Bjornstad had eight birdies and one bogey for the round of the day at The Dominion Club, recovering from his early bogey at No. 1 to climb to 9-under-par 135. He is one ahead of four players who are tied for second place at minus-8.
'I played fantastic today,' the 27-year-old Norwegian said. 'I'm driving it well, hitting my irons well and putting it really good.'
Todd Demsey had a 7-under 65 to climb from 35th place overnight into a share of second place at 8-under 136. He was joined by Todd Fischer, Michael Letzig and Nick Flanagan, who each shot 66.
Bob Burns (67), Chris Baryla (68) and Chad Collins (68) were a shot further back at 137.
Baryla had a hole-in-one with a 5-iron at the 199-yard ninth hole, the second ace of the week after Mike Heinen's at No. 11 on Thursday.
Bjornstad didn't need such dramatics. A slow and steady attack gave him the lead.
For the second straight day he found trouble at his first hole: After making a triple-bogey on No. 10 Thursday, he bogeyed No. 1 Friday. He followed the bogey with four birdies in his next six holes, then later made four birdies on his last seven holes.
'I had a rough start yesterday and another bogey on the first hole today. But I didn't let those holes bother me because I knew I had a lot of good golf in my body,' said Bjornstad. 'There is still a long way to go, but I'm just happy with how I've been playing.'
Bjornstad finished 152nd on the money list during his rookie season on the PGA TOUR last year. He has made four cuts in seven starts on the Nationwide Tour this season and finished tied for ninth place four weeks ago at the Livermore Valley Wine Country Championship.
'I'm just happy to be playing well,' he said. 'I've been searching for my game for over a year now. I had been thinking way too much about technique the last year. The last couple of weeks I just told myself I was going to feel comfortable over the ball and just swing it.'
Overnight-leader Denis Watson struggled to a 73 in the second round and tumbled into a tie for 24th place at 4-under 140.
The cut line fell at 1-under 143 with just 60 players moving on to the weekend.