Elton appeared via video on the Howard Stern Show and brought one of the guests to tears.
He explained why he volunteered to play piano on Metallica's Nothing Else Matters, which he called ''one of the best songs ever written.''
The 74-year-old, who appears on the new covers album, The Metallica Blacklist, brought the band's frontman, James Hetfield, to tears when he said: ''It's a song that never gets old. And playing on this track, I just couldn't wait. The chord structure, the melodies, the time changes, it's got drama written all over it.
''I have to say, it was a no-brainer playing on this song. Metallica are probably the creme de la creme of those kind of bands. You can't really define them. They're not a heavy metal band. They're a musical band. Their songs aren't just heavy metal. They're beautiful songs. This is such a melodic songs--it's f****** great, actually.''
Drummer Lars Ulrich responded: ''If you could have said 40 years ago when James and I started . . . that 40 years later we would still be functioning, we'd be sitting here with one of the greatest stars in the world (Cyrus) next to us, with Elton on a Zoom, and doing your radio show, and talking about the accomplishments and what this record means to so many people, that would be the mindf**k of all mindf**ks.''
He added: ''To be able to hear what you just said, Elton, sitting next to James . . . the reaction I can feel in his body language when you said it was one of the greatest songs ever written . . . it's just incredible to hear.''
Thursday's Howard Stern Show also saw Metallica and Miley Cyrus performing Nothing Else Matters, with the young woman taking lead vocals and James backing her up.