1. Cranks are not internal or external. They are just cranks. How the assembly needs to be balanced is partially determined by crank design and material. cast cranks and heavy pistons and rods generally need to have external parts to balance them, but any crank can be internally balanced by adding heavy metal (called Mallory metal) to the counterweights. I don't believe a true performance engine build should have externalbalancing. But that's me.
2. I'm aasuming you mean for a small block? If you're used to GM castings (sorry 67
) then you know why they needed 4 bolt caps. The iron is less than perfect. Mopars have always used better iron in thier blocks. Also, the main bolts IIRC are larger in Mopars. The factory small blocks do not have the provision for 4 bolt mains. That is, they dont have the extra meat to really make 4 bolt caps stronger and more stable. T/A version blocks have this extra meat, but were 2 bolt from the factory. The newer MP X and R blocks have 4 bolt mains because they were designed with them in mind. Run a good set of caps (I use Pro Gram), run studs as opposed to bolts, and you are fine for up to 600hp N/A. More than that, you really should be buying a new block anyway.
3. All depends ont eh package. A good crank is a heavier (more dense) material, and so weighs more. That means the counterweights can be smaller, or shaped for better windage control. Also, lighter pistons and rods mean the counterweights are too big anyway (what you want...) and then balancing is only drilling out material. In all cases, I prefer my own mahcine sho pto balance, because I know what equipment they use, and I have seen them use it. My assemblies are always internally balanced to within .2grams. A factory assembly (say an internally balanced forged crank 340) will be + or - 25 grams. And I've seen work from other shops where they considered being + or - 10 grams as "In balance". The finer the result, the smoother, more powerful, and longer lived the engine will be.
4. You cannot do the work yourself. Period. If you have the fixture to seperately weight the big and small ends of the rod, and a scale that is accurate to .001grams, you can make the process cheaper. But...Once you have all the weights, and plug them into the formula, then you still have to fit the crank with the bob weights, and spin it to find the ehavy spots, then be able to remove the proper amounts from the exact right spot. Old machines use a strobe light to mark the right spots. The place I use has a computer run spin balancer, that tells you how big a diameter hole based on material, and how deep t go to remove the exact amount. It costs me $300 for internal balancing, and additional for any Mallory metal used. If you are running the MP 4" cast crank, you will need Mallory metal.
