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Smith, a two-time ARCA East series champion, held off an unexpected late charge from Ryan Truex and took the checkered flag by 0.641 seconds to earn his first Xfinity Series victory.
Smith, driving the No. 18 Toyota for Joe Gibbs Racing, grabbed the lead for the final time on Lap 149 after getting around veteran Kyle Busch.
Smith, 18, then had to hold off various challengers over three more restarts to cement the win. Smith led a race-high 92 laps. He had led a total of 32 laps in 12 previous starts.
“It’s amazing, just a dream come true,” said Smith, who is also finishing up his senior year in high school. “Thanks to all the guys on this JGR team for giving me this opportunity.
Truex, who is running a partial schedule for JGR in the No. 19 this season, said he was caught off guard by the changes in the track.
“In that long run, we were really bad and I was just hanging on,” Truex said. “But Jason (Ratcliff, crew chief) and these guys let me complain on the radio and made the right adjustments and had a good restart at the end.”
Sheldon Creed rallied from a spin earlier in the race to finish third, Riley Herbst was fourth and Chandler Smith rounded out the top-five.
Completing the top-10 were John Hunter Nemechek, Austin Hill, Josh Berry, Busch and Daniel Hemric.
Following the break between Stages 2 and 3, a handful of cars including Justin Allgaier elected to pit but Hill remained on the track and inherited the lead.
The race returned to green on Lap 101 with Hill followed by Hemric and Sammy Smith. Allgaier lined up 30th.
Busch used a three-wide move on the restart to power into the lead.
Sammy Smith went to the inside Busch and came away with the lead on Lap 103.
By Lap 118, Allgaier had made his way into the Top 15 as Sammy Smith built a more than 2-second lead over Busch.
On Lap 139, Gray Gaulding hit the wall and came to a stop on the track which brought out the eighth caution of the race.
The lead-lap cars all pit with Hill the first off pit road. On the restart on Lap 148, Hill was followed by Busch, Sammy Smith, Nemechek and Herbst.
Just after Sammy Smith reclaimed the lead, Creed spun out in the middle of the field to put the race back under caution on Lap 150.
The race resumed on Lap 157 with Sammy Smith out front and followed by Busch, Hill, Nemechek and Allgaier.
Anthony Alfredo spun in Turn 2 on Lap 171 to bring out another caution. The race returned to green on Lap 177 with Sammy Smith still out front and followed by Nemechek and Busch.
Kaz Grala and Allgaier wrecked after contact from Brett Moffitt just past the start/finish line to bring out the 11th caution of the race.
Sammy Smith remained the leader on the restart with 15 laps to go. He was followed by Busch, Berry, Hill and Herbst.
With 10 laps to go, Sammy Smith remained out front but Truex had moved into second, less than a second behind the leader.
Allgaier held off Hill on fresher tires to take the Stage 2 win, the eighth time in his career he has swept both stage wins in a race.
Hemric was third, Sammy Smith fourth and Busch rounded out the top-five.
Following the break between Stages 1 and 2, most of the lead-lap cars pit but Sam Mayer was among those who stayed out and inherited the lead.
On the restart on Lap 56, Mayer was followed by Nemechek and Hill.
The race almost immediately was placed back under caution when Moffit spun around in Turn 4 after contact with Jeremy Clements. Nemechek was ruled the leader when the caution came out.
When the race resumed on Lap 63, Nemechek was followed by Mayer and Hill.
Connor Mosack spun and wrecked in Turn 2 on Lap 69 in an incident that also collected Josh Williams to bring out a caution.
The race returned to green on Lap 75 with Nemechek still in the lead.
Nemechek was black-flagged on the start for going below the yellow line before taking the flag and had to serve a pass-through penalty. The penalty handed the lead to Hill.
On Lap 84, Berry spun in Turn 2 after contact with Mayer to bring out the sixth caution of the race.
Most of the lead-lap cars elected to pit with Hill first off pit road. Allgaier remained on the track and inherited the lead.
The race returned to green with two laps remaining in the stage.
Allgaier held off a late-charge from Creed in a two-lap shootout to claim the Stage 1 win.
Sammy Smith was third, Herbst was fourth and Hemric rounded out the top-five.
Cole Custer started on the pole and took command early, running out to a 2.6-second lead over Allgaier after 20 laps.
On Lap 29, NASCAR was forced to display a caution for debris in the dogleg area of the track, which appeared to be the rear bumper cover from the No. 45 car driven by Leland Honeyman.
Most of the lead-lap cars elected not to pit and Custer remained in the lead when the race returned to green on Lap 36.
Allgaier went to the outside of Custer on Lap 38 and claimed the lead and the race went back under caution on Lap 40 when Berry spun around in Turn 3.
Several lead-lap cars pit but Allgaier remained on the track and in the lead. On the restart on Lap 44, Allgaier led the way followed by Custer and Sammy Smith.
Busch had to start the race from the rear of the field as he did not take part in qualifying. His No. 10 Chevrolet had issues with its fuel pump.