2000 M6 141k, transmission fluid changed at 90k and looked brand new. Car is driven fairly gingerly but often. Mods are fairly tame(check my profile)

Was cruising a few days ago around 40 MPH and I am almost certain I was in 4th gear when my wife and I heard a hollow thunk from underneath the car. I thought I had run over debris but I saw nothing in the mirrors. Got out, checked everything, seemed okay. Then we went to get on the highway and 5th gear would grind and 6th was locked out. I figure the 5th/6th synchro died, so we drove home which was not far, just a few miles. The feeling I had was confusion as I was in 4th. 1-4 and reverse all work fine. Very rarely 1st doesn't want to engage at a stop but rights itself when I just shift into 2nd and back to 1st. No noise from the transmission.

I've done some reading and getting the feeling I need to pull this thing and tear it down in order to determine a route for replacement parts

Is this correct?

I'll assume I will need to swap the slave cylinder, bleeder kit, billet dog bone, upgraded rear plate + new seal, rebuild the torque tube(?) and also replace the clutch with an LS7 unit or something.

A T56 magnum was planned for when the clutch died a bit later, however I wasn't planning on spending that kind of money this soon(I could afford it, just haven't warmed up to the idea). So, I was thinking just rebuild to stock? Any opinions will be useful.

Thank you for reading!

 

Spend your money on things that make you happy. Do you plan on keeping the car, and maybe put another 100k miles on it.

Are you doing the work yourself.

One and done my friend.

Buy that trans, clutch, and rebuild the torque tube. Call RPM transmission.

 

The car will be kept forever and 100k, yeah easily probably.

I do all the work myself.

 

Check out the T56 rebuild video series from XPs Garage on YouTube. He also had problems with 6th gear. Great step by step instructions. If/when I decide to go into my trans or rebuild a replacement unit these videos will be my go to reference. Tick makes a good rebuild kit.

 

OP, before you get carried away, consider that it might be a simple problem. IMHO, step one should be to check and see that your shift linkage is still in place and tight.