In June 1973, the British hard-rock quartet UFO landed in Germany for a tour — as a trio, because their guitarist suddenly quit. UFO had already gone through two other guitarists since forming in 1969. But it was fourth time lucky when singer Phil Mogg, bassist Pete Way and drummer Andy Parker borrowed local teenage dynamo Michael Schenker, then in Scorpions, for the German dates. Schenker soon joined UFO full time, flanking Mogg’s tough-glam bray with meaty, melodic riffing, and charging the band’s boogie locomotion with lethal, articulate soloing on a run of albums — 1974’s PhenomenonForce It and 1976’s No Heavy Petting, all reissued with bonus tracks (Chrysalis/EMI) — that became holy text for the spandex boys just around the bend, including Def Leppard, Iron Maiden and Guns n’ Roses/ UFO were, at the start, better jammers than composers (ballads were essentially breathers between cannonballs), and Phenomenon opens tentatively: “Oh My” sounds like it reads. But that album’s heavy-Yardbirds assaults, “Doctor Doctor” and “Rock Bottom,” are two of UFO’s — and Schenker’s — best moments on record. By Force It, the writing was sharper (“Let It Roll,” “Shoot Shoot”) and bolder (“Out in the Street”). No Heavy Petting came with keyboards and a poised mix of crunch and radio-wise pop that paid off in later FM hits like “Lights Out” and “Only You Can Rock Me.” Those two songs are on The Best of UFO (1974-1983) (Capitol), out soon to coincide with a U.S. tour by the current lineup of UFO (which will not include Schenker, who has been in and out of the band a few times since 1979). That set has everything for the novice, but the reissues — with seventeen extra tracks spread over them, including period demos, outtakes and hot, live juice — are the real phenomenon.