Halifax took the lead against the run of play in the 51st minute through miniature midfielder Jamie Paterson. But with the Rebels having missed three great opportunities in the first half, it needed West to keep Slough in the competition with a goal six minutes from time.
Anything less than a draw would have been an injustice after Slough had given the Conference leaders their toughest test at home this season. The Rebels should have gone ahead after four minutes when Corey Browne cut in from the left and fined across the face of goal from an impossible angle with unmarked strike partner Gary Abbott begging for a pass.
When Halifax mounted a rare attack after half an hour of one-way traffic, Paul Wilkerson brought off a tremendous save from Halifax's 28-goal marksman Geoff Horsfield who turned in the box to fire from point blank range.
Abbott then had a drive blocked on the line by Joe Brown and Paul Hardyman smashed the rebound at the opposite post where Bolt missed by a Rizla getting the necessary touch. Desperate remedies were required and Halifax boss George Mulhall's prescription included the addition of ex-Coventry defender Brian `Killer' Kilcline.
The arrival of the charismatic Kilcline gave Halifax a new, but temporary, lease of life and the Shaymen took the lead with a well-worked move after an imaginative crossfield pass from Kilcline gave Horsfield a run down the left.
The ball was cleverly worked out of a tight situation and Halifax's latest signing from Leyton Orient, Dave Hanson, flicked the ball on for Paterson who controlled well before tucking it away from the edge of the box.
But if the appearance of Kilcline changed the course of events, so did that of West who had scored twice against a star-spangled Reading side three days' earlier. Three minutes after coming on he shot into the side-netting after Bolt had ripped through the midfield before slipping to ball into his path on his right.
But Halifax keeper Lee Martin was nowhere when Smart's free kick in the 84th minute was flicked on by Bolt to the perfectly placed West who headed across goal into the right corner. Slough immediately settled for the replay, but needed a goal-line clearance from Terry Angus to make sure when Kilcline gave Mark Bradshaw the opportunity to kill the Rebels off in the dying moments.
The result made Slough the only other club apart from Hednesford Town to deny the Shaymen victory in front of their own fans this season.
Halifax Town Line up
Martin, Thackeray, Bradshaw, Mu O'Regan, Stoneman Paterson, Hulme, Han Horsfield, Brown. SUBS: Lyons (87, for Hansoi Homer, Kilcline (45, for Hulme).