I don't think there's any question that the longer the extension, the higher the liklihood of problems caused by rotational forces. Keep it as short as possible and still allow proper engine cooling. Lighter is better, so aluminum rather than steel. Normally 2024 is best, but I used 6061-T6 and it's fine too. Others with experience on lengths that have worked on various engines are welcome to chime in, I just thought I'd throw out an overview. Bottom line is that unless you are building one exactly by somebody's proven methods (GPASC or William Wynne), we are all experimenting and act as test pilots. Below is something I posted to CorvAircraft this morning:
> See http://home.hiwaay.net/~langford/corvair/crank.jpg . WW was here > last > night (happened to be on the way back from SAA) and suspects the prop, > since > it's not a CNC prop, and has been repitched by hand. The break is just > inside the front of the case, right through the edge of the rod journal > (the > radius). You can see from the location of the break that if the prop was > the problem, the long moment arm of the extension will magnify it. He > doesn't attribute the problem to the rear starter setup at all. This > crank > was done by him, so the radiusing is done right (I'd forgotten that > earlier). Right now I think I'm going to build a 2700cc engine, and use > the > same rear starter setup (but stiffen the ring gear some more). I'm still > thinking on the prop hub thing. WW says he put it back together with a > known good crank and prop and try it again. But that prop was magnafluxed > (not to say what quality) by the local machine shop. He's going to sell > me > one that he knows was done right, and I'll put a CNCd prop on it. Still > wondering about the prop extension though. It's tempting to keep it stock > WW. > > Three crank breaks have been in the same place (first rod journal), the > other (Bob's second) was one journal further rearward. WW thinks that > since > the three are all broken at the front, torsional vibration isn't the > cause. > I'm going to contact a tosional vibration expert at TCM and see what he > thinks. At this point, I'm tempted to pay him to take a hard look at it > (others may want to get in on this), and make sure we're not doing > something > stupid with the rear starter setup. I'll also start looking into DYI > torsional testing, if there's any such thing. > > More later, but this weekend I'll tear down my 95 hp engine to figure out > what I need to order Monday morning. Tearing this one down will be later, > but I don't think I'm going to find any spun bearings or that kind of > stuff. > Oiling has been fine with this thing... Mark Langford, Huntsville, Alabama see KR2S project N56ML at http://home.hiwaay.net/~langford email to N56ML "at" hiwaay.net
