Donc – not my mechanicals, my Singular, Niner, Hope, Thompson and Shimano combination worked perfectly, even though it was built as a geared bike on Thursday night and I'd only riden it round the campsite before the race.
I stopped to help a guy who'd blown his tubeless about six miles before the first feed station (long fast fire road descent) and couldn't get the valve out. I couldn't let him walk carrying his bike when I had pilers.
Then I singlespeeded a guy's bike, and then fixed his chain, and then re-did the singlespeeding when he bounced/kicked it into a different ratio.
Spent about an hour fixing other peoples bikes – without which I'd have been marginal on the first cut off, but including it I wasn't close.
Interestingly the Garmin shows where I decided I couldn't make the cut off. Both my heart rate and my speed drop as I go into "admire he view" mode just after the second time at the third water stop (around 44 miles).