sherpa325, on Apr 21 2010, 08:40 AM, said:
Hi , I have to agree with Jon , I have two 199A's that have been bored to 340 and both were done with 1.5 thou clearance. I took great care running them in, ie heat up and cool down several times using an oil rich mixture. Both bikes are very quiet and run perfectly. My experience is that the larger bore motors can develop noisy piston slap with clearances that wouldn't cause noticable noise in a 250. My advice is to keep clearances on the small side and be careful running in and you will enjoy a quiet motor and long lasting rebuild.
I spoke with a retired machinist friend who has several top-notch Bultacos (stock, modified frame and a Sammy Miller framed example) and is an expert on rebuilding the Bul engines. He said he sets up shirt clearance at .0015" and has never had a problem. I really trust his judgement so this is why I passed on that recommendation.
On the subject of port chamfering, I often see the port edge cut sharply at a thin, 45 degree angle (just enough to "break the edge", as some machinists say), which will keep the ring edge from catching, but promotes damaging harmonics and wear by slamming the ring back into the land, and in a place where the ring experiences the most heat. In my experience, a better way (using an exhaust port for an example) is to use a 15-18 degree angle with the chamfer about 2-2.5mm in width in the top/center of the port and tapering out to the edges. This way the ring is eased back into the ring land, rather than slammed back when it catches the center of the 45 degree slope. This type of chamfering is mandatory in a roadrace engine, for instance, with big ports, high RPM and high heat, but I've found it promotes longer ring life in any two-stroke. Intake ports are a little less critical as they are usually narrower and have the cooling effect of the intake charge, but I think you'll get the idea.
Edited by JSE, 23 April 2010 - 04:59 AM.