In August, I blew a rear bearing in my 424 engine. I foolishly didn't realize that it had the old style rear bearing (lesson learned). At the time it was a very good engine, peaking at about 17.2-17.3 on an APC 9-6. In rebuilding it, I replaced the Piston/Liner, cleaned up the head and of course the bearings. I replaced both bearings with a set from RCBearings.com http://www.rc-bearings.com/catalog/p...roducts_id=748
Prior to the rebuild the engine had noticeable endplay. Now it does not. I even bought a bearing seating tool from Darrol Cady and re-set them again. The bottom end is very free, and the crank rocks back and forth nicely. I have about a gallon of 15% through it at this time and 4 heat races. It was re-broken in on an APC 8-7 running rich at between 16.8 and 17.3K and then with a 9-6 at best mixture. The engine seems fairly strong, though not a world beater. HOWEVER, as the engine heats up, it slows down noticeably. For the first minute after starting, it will run strong at about 17-17.2 and then as it heats up more, it will slow down to 16.7 or so. I noticed this during the heat races as well, it started out well but slowed down as the race progressed.
The RC Bearings replacement front bearing is listed as being .281 inches wide, Tower shows the stock front bearing as .28 wide. http://www3.towerhobbies.com/cgi-bin...&I=LXAHZW&P=ML
There is also text saying that it is 7mm wide. According to my conversion program, 7 mm is 0.2755906 not .281.
Since the coefficient of linear thermal expansion of aluminum (the case) is almost twice what it is for steel (the crank), it is possible that I am getting a bind as the case heats up and that it is what is causing the engine to slow down?
Other than buying a stock replacement front bearing from Tower, I can't think of any "legal" solution to this problem (it's not legal to mill out the front bearing recess). Any thoughts on other ways to fix this? Thanks!