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If your AR5 is experiencing shift blocking or lock-out, your synchronizers are worn (or your clutch is dragging). At around the 200K mile mark, I could not downshift into 4th from 5th but I could go from 3rd to 4th no problem. The only way I could downshift into 4th was to double clutch or as a quicker alternative quickly "bump" 3rd gear and quickly shift to 4th in one fluid motion. This is how I knew the 4th synchro was toast. Bumping 3rd (essentially short shifting) eventually killed its synchro too. A few weeks ago the clutch hydraulic line burst, so I decided to put in a new transmission since I had to pull it to fix the hydraulic line anyways. I have completely disassembled the AR5 and will post a full tear-down guide next week but for now here are pictures of the worn out 4th synchro.

What has happened is the brass blocking ring drive "lug" has worn into the steel hub. This allows the blocking ring to index too far and thus preventing the slider from moving forward even though the synchro has matched speeds. The synchro hub must be made from some really ****ty steel for brass to wear into it.
 

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Thanks for taking the time to post those pictures, I experianced the same issue on my car, but with the LS2 that is not surprising. I am told if you replace the early Brass parts with the carbon fiber parts used in the Hummer versions of the AR5 they hold up better. I skipped the issue and installed a Tremec 6060

Are you rebuilding yours?
 

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Sorry for taking so long to reply. The hummer synchros wont help. They are just carbon fibre lined brass rings. GM did this to help absorb the additional kinetic energy from the bigger clutch disk.

The failure you see in my photos is due to a hardness mismatch between the brass ring and the steel hub. The brass is harder than the steel hub because the brass is likely an aluminum-brass alloy and the steel hub is probably made from an easily forged low carbon mild steel. Reference- I'm a machinist.

If we want a better synchros for the AR5 it's going to cost a pile of money for CNC billet parts. Better just to swap in another transmission.

As for rebuilding the AR5, the answer is NO unless you have large hydraulic press. Everything is pressed on the shafts, and I mean pressed heavily! This is not something you're going to knock out in your garage and I have rebuilt automatics in my garage no sweat. The AR5 is a whole 'nother animal! My 10 ton shop press couldn't budge the 2nd gear off the countershaft. I gave up and sliced it in half with a cut off wheel just to inspect the synchro.
 
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