Getrag 420G rebuild?

[Chanman1029]

Does anyone have any resources for companies who might rebuild our 6 speeds? I have stumbled across a shop called metric mechanics, that seems to do rebuild at a rate of $4565, plus your core.

I have this terrible knocking and lashing back and forth feeing that is coming from my gearbox, down hill coasting at low rpm will make the car lurch forward and back as it rolls down the, at any speed, in any gear.

I’d really like to avoid buying another transmission that could have its own issues. What do yall got??

[heinzboehmer]

Couple thoughts:

1. Are you sure it's not your diff? The transmission in these cars is pretty stout and the issue you're describing is usually caused by the diff (or at least that's what the forum hive mind has agreed upon).

2. As far as I know, no one sells the internals for these transmissions, so you either have to piece one together from multiple units or make everything yourself. Probably why that rebuild is so expensive.

3. If you end up swapping your transmission for a used unit, you can consider buying an SMG one. They go for cheaper and should be less abused, as they've been shifted by a computer all their life. Just gotta machine the bell housing, add a couple detents and sensors and you're good to go.

[Chanman1029]

Couple thoughts:

1. Are you sure it's not your diff? The transmission in these cars is pretty stout and the issue you're describing is usually caused by the diff (or at least that's what the forum hive mind has agreed upon).

2. As far as I know, no one sells the internals for these transmissions, so you either have to piece one together from multiple units or make everything yourself. Probably why that rebuild is so expensive.

3. If you end up swapping your transmission for a used unit, you can consider buying an SMG one. They go for cheaper and should be less abused, as they've been shifted by a computer all their life. Just gotta machine the bell housing, add a couple detents and sensors and you're good to go.

I’m 95% certain its not coming from the diff. My diff bushings are poly, I’ve had the car on the lift before and used a stethoscope to listen to exactly where the sounds came from, when the stethoscope was on my bell housing, it wasnt very audible, but when you put it on the bottom of the gearbox, it was extremely audible through the stethoscope. I also highly suspect the transmission because even at a stand still, simply shifting into first makes a loud noise that transmits some sort of energy all the way through the drove line, a very similar noise to the downhill coasting thing.

[heinzboehmer]

Don't want to continue pushing on the diff thing, especially because it sounds like you've done a lot of troubleshooting. But, from what I've read, forum consensus is those types of noises are caused by internal play in the diff. Could be a worn clutch stack or tolerances that have loosened with time, causing it to clunk when the gears mesh (and unmesh).

If I'm being honest, both my transmission and diff are original to the car, so I can't personally add to the discussion. Just reiterating what I've read around here.

Did you by any chance put the stethoscope on the diff when you had the car on the lift? I'm interested to see what ends up fixing your issue, as my car also does every single thing you've described (although the intensity of the clunks descreased SIGNIFICANTLY when I changed out all suspension wear items).

[Chanman1029]

Don't want to continue pushing on the diff thing, especially because it sounds like you've done a lot of troubleshooting. But, from what I've read, forum consensus is those types of noises are caused by internal play in the diff. Could be a worn clutch stack or tolerances that have loosened with time, causing it to clunk when the gears mesh (and unmesh).

If I'm being honest, both my transmission and diff are original to the car, so I can't personally add to the discussion. Just reiterating what I've read around here.

Did you by any chance put the stethoscope on the diff when you had the car on the lift? I'm interested to see what ends up fixing your issue, as my car also does every single thing you've described (although the intensity of the clunks descreased SIGNIFICANTLY when I changed out all suspension wear items).

I cannot remember if I put the stethoscope on the diff, as this was a couple years ago. I would like to believe I did, however in the next couple weeks I am going to take it back to the shop, throw it on a lift and do the check all over again, this time paying more attention to the diff and axles.

This springy sort of back and forth motion my car does while is rolling forward down hills can get pretty violent, so much so that I have to push the clutch in to stop it or else I fear it will start messing up the engine

[jet_dogg]

Stethoscope is such an underrated instrument.

If you have a dealer hookup you could get a reman from bmw for 6200 rather than paying 4600 for some dude to tear yours apart.

[simonnim]

The springy back forth has happened to me too. When there's a drop in the road at slower speeds. Or sometimes off a red light and turning. Pretty infrequently, but it happens. I'm 99% sure it's the diff. It almost feels like you're just stomping the has pedal and letting go right away.

At times on tight right turns where I'm almost to lock I'll get grinding coming from the diff. Or some weird sound. Also my right rear input seals have a little bit of fluid seeping out.

The only other thing is the axle is bad, which I think it is also causing some weird engagement. Anyways, I really think you should look into the diff.

[sapote]

I have this terrible knocking and lashing back and forth feeing that is coming from my gearbox, down hill coasting at low rpm will make the car lurch forward and back as it rolls down the, at any speed, in any gear.

I don't think a tranny can do this. Next time just turn off the engine and I bet it won't jump forward and back.

Don't go around and ask people to rebuild it as they will be happy to get pay. If tranny is not bearing noisy, and not grinding when engaging the gears, then leave it alone and hold tight on your money.

[Mr.wReckless]

Not sure how much a used SMG trans goes for, but could you not get one, swap your bell housing over and see if the noise persists? Surely got to be cheaper than 4.5k?

FWIW I have redone my clutch and diff, and with wheels off the ground in neutral the wheels turn slightly, but I can stop them spinning with my hands. I can only attribute this to internal friction in the trans that allows for a slight level of drive to go through even in neutral, but it does explain the slight driveline jolt/noise you feel when stationary and engaging 1st gear.

Regards

[tnord]

this is one of those things that's "common knowledge" that I just can't accept. My synchros are very slow and have basically given up at this point. I don't know if I'll find a used one to throw in, or just go whole hog and get something like this;

https://www.turnermotorsport.com/p-339947-rebuilt-manual-transmission-e46-m3/?pdk=AQE

somebody is out there rebuilding them and offering parts for it if this is available.

[Slideways]

this is one of those things that's "common knowledge" that I just can't accept. My synchros are very slow and have basically given up at this point. I don't know if I'll find a used one to throw in, or just go whole hog and get something like this;

https://www.turnermotorsport.com/p-3...46-m3/?pdk=AQE

somebody is out there rebuilding them and offering parts for it if this is available.

I'd imagine it's a shop that specializes in transmissions located near TMS. Otherwise, OP mentioned these guys -

[Slicktop]

this is one of those things that's "common knowledge" that I just can't accept. My synchros are very slow and have basically given up at this point. I don't know if I'll find a used one to throw in, or just go whole hog and get something like this;

https://www.turnermotorsport.com/p-339947-rebuilt-manual-transmission-e46-m3/?pdk=AQE

somebody is out there rebuilding them and offering parts for it if this is available.

I really need to figure this out too. My synchros are bad in 3rd and terrible in 5th. I can't for the life of me find a way to re-build them because of how these trans have been made.

[Chanman1029]

I don't think a tranny can do this. Next time just turn off the engine and I bet it won't jump forward and back.

Don't go around and ask people to rebuild it as they will be happy to get pay. If tranny is not bearing noisy, and not grinding when engaging the gears, then leave it alone and hold tight on your money.

this transmission does have a slight grind going into second sometimes, and very rarely will it be hesitant to go into a gear, where I will have to put it in a different gear and then go back to my desired gear. Not sure if that changes your opinion on things.

[Chanman1029]

Stethoscope is such an underrated instrument.

If you have a dealer hookup you could get a reman from bmw for 6200 rather than paying 4600 for some dude to tear yours apart.

It’s definitely extremely helpful, one of my favorite least used tools in my arsenal.
I do have an old friend that works at a dealership, but just haven’t figured out if id rather buy a whole reman one or a rebuilt one. Of course, first I need to do a litte bit more investigation into this.

[Chanman1029]

I really need to figure this out too. My synchros are bad in 3rd and terrible in 5th. I can't for the life of me find a way to re-build them because of how these trans have been made.

Ive come to the conclusion that a diy rebuild is pretty much impossible. These transmissions were supposedly assembled in a manner that tolerances are so tight that special pulling tools are required to pull apart and press everything back together.