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DIY: SMG to 6MT Bellhousing and Transmission Modification - Revised 2020

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  • DIY: SMG to 6MT Bellhousing and Transmission Modification - Revised 2020

    This DIY will be mostly a rehost of Rontgen's post on M3Forum from 2014. There some changes I will be adding to correct issues with this original post.

    I've recently coverted an SMG transmission bellhousing for the use with a 6MT swap. Most of this info is on the internet, but it's scattered all over the place. Nothing in this DIY is necessarily a breakthrough, but just and effort to consolidate all of this info into one spot.

    Background

    Our E46 M3 use the same Getrag 420G transmission in both 6MT and SMG variations. While they are the same transmission, there are differences between them - notably in the bellhousing section. There are two cast holes in the bellhousing for all 420G bellhousings with Getrag only machines those holes in the 6MT units. In order to prepare an SMG transmission to be used as a 6MT, these holes must be machined and the associated hardware added.

    SMG Transmission



    Manual Transmission



    The portion we'll be adding to the SMG bellhousing is the centering assembly. This is what provides the "spring" function as you move your shift lever from side-to-side (laterally). The following pictures detail what happens when you laterally move deflect the shift lever.



    As you move the shift lever, the arm rotates. The two machined stops (labeled stop in the above photo) provide the stop when you reach the far ends of the shifter movement. Keep in mind these stops are already present in the SMG unit, so no need to do anything with them.

    The two locking pins (labeled P in the above photo), provide the resistance you feel when you shift toward 5th or Reverse.

    The torsion spring provides the resistance you feel when shifting from the 3-4 gate to the 1-2 gate. This spring is what causes the shift lever to center on the 3-4 gate when you drop into neutral.

    That's the basics of how this all works, now onto the parts and process needed to convert the bellhousing to manual.

    Parts you will need from BMW
    (1) 23111222979 - Locking Pin
    (1) 23311228393 - Compression Spring
    (2) 23127527439 - Sealing Cover
    (2) 07119934624 - Lock Ring
    (1) 23311224130 - Locking Pin
    (1) 23111222720 - Compression Spring
    (2) 23117542726 - Pin Bushing Rebuild Kit

    Parts you will need from alternate sources
    Torsion Spring - McMaster-Carr (www.mcmaster.com). Part Number 9271K125
    Bearing Wheel - McMaster-Carr (www.mcmaster.com). Part Number 5972K91
    Spring Detent Pin - Fastenel Part Number 40068*

    * The spring detent pin is the only part that you cannot buy off the shelf ready to use. This part comes as a steel 30mm long rod that needs to be cut down and a groove added. Use the pictures and your own transmission to figure these dimensions out. You can use a drill and a file to make your own lathe or you can have a machine shop turn you out this part.

    Machining the transmission bellhousing

    Now you will need to find a machine shop willing to tackle this. Here is the drawing you will need to give them to explain the machining process.



    This is obviously in millimeters, but you may want to point that out to them to ensure that they are capable of machining everything to the exact spec.

    The other piece of this machining process is to add the 8mm diameter hole for the shift pin. The hole needs to be .75" deep.

    Assembly

    Once you have to all back, it's time to put it together!

    Thoroughly clean the bellhousing use solvents, water and compressed air. Ensure all metal chips from the machining process are removed. Also ensure all solvents are rinsed away and that all water is 100% dried. Coat your internals with MTF if you plan to let this sit. The components inside are mostly raw steel and they will start to rust if you do not do this.



    Start by driving the locking pin sleeves into place. They are driven all the way to the bottom of your newly machined 18mm holes. The sleeves are intended to be driven in with a detent tool 234023 tool found in the ZF 320z detent repair tool kit.



    Locking pin placement
    23111222979 locking pin with the groove on the tip is for 5-6 gear detent and it uses the softer 23311228393 compression spring.
    23311224130 locking pin without the groove on the tip is for Reverse gear detent and it uses the stronger 23111222720 compression spring.

    Insert the locking pins and use a small amount of MTF fluid on the pins and in the sleeves. Ensure the pin slides freely through the entire sleeve and all the way into the stop. You may have to clean up deformed edges on the sleeve if you drove them in without the specific detent tool. You can use a fresh exacto knife or a dremel just be mindful of the coating applied to the inside diameter of the sleeves. Install the correct compression spring behind each locking pin.

    Install the sealing cover and lock ring. This is easier said than done, particularly with the Reverse pin since the spring pressure is considerable. This job is made easier with the detent tool 234021 found in the ZF 320Z detent tool set. Make sure your snap ring is also fully installed into the groove.

    Press the 22mm bearing on the shift arm

    Remove the arm from the transmission and press the bearing on to the shaft. The bearing is correctly installed when it is fully bottomed out to the base of the shaft. Press from the inner race of the bearing or you risk damaging it. Lubricate the bearing and the shaft base with MTF and reinstall into the transmission.



    Install the 8mm pin (if not done by your machine shop)

    Freeze the pin for a few hours before installation. Install can be done with hammer. Drive in until fully seated and the spring catch groove is at the same height as the corresponding pin found on the shift arm. My machinist turns the pin to size/shape and installs it for me as part of his service.



    Torsion Spring

    This is where Rontgen's post directions were wrong. Failure to match the new spring to same style as the spring found in an original manual transmission will lead to poor shifter feel (too soft).

    Original versus McMaster



    Long Leg of the Torsion Spring

    You can start by cutting the long leg to length. You visualize this using the shift arm and 8mm pins for reference. Cut correctly the length of the long leg falls just past the outer pin as shown in the pictures.

    The bend on the long leg starts just before the shift arm pin. The angle of the bend eyeball to match what you see here. When bent correctly, the 22mm bearing is gently pressed against the Reverse lock pin so there is no slop (you check this with the torsion spring fully installed and pull it back out to correct it if not).


    Short Leg of the Torsion Spring

    Here is what the guides get wrong. Notice in the Original BMW manual spring that the short leg is 90 degrees from the direction of the long leg. You have to unwind this side of the spring 45 degrees to match this angle. This is a bit difficult to do and will require a vice. Clamp the spring in the vice by the short leg. Take a long 3/8 extension and pass it through the spring inner diameter. Use the extension as a handle to pull up the spring to unwind 45 degrees out. You will have to straighten that leg out once done as it will still have some of wound arc to it. You are matching the 7 windings that the original BMW spring has by doing this. You are not adding any more or less pressure than the OE part. You are modifying the McMaster part to match it.

    After you have corrected the short leg, you can visualize where to cut it and then add the small bend that the original had. Eyeball this, it isn't super critical.






    Installing the torsion spring, with the correct OE shape on it, is a much harder task than the way the spring was setup in this original guide. There is much more spring tension which is going to equate to a factory manual transmission feel. You'll have to fuss your way through installing the spring. I'd recommend a long 1/2 extension in the retaining sleeve to help you draw the assembly into the wall of the transmission so the bolt can be reinstalled.



    At this point you are done with the bellhousing side of the conversion.

    MSportParts | Braymond141
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