I have rebuilt stock twins successfully. They are 100% rebuildable items... 50% of the time . But just like any other mechanical item there is a point of no return. It would suffice to say that if they are already smoking, new seals and bearings aren't going to help. That's because the groove in the turbine shaft is probably worn out as well as the seal.

If one could buy a new turbine and center housing, I would imagine rebuilds would be much more successful, but you can't and I think the reason they so often fail is that builders use cores that aren't rebuildable in the 1st place.

People make it sound like there is some mysterious phenomenon that makes a stock twin rebuilds work 1/2 the time and not work the other 1/2. That's BS. They're pieces of metal. They need to be spec'd out and not just eyeballed and when in doubt high wear parts should be replaced with low mileage parts from another core set.

and as far as those things go, I'd stick with the stock turbines....