Xflow crank/flywheel balance

[Olgawld]

Was looking into whether I need to get the crank and flywheel balanced on my Supersprint 1700 Xflow rebuild. Noticed there are already a few holes drilled into the weight parts of the crank and arrows in the crank, flywheel and clutch cover to match things up. Does this mean it's already been done at the factory? I know all cranks are roughly balanced but is it important to get it balanced more accurately when rebuilding to a higher spec? Been quoted £180 to get it balanced which I'd rather spend elsewhere if it's already been done good enough. Thanks

[Roger King]

It's an impossible question to answer because it depends on how well it was done originally. Personally, I always balanced the assembly so I could be certain, but some needed more work than others.

If you do decide to balance, make sure that the crank is done first and then add the flywheel. That way in theory at least, you can swap the flywheel for another balanced one and things will still remain correct. I have encountered some balancers who do the whole assembly in one go, but this means you can't change anything without starting again.

[Olgawld]

Thanks for your reply as always Roger. The rods have been lightened and balanced with the pistons, all within .5 gram of each other. The crank looks like it's been balanced as has several holes drilled in various places and marks to line up with flywheel and clutch cover. Am I right in thinking though that the crank needs balancing together with the rods and pistons and as the pistons I'm using are new forged so lighter, then it would all need balancing again? Thanks

[Roger King]

No, on a straight four cylinder engine, the crank and flywheel assembly should be in balance as an assembly, but it is only necessary to have all pistons/rod assemblies balanced so that they are the same as each other. By this I mean that all pistons should weigh the same as each other, and all rods should weigh the same as each other (and each rod should have similar mass distribution end to end, or at least as near as is practical. What I'm trying to say here is that you don't want rods of equal weight, but with some having a lot more mass at the little end and some having more at the big end).

If you think about it, the crank and flywheel assembly will be in balance without the pistons/rods attached, but when you fit them, two piston/rod assemblies go up at the same time as the other two are going down, thus canceling  each other out; therefore their absolute weight is not important. In any case it's impossible to get any engine in perfect balance in all respects, and some, such as a V6 are much more involved.

In all likelihood, the crank will have been drilled by Ford during manufacture because, contrary to popular opinion, they do balance them at the factory, although the alignment marks suggest the assembly may have been done again at some point.

[Olgawld]

Thanks Roger. What you say of course makes perfect sense. Always grateful of your advice, cheers.