Quote:
Originally Posted by undy
I guess I need to tell my builder to dump his 5 axis CNC block machine and start looking for the "accurate" Ford core-shift specials.. I'd heard for every "good" 427 block that was made one went back at the factory for scrap and melt down due to tolerance/casting inconsistencies. I'll take any knowledgeable qualified, CNC equipped shop over Ford of yester-year (60s & 70s). For quality, metallurgy and a host of reasons.
Dave
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Those 5 axis CNC machines didn't stop Genesis from sending slews of mis-machined blocks to all sorts of engine shops - among a number of problems, cam tunnels were so bad they came in from one end half way and then from the other end to meet up and they were off some horrendous amount! The important aspect is shops like Genesis, Dove, etc send stuff out like this, vs Ford would tend to catch this so that when you received a part it would go together without jumping through major hoops.
Core shift is a tolerance just like any other dimension and parts at large companies like Ford that end up outside the BP range are reviewed by educated specialists to determine whether they are acceptable to send out as is, rework or scrap......I would rather have that scenario than a high school educated machinist at a small shop with a relatively narrow scope of experience to be making such calls. Whether it be aircraft or autos, parts are custom selected for the most demanding applications and always have been. Presidential aircraft are not allowed to use components that have been through the "MRB" (Material Review Board) because of non-conformities and even the best automotive race engine parts made today are individually inspected and picked by the top NASCAR teams.
There is alot of BS out in the automotive field. I did some fairly extensive research and analysis of rocker arms for a vintage factory Shelby American race engine I recently finished. The general philosophy with the engine was to stay as stock as possible, but not expose myself to excessive risk that could destroy castings, etc that could not be replaced. My initial thought was to go with a steel roller tip rocker, but after multiple spread sheets of rocker kinematics, mock ups and phone conversations with the top designers at most of the rocker arm companies, leading race engine shops (incl NASCAR and an F1 designer) and others, the stock Ford rocker surfaced as the best solution - just with some minor tweaks like polishing, different adjuster and special coatings. It turns out that roller tips do initially roll, but as engine speed increases, the inertia of the roller tip going in one direction can't be immediately stopped and started in the opposite direction, so they skid. This negates the benefit and puts excessive contact stresses on the valve tip due to the much more localized point contact of the relatively small radius roller. In addition, the extra mass of the roller and it's supporting structure force you to run notably more spring pressure to reach the same RPM.....which puts more stress on the rest of the valvetrain components. If you look at the rocker arms used by the top NASCAR teams, they are very similar to the almost 50 year old FE part being made out of a steel alloy with a pad tip and "fin" on the backside as the most efficient way to generate stiffness. One top designer at a rocker arm company said he doesn't want his identity revealed, but they mainly offer roller tip rockers (often aluminum, which is not good for this) to meet demand from the low buck engine builders who aren't able to determine optimal configurations.