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Encountered similar issues in my '17 1SS with the M6. Acquired the car "nused" with only 20,500 miles in Jan 2022. Upon 1st drive, I noticed 1-2 shift synchro issues. Thought it was me but issue repeated. Took to local Chevy dealer/service dept and got the "cannot duplicate." Car was just outside of the 5yr powertrain warranty. Chevy was willing to fix it and work with me on the bill since it was only outside warranty by 3 months, but they refused to allow me to use Tremec's carbon hybrid synchros that I wanted.
Took it to a local performance shop and they initially said the same "cannot duplicate." Had the 3rd party warrantor's underwriter come to shop for a ride along with the mechanic. Same result. I insisted on the removal and tear down of the trans to show the underwriter the problems first hand. Though the car had really low miles, someone had been hard on it. I don't know if it was rough 1-2shifts from the beginning, trying to force through the skip shift, or they were just unusually hard on the trans. Results: Worn synchros, worn teeth on 2nd and 4th gears.
Replaced the worn gear sets and upgraded to Tremec's carbon hybrid synchros for 1st thru 4th. Went with bronze shift pads, new flywheel, Tilton clutch and Hurst short throw while trans was out. No issues since. ...11K mi later, reverse didn't work. Troubleshot skip shift eliminator, reverse lock-out solenoid and back up light switch. Turned out a magnet from trans case came apart and lodged itself to block shifting into reverse. I'd recommend removing the magnets and going with magnetic drain plugs if you ever open up one of these for any reason. Other posts have addressed this magnet issue before.
Note: the 1-2 shift feels worse if you pull it back from 1st to 2nd with side pressure towards you as the driver. The 5th to 6th shift with side pressure towards the pass seat can feel equally crappy. Learn the angle and pull straight from 1st-2nd and 5th-6th... smooth like butter.
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