Hey Anon,
I designed and made main caps (splay bolt style) for our 525 ci Supercharded Methanol Pro-mod Ford for the last season. The block is a cast aluminium piece, and the caps that came with it were made of 7075-T6. The caps would only last about a half a dozen runs, (not bad with over 2500 horsepower, but it get's expensive). These engines regularly see over 10,000 rpm. The new caps were supposed to be stronger, while keeping close to the originals in weight.
Originally I was going to use 4340, but I couldn't get any in time, so I had to settle for 4140. I machined them in the annealed state, then heat treated them to about 45Rc. They were also stress relieved. The strength at that level is about 150,000 psi, if I remember correctly. I bolted two caps together so that the mating surfaces wouldn't move around that much in heat treating. I also left about 0.010 on the mating surfaces for final grind, and about 0.030 for the final align bore. ON the fastener front, we always use ARP fasteners, and follow their torqueing procedure to a "T". We actually made a jig, and tested their recommended procedure against other commonly used procedures while measuring actual stretch, and it makes a huge difference.
The good news is that they held well, as long as the torqueing procedure was followed. There is only a difference of 2 pounds using the steel caps, and they don't break. Now, the top end.........then a forged block........then a..........and then.......
Good luck,
Fish  |