Where portion and part are not interchangeable is the times when you can not cut the devision just anywhere. That is why in Bretts example, a 'part' of a motor is not a portion. The part came off and can go back on. If you can take a portion, you can take .1% through 99.9% without destroying it.

Can somebody tell me the difference between 'part' and 'portion' Sometimes you can use either word. 'You can take a portion of credit for the success' OR 'You can take part of the credit.' The word 'portion' is often used to describe food; 'a portion of pie,' for instance. 'Part' can be used to describe a piece of machinery; 'This part comes from a motor.' Also you can say 'part of the problem,' not 'a portion of the problem,' but you CAN say 'a portion of the blame,' OR 'a part of the blame.' I hope this is some help, I agree it is confusing.