Internal balance vs external balance
 

I am rebuilding one of my twin turbo engines and I am probably going to get a Eagle rotating assembly. Here is the whole story
The port engine was a draw thru carburated twin turbo system. No matter what I did with the old carb I could not lean it out on the top end and it would run pig rich under boost (10:1, 14 psi) I pinched a ring land from detonation and destroyed 6 pistons. The block, crank, rods and pistons are at the machine shop now. What Im being told is that the block will need to be bored to .060 and the crank needs to be turned to .020. Instead of using old parts, I'm going to buy a Eagle boosted application rotating assembly. The flywheel Im going to use is a external balance from a 330 HP BBC and it has the ring gear towards the block (block mounted starter) . Id like to stay with the external balance because I may want to put these engines in my other boat and that application uses a external balance flywheel with the ring gear away from the block (bell housing mounted starter).

Correct me if I am wrong but to balance the new rotating assembly as an external balance, I can use my existing balancer, I may have to have metal drilled out of the flywheel (the flywheel was on a cast crank cast rod 330), but the metal removed would be marginal?

If I do an internal balance crank, I would need to get a new balancer and have my flywheel extensively reworked.

What is the preferred method for a boat? Internal balance or external.

Internal balanced rotating assembly with a neutral balancer and fly wheel is what Id do, have you checked with scat to see if they have int and ext assemblies

I would definitely have it internal balance. Yes you will have to buy a new Damper but it is a very easy job to make the flywheel internal balance. You just have to mill the weight out of it and then touch up the final balance.

Not really sure I would want to go .060 on the block without sonic testing it, especially if it is raw water cooled. Just depends on the amount of corrosion on the back side of the cylinder walls. Around here they get pretty terrible but if you in fresh water only it might not be too bad.

What block is it, and what will the final bore size be..... agreed .060 is a big chunk

It can be done either way and both are acceptable. Internally balanced will cost some extra dollars in mallory metal. I'd be more concerned about making sure the person doing the balancing does it right.

The block is already .030 over

Internal balance will be preffered by most, if the flywheel is an issue you can have them balance both styles or just move starter ring, I would ask around how Eagle has been I was not too thrilled with mine as the clearances were all over and it was a nightmare to get them where I wanted them to be, once crank was real close the other varied a ton! If I did it again I would look at other brands.

totally agree,i no longer use the eagle cranks because the journal size is all over the place.look into scat.

You can get a Callies Compstar for a bout 850 bucks if its the budget, might be a better fit for a boosted application

Are you increasing the stroke as well? If not, and sticking with a 4" stroke, id just use a ext balance GM forged crank. But if youre going 4.25, and it comes setup for nuetral balance, id keep it that way. Whatever you spend doing balancing work to modify the new setup, will help pay for a new flywheel setup and damper. Nuetral balance is superior imo.

As far as your carb issue, Id look into a blow thru carb from Roger at c&s. I think Mike Tkach would agree

FYI, when i upgraded motors from 4 inch stroke to 4.25 , i bought rotating assembly,. motors were internally balanced and i keep them that way.
think crank is scat,,, rods eagles,,, pistons srp's...

Port engine is stock 420 Merc setup ext balance. Stbd engine is internal balance from a previous failure. We snapped a crank in 2 pieces 7 seasons ago on the internal balance engine. It was an eagle 4340 4" stroke crank. I believe internal balance is the only way to go when using non stock combinations but I must stress that execution in bearing fitment and balance are crucial. At 4" stroke either way is acceptable just make sure whoever does your balancing knows their chit.

Lunati Voodoo is a nice crank at a reasonable cost.