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@IDoNothing
Too long can’t be arsed to read summary: I like powerbites.
I run primo powerbites on my polo bike, mainly as I already owned a set. But thats because imo they are the best bmx cranks (having also run on bmx profiles, wethepeople royals and éclat tibias), larger 22mm axle, stiff one piece arms, pinchbolt design helping limit excessive side load on the bb bearings if your tube spacer isn’t perfect, no chance of developing 'wobble'(48 spline cranks shouldn't either if you look after them properly) and generally cheap to pick up second hand (£25-40). They aren’t without their flaws, the pedal inserts can come loose (loctite can sort this though), wider Q factor than many others, a bit porky, and the combination of a 22mm axle and euro bb shell results in the bb having very small bearings.
The last point kind of brings up the external bb issue. On the whole external bb’s for bmx cranks do not exist as it was quickly realised that euro bb’s just weren’t strong enough for bmx especially for 22mm cranks hence the increase in size again to spanish and mid bb shells. So there is now no major demand for external bb’s for bmx. There are a few exceptions like the profile one - http://www.alansbmx.com/product_info.php?products_id=9490 or a few short lived attempts at a spanglish bb.
So ever since I put the powerbites on my polo bike I’ve been concerned about the life of the bb bearings and have been looking for an alternative. I recently learned that the OD on the bearings found in many mtb external bb’s is the same as the OD of Spanish bmx bb bearings, so the mtb bearing cups can be used to hold the spanish bmx giving you an external bb for the bmx cranks. The only problem with this is that this set up will push the chainline out (not sure by how much exactly yet as I’ve not fitted the one I put together, but likely around 5mm), a bit of machining to the cups could definitely cut this down a bit. Or my other thought was as I use a 110mm spaced bmx hub in the rear I could run both the 5mm washers (currently one either side of the hub inside of the 120mm spaced dropouts) on the non drive side of the axle and dish the wheel to move my chainline over.
However, in reality it seems polo does not put anywhere near the same demands on the bb and my euro bb bearings are still in great condition after going on 5 months now. So unless anything drastic happens soon I’m beginning to think maybe the £25 for a new euro bb is easier than dicking around with trying to make an external bb work for bmx cranks. Although I kinda still hope I can get it to work.
If an external bb is more important go with mtb cranks.