is the piston expands and begins to drag in the bore this then requires more throttle to maintain the same speed, this very quickly becomes a vicious circle with the result you have seen, the answer is to increase the bore size to prevent this happening again, or you can run the bike rich which does two things the extra petrol thins the oil on the cylinder wall causing it to run in faster also the extra petrol cools the piston to a fair degree keeping it from expanding so much, with experience you can feel the engine begin to loose a little power and pull the clutch in and stop to let it cool down before any damage is done. ------ I have had two BSA singles already this year with the same problem, this is why I do not dispute that you had the bike set up correctly but what I do say it that correct in this case is not what you want, let it run rich until fully run in then put it back to standard and run very carefully feeling for any sign of that power loss, if you detect it tightening run it rich again for some more miles. All the best and good luck with it.