12 BMW 1977 R100RS Refurbish & Rebuild Starter Motor

I replaced all the engine electrics (alternator, diode board) all the engine wiring mounted to the timing chest cover. You can read about that work here:

Due to the amount of rust on the motor housing and corrosion I found in the front engine cover, it’s possible there is corrosion inside the motor as well. I had some problems starting the bike. So I refurbish the starter motor; I remove the rust, paint the starter motor and starter solenoid, replace the brushes and install new armature bushings.

Rusty Starter Motor with Cob Webs

Rusty Starter Motor with Cob Webs

Rusty Starter Motor

Rusty Starter Motor

Cracked Insulation on (+) Battery Cable; "Red" Wire Wrapped with Electrical Tape

Cracked Insulation on (+) Battery Cable; “Red” Wire Wrapped with Electrical Tape

Starter Motor Front Mounting Bracket

Starter Motor Front Mounting Bracket

Engine Breather Hose

Engine Breather Hose Location

The starter motor is a Bosch type 157, model 023. That means it is a 9 tooth starter motor which ONLY works with the 111 tooth flywheel, (part#:  11 22 1 336 380)    . It develops just about 1 Hp (0.7 Kw).

Bosch Stater Motor Model Showing Type and Model Number

Bosch Stater Motor Model Showing Type and Model Number

Parts List

Euro Motoelectrics provides a kit to refurbish a Bosch type 157 starter motor. They also stock many of the parts to repair the motor and complete replacement motors.

Part # Description
BO157-TKPLUS Starter Kit  12 41 1 352 536, 527, 537_Bosch 157 Series

Resources

Bob Fleischer has material about Bosch and other replacement starter motors.

After I finished this work, I stumbled across a series of YouTube videos by Motor Phoenix about the starter motor. Although you do sit through removal of bolts and nuts and the focus is lost on occasion, it’s a good series by a knowledgeable mechanic.

Remove Starter Motor

The starter motor is under the top engine cover. It can be removed with the engine in the frame, but it’s a tight fit with little room to work. It’s very easy when the engine is out of the frame.

The starter motor mounts on the top of the engine with two Allen head bolts at the rear and a hex head bolt that attaches a bracket to the front of the engine block. The rear Allen bolts are tucked under the motor. I can get the long leg of an Allen wrench on them and use a socket to remove the nuts. The nuts secure a wire bail the holds the plastic cover over the Bendix starter pinion.

Starter Motor Left Rear Mounting Bolt

Starter Motor Left Rear Mounting Bolt

Starter Motor Right Rear Mounting Bolt

Starter Motor Right Rear Mounting Bolt

Starter Motor Rear Mounting Bolt Nuts

Starter Motor Rear Mounting Bolt Nuts

Starter Motor Removed Showing Hardware

Starter Motor Removed Showing Hardware

The shelf the starter motor mounts on is filthy so I use some engine cleaner to remove the greasy muck.

Cleaned Up Starter Motor Bay

Cleaned Up Starter Motor Bay

I use a wire brush to get the loose rust off the motor and starter solenoid.

Starter Motor Assembly with Loose Rust Removed

Starter Motor Assembly with Loose Rust Removed

Remove Starter Solenoid

All the fasteners have rust on them so I apply some Kroil penetrating oil and let it sit for a bit before removing them.

There is a braided copper wire attached to one of the threaded terminals on the starter solenoid. I remove it. After I remove it, I leave the nut and lock washer on the other threaded terminal the (+) battery cable attaches to. That way I’ll position the starter solenoid in the correct orientation when I reassemble it so I don’t get the terminals switched.

Field Coil Copper Wire Connection on Solenoid Threaded Terminal (Left)

Field Coil Copper Wire Connection on Solenoid Threaded Terminal (Left)

Removing Starter Motor Field Coil Lug Nut

Removing Starter Motor Field Coil Lug Nut

Starter Motor Field Coil Lug and Mounting Hardware

Starter Motor Field Coil Lug and Mounting Hardware

Starter Motor Field Coil Lug

Starter Motor Field Coil Lug

The stater solenoid has a bolt that attaches it to the cast housing for the Bendix gear. I remove it.

Solenoid Bendix Actuating Arm Bolt

Solenoid Bendix Actuating Arm Bolt

Removing Solenoid Bendix Actuating Arm Bolt

Removing Solenoid Bendix Actuating Arm Bolt

Solenoid Nut and Washer Go on Side with Machined Recess

Solenoid Nut and Washer Go on Side with Machined Recess

Solenoid Hardware Securing Bendix Actuating Arm

Solenoid Hardware Securing Bendix Actuating Arm

Then I remove the two screws that secure the starter solenoid to the cast Bendix gear housing with an impact driver.

Removing Starter Solenoid Housing Screw with Impact Driver

Removing Starter Solenoid Housing Screw with Impact Driver

Starter Solenoid Housing Screw

Starter Solenoid Housing Screw

I slide the solenoid out of the cast Bendix gear housing. There is a rectangular rod with a hole in it which engages a lever to push the Bendix gear in and out so it engages the flywheel.

Starter Solenoid Removed From Cast Bendix Gear Housing

Starter Solenoid Removed From Cast Bendix Gear Housing

Disassemble Starter Motor

NOTE:
You will note the starter solenoid is still attached in the following photos. I should have removed it first but didn’t. So I organized the write-up in the proper sequence.

The body is secured with two long bolts that have captive nuts. These are the same bolts the front mounting plate attaches to.

Rod with Captive Nut Securing Field Coil Cover to Cast Bendix Gear Housing

Rod with Captive Nut Securing Field Coil Cover to Cast Bendix Gear Housing

Rod Securing Field Coil Cover to Cast Bendix Gear Housing

Rod Securing Field Coil Cover to Cast Bendix Gear Housing

I separate the motor housing from the cast Bendix housing, but I have to remove the armature retaining C-clip under the protective cover of the brush holder on the other end of the motor before I can remove the armature from the motor housing.

Field Coil Housing Separated From Cast Bendix Gear Housing

Field Coil Housing Separated From Cast Bendix Gear Housing

I remove the two screws that secure the alternator bushing cover.

Remove Armature Shaft Cover Screws

Remove Armature Shaft Cover Screws

Armature Shaft Cover & Screws

Armature Shaft Cover & Screws

This end of the armature is secured with a large C-clip and I remove it.

Armature Shaft Retaining Clip

Armature Shaft Retaining Clip

Next are three shims and then a seal.

Three Shims-1 Thick, 2 Thin

Three Shims-1 Thick, 2 Thin

Seal

Seal

Here are all the parts in order of disassembly from right to left.

Parts On Brush Holder End of Armature In Order, Left-to-Right

Parts On Brush Holder End of Armature In Order, Left-to-Right

I push the armature through the brush holder.

Pushing Armature Shaft Out of Brush Holder Housing

Pushing Armature Shaft Out of Field Coil Housing

The armature is free from the field coil housing.

Field Coil Housing Removed Showing Armature

Field Coil Housing Removed Showing Armature

Inside the field coil housing are the coils along the perimeter and at the bottom is the brush holder.

Interior of Starter Motor Housing-Field Coils On Sides & Brush Holder at Bottom

Interior of Starter Motor Housing-Field Coils On Sides & Brush Holder at Bottom

There is a Bendix gear actuating arm inside the Bendix cast housing.

Bendix Arm Inside Bendix Gear Cast Housing

Bendix Arm Inside Bendix Gear Cast Housing

I pull the armature with the Bendix gear out of the housing. I didn’t notice the rubber seal which pops out.

Removing Armature From Bendix Gear Cast Housing

Removing Armature From Bendix Gear Cast Housing

This is the orientation of the seal in the cast Bendix housing.

Location of Starter Solenoid Housing Rubber Gasket in Bendix Gear Cast Housing

Location of Starter Solenoid Housing Rubber Gasket in Bendix Gear Cast Housing

Starter Solenoid Housing Rubber Gasket

Starter Solenoid Housing Rubber Gasket

As the armature comes out of the cast Bendix gear housing, the actuating arm comes off.

Bendix Gear Actuating Arm

Bendix Gear Actuating Arm

It has a groove in the arms of the yoke that fit on the round sleeve at the back of the Bendix gear assembly.

Bendix Gear Actuating Arm Detail

Bendix Gear Actuating Arm Detail

Actuating Arm On Bendix Gear Disk

Actuating Arm On Bendix Gear Disk

Remove Brush Holder Cover

The brush cover is supposed to “just slip off” the motor housing. Mine did not want to do that and I had to use a screw driver to drive it off the motor housing. I pushed in along the edge of the rubber seal surrounding the braided copper wire and then used a plastic hammer to loosen the cover from the motor and open up a crack between the cover and the motor housing.

NOTE:
As I found out when assembling the starter motor, it’s a good idea to put an index mark across the field coil housing and Bendix gear cast housing so it’s easy to align them again. The long bolts with captive nuts have to thread into holes in the brush holder. I had to rotate the field coil cover a bit to get the long bolts to align with the holes.

Forcing Brush Holder End Cap Off Starter Housing with Screw Driver

Forcing Brush Holder End Cap Off Starter Housing with Screw Driver

Then I twisted a screw driver blade in the gap all around the cover to get it loose.

Prying Brush Holder End Cap Off Starter Housing-Working Around the Edge

Prying Brush Holder End Cap Off Starter Housing-Working Around the Edge

Brush Holder End Cap

Brush Holder End Cap

Now I can access the brush holder on the end of the motor housing.

Brush Holder Attached to Armature and End Cap

Brush Holder Attached to Armature and End Cap

Disassemble Brush Holder

The brush holder is attached to the the field coil inside the field coil cover. I remove the brush cover by pulling the springs off the ends of the brushes and hanging them on the edge of the brush slots. Then I pull the brushes out of their slots.

Brush & Coil Spring Detail in Brush Holder

Brush & Coil Spring Detail in Brush Holder

Brush Removed From Slot in Brush Holder

Brush Removed From Slot in Brush Holder

There are four brushes; two of the brushes are attached to the cover and the other two are attached to the field coil.

Brushes Attached to Field Coils

Brushes Attached to Field Coils

Brush Plate with Brushes Out of Slots

Brush Plate with Brushes Out of Slots

Remove Old Brushes, Install New

The Euro Motoelectrics upgrade kit includes four brushes, coil springs and two new armature bushings.

Euro Motoelectric Bosch Starter Rebuild Kit

Euro Motoelectric Bosch Starter Rebuild Kit

I inspect the brush holder and I expect the brush pigtail to be soldered but that’s not the case. The copper is “welded” to the lug by being crimped at high pressure to the lug.

So I cut the pigtail and then file down the old copper strands.

Cutting Brush Pigtail Off Lug

Cutting Brush Pigtail Off Lug

Removing Old Copper Wire

Removing Old Copper Wire

I tin the end of the new pigtail and the lug and then solder the new brush pigtail to the lug.

Brush Soldered to Brush Holder

Brush Soldered to Brush Holder

The field coil brushes are attached to lugs on a bus bar that connects to the braided copper wire that was attached to a threaded terminal of the starter solenoid. The bus and field coils are coated in rubber insulation.

Old Brush Attached to Lug with Rubber Insulation

Old Brush Attached to Lug with Rubber Insulation

I clean the rubber off the lug.

Rubber Removed From Lug and Brush Pigtail

Rubber Removed From Lug and Brush Pigtail

Rubber Removed From Lug and Brush Pigtail

Rubber Removed From Lug and Brush Pigtail

I pull the ring buss off it’s holder so I can expose the end of the brush pig tail. I cut the pig tail off, and file the remaining copper strands down. I tin the end of the pigtail and the lug and then solder them together.

Rubber Removed, and Bus Pulled out of Bracket to Expose Brush Pigtail

Rubber Removed, and Bus Pulled out of Bracket to Expose Brush Pigtail

Field Coil Brushes Soldered to Lugs

Field Coil Brushes Soldered to Lugs

After the new brush pigtails are soldered to the field coil bus lugs, I push the bus back into the clamp and squeeze the clamp tight on the bus. Then I coat the exposed parts of the bus and the lugs with black automobile touch up paint.

Painted Field Coil Bus and Brush Lug

Painted Field Coil Bus and Brush Lug

Clean Up Commutator & Paint Parts

I  repainted the field coil cover, Bendix Gear cast housing and solenoid.

Starter Motor Parts Painted

Starter Motor Parts Painted

I use some copper polish and then some 1000 grit sand paper to clean the commutator on the alternator.

Commutator Cleaned Up

Commutator Cleaned Up

Assemble Starter Motor

I replace the armature bushings and then assemble the motor and solenoid.

Replace Armature Bushings

The armature mounts in two bushings, one in the Bendix gear cast housing and the other in the brush holder cover. I soak the new bushings in motor oil overnight.

Bushings Soak in Motor Oil Over Night

Bushings Soak in Motor Oil Over Night

The bushings have different thickness. The thick one goes in the brush holder and the thin one in the Bendix cast holder.

Brush Holder Cover End of Armature-Outside Bushing Detail

Brush Holder Cover Outside Bushing Detail

Brush Holder Hosing End of Armature-Bushing Detail Inside

Brush Holder Cover Inside Bushing Detail

Front End of Bendix Cast Housing End of Armature-Bushing is Recessed

Front End of Bendix Cast Housing-Bushing is Recessed

I drive out the old bushings using an appropriate size socket.

Driving Bushing Out of Bendix Cast Housing

Driving Bushing Out of Bendix Cast Housing

Old Bendix Housing Bushing-Thin One

Old Bendix Housing Bushing-Thin One

Old Brush Plate Cover Bushing-Thick One

Old Brush Plate Cover Bushing-Thick One

To install the new bushing in the brush holder cover, I invert the socket to the end the extension mounts to is against the bushing. This helps prevent bending the edge of the bushing.

Ready to Drive New Bushing in Brush Holder Cover

Ready to Drive New Bushing in Brush Holder Cover

Thick Bushing Depth-Outside of Armature Housing

Thick Bushing Depth-Outside of Brush Holder Housing

Thick Bushing Depth-Inside of Armature Housing

Thick Bushing Depth-Inside of Brush Holder Housing

The Bendix gear cast housing has a sharp edge on the outside and a bevel on the inside. I install the new bushing from the inside so it will not be damaged. I invert the socket as I did when driving the bushing in the brush holder cap. I use a long extension inserted into the front of the socket so I can drive it into the Bendix cast hosing. By the way, this bushing is longer than the original one so it fills the hole but the original is recessed so the bushing is flush with the inside edge of the hole.

Bushing Ready For Driving In Bendix Housing

Bushing Ready For Driving In Bendix Housing

Thin Bushing Depth-Inside of Bendix Cast Housing

Thin Bushing Depth-Inside of Bendix Cast Housing

Thin Bushing Depth-Outside of Bendix Cast Housing

Thin Bushing Depth-Outside of Bendix Cast Housing

Install Brush Holder

I insert the four new brushes into the slots in the brush holder.

Brushes Pulled Out So Armature Can Be Inserted

Coil Springs On Edge of Slot and Brushes Pulled Out So Armature Can Be Inserted

I insert the commutator end of the armature in the field coil housing and then set the coil springs on the end of the brushes.

Moving Brush Spring Onto Brush After Armature Inserted

Moving Brush Spring Onto Brush After Armature Inserted

Brush Spring Orientation on Back of New Brush

Brush Spring Orientation on Back of New Brush

Install Brush Holder Cover

I put the brush holder cover on so the armature shaft fits through the hole in the cover with the rubber grommet around the braided wire in the slot. It is supposed to slip on the field coil housing, but mine is too tight to slide all the way on. Later I use the long bolts with captive nuts to pull it down. Then I install the seal, shims and retaining clip. I hold the motor so the armature doesn’t slip out of the brush holder.

Brush Holder Cap Started on Field Coil Housing

Brush Holder Cap Started on Field Coil Housing

Brush End of Armature Pushed Through New Bushing

Brush End of Armature Pushed Through New Bushing

Seal Installed on Brush End of Armature Assembly

Seal Installed on Brush End of Armature Assembly

3 Shims Installed on Brush End of Armature Assembly

3 Shims Installed on Brush End of Armature Assembly

Brush End of Armature Assembly Secured

Brush End of Armature Assembly Secured

Install Bendix Gear Actuating Arm

I put a bit of grease in the crease of the Bendix gear actuating arm and on the slot of the starter solenoid arm the fits over the end of the actuating arm.

Grease on Circular Legs of Bendix Gear Actuating Arm

Grease on Circular Legs of Bendix Gear Actuating Arm

Grease on Solenoid Arm Slot

Grease on Solenoid Arm Slot

I insert the circular arms of the actuating arm on the disk of the Bendix gear and maneuver the a shaft inside the slot of the Bendix cast housing. This is a bit fiddly to do since there isn’t a lot of clearance between the cast housing and the field coil housing.

Bendix Gear Actuating Arm On Ring of Bendix Gear Assembly

Bendix Gear Actuating Arm On Ring of Bendix Gear Assembly

Inserting Bendix Arm Ready In Slot in Bendix Gear Cast Cover

Inserting Bendix Arm Ready In Slot in Bendix Gear Cast Cover

I insert the pivot bolt through the solenoid Bendix gear cast housing and the Bendix gear actuating arm and secure it with the nut and washer.

Insert Solenoid Arm In Housing Over End of Bendix Gear Actuating Arm

Insert Solenoid Arm In Housing Over End of Bendix Gear Actuating Arm

Actuating Arm Secured with Bolt and Bendix Cover Mated With Motor Housing

Actuating Arm Secured with Bolt and Bendix Cover Mated With Motor Housing

Install Starter Solenoid

I insert the stater solenoid so the slot in the actuating arm slips over the end of the Bendix gear actuating arm.

Insert Solenoid Arm In Housing Over End of Bendix Gear Actuating Arm

Insert Solenoid Arm In Housing Over End of Bendix Gear Actuating Arm

Pivot Bolt Pushed Through Bendix Actuating Arm

Pivot Bolt Pushed Through Bendix Actuating Arm

I install the rubber grommet and then push the Bendix gear cast housing onto the field coil housing. Then I secure the starter solenoid to the Bendix gear cast housing with the two bolts.

Bendix Arm Rubber Seal Inserted In Bendix Arm Cast Housing and Field Coil Housing

Bendix Arm Rubber Seal Inserted In Bendix Arm Cast Housing and Field Coil Housing

Solenoid Mounting Screws Installed

Solenoid Mounting Screws Installed

I put the tab on the end of the braided wire that powers the field coil on the threaded terminal, the one that I left the nut and washer off when I disassembled it so I’d get the field coil power on the correct solenoid threaded terminal.

Starter Solenoid Sends Power To Stater Motor via Braided Wire Lead

Starter Solenoid Sends Power To Stater Motor via Braided Wire Lead

I insert the two long bolts with captive nuts through the holes in the brush holder cover. I twist the field coil cover a bit to get the bolt to align with the threaded hole in the cover.

Twist Motor Housing To Align Case Bolts with Holes and Screw Down

Twist Motor Housing To Align Case Bolts with Holes and Screw Down

I clean the paint off the face of the rear mounting bracket as this serves as the ground for the motor.

Paint Cleaned From Face of Rear Mounting Bracket (It's a Ground)

Paint Cleaned From Face of Rear Mounting Bracket (It’s a Ground)

Testing the Starter Motor

I mount the starter motor firmly in my bench vice and connect a battery to the stater solenoid so I can test the motor before installing it and correct any mistakes I may have made. I connect the (+) battery cable to the threaded terminal of starter solenoid and use a bolt to through a rear mounting hole for the (-) ground cable.

WARNING:
At 1 Hp, the motor creates a large torque when starting to spin up and you don’t want it to get loose as it can hurt you and damage the motor.

Also, don’t allow the cables of on the battery to touch each other or the metal vice. There is enough power in the battery to metal the ends of the terminals.

Starter Motor Test Setup

Starter Motor Test Setup

Ground Wire Connected to Rear Motor Mount

Ground Wire Connected to Rear Motor Mount

I use a bit of wire and wrap one end over the spade terminal of the stater solenoid. If I touch it to the threaded terminal with the (+) battery terminal the motor ought to spin.

Solenoid Spade Terminal Wire Starts Motor When Touching (+) Terminal

Solenoid Spade Terminal Wire Starts Motor When Touching (+) Terminal

Here is a short video of the test.

VIDEO: CLICK TO RUN

Mount Starter Motor

Before I mount the starter motor, I spray more paint to cover the mounting bolts and the threaded terminal with the braided copper lead to the field coil. This was the way I found things when I removed it. I suspect Bosch wanted to reduce the opportunity for water to seep into the inside of the motor and solenoid. I also make sure the circular edge of the engine block the Bendix cast housing mounts to is clean so the motor sits square to the flywheel.

Brush Holder Cover Painted Including Hardware

Brush Holder Cover Painted Including Hardware

Painted Starter Motor

Painted Starter Motor

Here are is the hardware.

Starter Motor Mounting Hardware

Starter Motor Mounting Hardware

The front mounting plate has an offset in it. The orientation against the motor is as shown below.

Front Starter Motor Bracket-Bend Goes Toward Rear

Front Starter Motor Bracket-Top of Bend Goes Toward Rear

Here is the hardware orientation to mount the bracket to the motor.

Mounting Front Starter Motor Bracket to Motor

Mounting Front Starter Motor Bracket to Motor

Mounting Front Starter Motor Bracket to Motor

Mounting Front Starter Motor Bracket to Motor

The Plastic Bendix gear cover goes over the exposed gear on the back of the motor.

Bendix Gear Cover Fits Over Gear Housing

Bendix Gear Cover Fits Over Gear Housing

I install the Allen head bolts with the bolt head inside the starter motor cavity. The nuts have a slot on one end and it goes against the engine block with the ends of the wire bail that secures the cover in the slots.

Bendix Gear Cover Installed

Bendix Gear Cover Installed

I install the bolt through the front bracket and orient the bracket so it won’t interfere with the gasket under the timing chest cover.

Starter Motor Front Mounting Bracket Installed

Starter Motor Front Mounting Bracket Installed

Then I tighten the rear Allen bolts and nuts and recheck that front bracket bolts and nuts are tight.

Tightening Rear Starter Motor Mounting Bolt

Tightening Rear Starter Motor Mounting Bolt

Installed Starter Motor

Installed Starter Motor

Installed Starter Motor

Installed Starter Motor

Last I install the engine breather hose so it routes over the top of the motor.

Oil Breather Pipe Mounting Detail

Oil Breather Pipe Mounting Detail

Oil Breather Pipe Installed

Oil Breather Pipe Installed

2 thoughts on “12 BMW 1977 R100RS Refurbish & Rebuild Starter Motor

  1. 1980 R100RT Starter Motor Front Mounting Bracket had a slot cut in it so the fixing bolt did not have to be fully removed. Removal of oil cooler to get the front cover off helped to gain access to this bolt. Starter mounting bolts were not socket head nor are those shown in the fiche. I had to remove air box to gain access to these bolts.
    Can not find anywhere, except fleischer, any info about oil breather which is why I removed the starter. Where is that oil return hole that should be cleared?

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