46 BMW 1973 R75/5 Install Rear Sub-Frame, Shocks, Key Lock & Seat Latch

With the engine, transmission, swing arm  and rear drive installed, I’m ready to install the rear sub-frame and the shocks.

Rear Frame

Rear Sub-Frame, Upside Down

The sub-frame legs mount to the brackets that hold the exhaust mounts and inside the tabs at the top of the frame. So it is very easy to scratch the new powder coating on the frame. I found that the legs were too tight and I pulled them apart with my hands to expand them enough so they would pass over the exhaust bracket and not scratch anything.

Parts List

I renewed the hex bolt that secures the grab handle to the frame and sub-frame with a new stainless steel bolt from Hucky’s.

Part # Description Each
46 51 1 231 093 SS Hex bolt, M8x25, Stainless Steel, Handhold 1

Mounting Hardware

Here is the shock mounting hardware.

Shock Mount Hardware

Shock Mount Hardware

There is a fine thread, short bolt that goes into the top of the right side shock. The two longer bolts go on the left side shock. The nut, large flat washer and wave washer go on the lower right stud in the rear drive.

Here is the sub-frame mounting hardware.

Rear Sub-Frame Mounting Hardware

Rear Sub-Frame Mounting Hardware

The grab rail front mount bolt goes through the top, left sub-frame hole. The rear grab rail is tapped on the other side for an Allen bolt. The other three bolts mount the lower left and right sides of the sub-frame to the frame.

Shock Orientation

The stock Boge shocks have spring pre-load adjustment via a lever and notched ring. Each ring is stamped to indicate which side they go on.

Marking on Right Shock Pre-Load Adjuster

Marking on Right Shock Pre-Load Adjuster

Marking on Left Shock Pre-Load Adjuster

Marking on Left Shock Pre-Load Adjuster

I put jack stands under the swing arm on the left side and the exhaust bracket on the right side with some padding to protect the powder coat.

Jack Stand Supporting Swing Arm

Jack Stand Supporting Left Swing Arm

I install the shocks in the lower mounts and finger tighten the nuts. This makes it easy to swing the shocks up into the upper mounts of the sub-frame. I used some padding under the left shock so it wouldn’t mare the power coating on the swing arm. The right side lower mount is a stud on the rear drive.

Securing Shocks by Lower Shackle

Securing Shocks by Lower Shackle

Here you see the large flat washer and wave washer with nut on the rear drive stud.

Lower Right Shock Shackle

Lower Right Shock Shackle and Hardware on Rear Drive Stud

Installing the Rear Sub-Frame

It’s easier to install the right side of the frame first. I eased it into the top tab and ran a bolt through and the the bottom tab.  I finger tighten the nuts so I can adjust the sub-frame to get the left side mounted.

Right Side Sub-Frame Bolts Installed

Right Side Sub-Frame Bolts Installed

For comparison, here is how this looked when I disassembled the bike-major improvement 🙂

Battery Box Rust

Rear Sub-Frame and Battery Box During Disassembly

The left side uses the grab rail with a bolt to hold the top of the sub-frame.

Grab Handle Hardware

Grab Handle Hardware

The Allen bolt goes in from the inside of the frame into the tapped hole in the grab rail.

Gab Handle Allen Bolt

Rear Gab Handle Mount Allen Bolt

I adjusted the jack stands and the motorcycle lift so the top shackle on the left shock is a little lower than the hole of the mounting bracket when I swing the shock up into the bracket.

Inserting Left Shock Top Shackle in Bracket

Inserting Left Shock Top Shackle in Bracket

Then I can lift the swing arm with one hand while inserting the bolt with the other to secure the top of the left shock in the frame. This supports the swing arm. I swing the right side shock up into the housing and insert the shorter, fine thread bolt to secure it.

The spring pre-load handles should be on the inside of the swing arm as you turn them. If they aren’t, you can put the top shackle in a vice (use rubber jaws to avoid chewing them up) and push down on the adjusting handle and rotate the handle as needed. The handle is slotted and can be repositioned on the bottom of the stepped adjusting ring. (Yeah, I had to do that) 🙂

Right Shock with Handle on Inside

Right Shock Handle on Inside (click to enlarge)

Left Shock with Handle on Inside

Left Shock Handle on Inside (click to enlarge)

Torque Shock and Sub-Frame Bolts

The Haynes manual shows a lower torque setting for the lower left bolt, 24 FOOT/pounds, than for the other three shock mounting bolts, 27 FOOT/pounds.  The sub-frame mounting bolts are set at 18 FOOT/pounds.

Install the Seat Lock

Here is the hardware for the seat latch and the key lock when I removed them during dissasembly.

Seat lock outer cover removed

Seat Key Lock and Latch Assembly During Dissassembly

And here are the cleaned parts ready for installation.

Seat Latch and Lock Hardware

Key Lock and Seat Latch Hardware

The key lock has a retaining ring on the rear that fastens to the frame with a small machine screw.

Key Lock Hardware

Key Lock Hardware

Remove the retaining ring from the key lock barrel and insert the barrel through the frame. Rotate it so the key slot in the front of the lock is pointing down. From the back, the hole for the small screw that holds the retaining ring will be at the 11:00 o’clock position. The hole is inside the V-slot that orients the chrome retaining ring.

Lock In Frame

Lock In Frame

I slip the chrome retaining ring over the locking arm on the key lock barrel and then I rotate it 90 degrees to get it onto the lock barrel. It fits into a V-slot in the back of the lock.

Lock Retaining Ring on Lock Barrel

Lock Retaining Ring on Lock Barrel

I secure the retaining ring to the key lock barrel using the small machine screw.

Attaching Lock Retaining Ring with Screw

Attaching Lock Retaining Ring with Screw

Install the Seat Latch

The seat latch mechanism includes a spring that pushes the latch open. The button on the front pushes the latch open and the spring pushes the latch closed. Note one end of the spring goes on the latch lever and the other end extends past the latch housing. It gets compressed when the latch cover is installed.

Orientation of Seat Latch Spring

Orientation of Seat Latch Spring

The latch housing installs around the seat lock assembly with the latch bar pointing to the front.

Seat Latch Goes Around Key Lock

Seat Latch Goes Around Key Lock

Since the front of the latch cover has to compress one end of the spring, it is easier to insert the rear screw first. It aligns with a tapped hole in the frame and due to its length, it is best to look at the front of the frame as you wiggle the screw until it lines up with the hole and then finger tighten it. I screwed it in about half way and then pushed on the front of the cover until I got the seat latch spring compressed, inserted the screw and lined it up with the tapped hole in the frame. I finger tightened the front screw while holding the cover tight against the latch housing against the spring pressure and then tightened the front screw and then the back screw.

Seat Latch Backk Plate Installed

Seat Latch Backk Plate Installed

Final Product

Here is the rear end of the bike when I disassembled it.

Rear End Disassembly

Rear End Disassembly

Supporting the bike before rear end and swing arm removal

Rear End Dissassembly

And how it looks now. Major improvement 🙂

Rear Shocks Installed

Final Product

Lock & Grab Handle

Key Lock and Grab Handle Installed

6 thoughts on “46 BMW 1973 R75/5 Install Rear Sub-Frame, Shocks, Key Lock & Seat Latch

  1. Hi Brook,

    Your blog is very helpful to me as I am in the process of restoring a R50/5 1972.
    I was wondering what did you use to protect the VIN plate and all the thread hole on the frame from the powder coat.

    Thank you and Bravo for the restoration you did

    Gilbert

    • Hi Gilbert,

      I put masking tape marked with a “T” over the holes that were threaded. I told the powder coating company to protect the threads where they saw a “T”. They insert rubber plugs in those holes to keep the coating out of them and off the threads. That said, I had to clean some of the threads at the top of a hole and found Permatex gasket remover (documented in this write-up http://brook.reams.me/bmw-motorcyle-rebuilds/1973-bmw-r755-rebuild-project/51-powder-coating/) softened the coating so I could use the bolts to chase the threads and clean the coating from them.

      For the VIN, I think it’s best to remove it, which I did not do. I taped over it with two layers of masking tape, but that didn’t really protect it and the result was the black paint was lightened to a dark brown. The VIN is riveted to the steering stem tube and you can get new rivets and then install the VIN plate. This is the best method.

      I hope this helps, and thanks for the kind words.

      Best.
      Brook.

  2. Hi Brook,
    Thanks very much, the pics and description of the seat lock was just what I was looking for. My lock cylinder just started to spin in its hole in the frame, this info will help me fix it!

    Thanks again!
    – Bob

    • Hi Bob,

      Cool beans. I suspect that little screw on the inside went missing. I’m glad this content helped you out.

      Best.
      Brook.

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