Access to the oil pump requires removal of the transmission, clutch and flywheel. You can see how I do that here:

• 23 BMW 1983 R100RS Remove Transmission
• 21 BMW 1983 R100RS Remove Clutch
• 11 BMW 1983 R100RS Remove Flywheel & Rear Main Seal

Video

I shot a short video of highlights of the procedure documented in this write-up

VIDEO: 1983 R100RS Remove and Inspect Oil Pump

Remove Oil Pump Cover

The oil pump is behind the cover at the bottom of the bell housing behind the flywheel. It is secured by four M6 bolts. The top edge of the oil pump cover is beveled. The cover bolts are not symmetric so the cover can only mount in the correct orientation. This is necessary due to the cavities machined into the cover that must be in the proper orientation to the internal pump parts.

The pump cover o-ring is installed in a groove on the back of the cover. A leaking oil pump cover o-ring is a source of oil leaks from the rear of the engine.

The two rotors are mounted inside the oil pump cavity that is machined into the engine block. The inner rotor attaches to the rear end of the cam shaft and has four lobes while the outer rotor has five.

As the cam shaft spins the inner rotor, the inner rotor turns the outer rotor inside the oil pump cavity. As you see in the picture above, the gap between the inner and outer rotor vary around the circumference of the inner rotor. As oil enters a large cavity, it is compressed as that cavity gets smaller increasing the oil pressure.

Oil Pump Measurements

There are several measurements to make in order to assess the condition of the oil pump. In the case of measuring clearances, I use feeler gauges, but mine are in 0.001 inch increments with the equivalent metric measurement in mm shown on the feeler. The specifications state a range for the acceptable clearances in mm.

I start with the feeler gauge closest to the smallest clearance in the range in mm and work my way up until I find the gauge that is very tight or won’t slip into the gap. I write down that gauge and the one that was just smaller as the measured range for the actual clearance.

Outer Rotor-to-Case Gap

I use feeler gauges to determine the size of the gap between the outer rotor and the case. The allowable range is (0.100 – 0.171 mm ). I found the gap to be between (0.152 – 0.178 mm).

Inner-to-Outer Rotor Clearance

I use feeler gauges to determine the clearance between the inner and outer rotor lobes. The allowable range is (0.120 – 0.200 mm). I found the clearance to be between (0.152 – 0.178 mm).

Rotor End Play

The outer and inner rotors are slightly recessed inside the engine block pump housing which provides some end play for the rotors so they aren’t grinding on the pump cover. I use an Allen wrench as a gauge block across the engine block and the face of a rotor.

I place the Allan wrench so the foot faces outward and the flat face on the back is flush against the body of the oil pump housing in the flat surface of the pump body housing.

WARNING:
If you aren’t careful, you can end up with the edge between two flat faces of the Allan wrench against the pump housing. This will result in an incorrect reading with too large an end-play gap measurement. Be sure a flat face of the Allan wrench is placed across the pump housing with the rotor underneath that face as shown below.

I slide the feeler gauges beneath the Allan wrench face and the outer rotor until I find the one that won’t slide into the gap.

I remove the outer rotor and put the inner rotor on the cam shaft and make the same measurement for the inner rotor.

WARNING:
Avoid putting the Allan wrench across the end of the cam shaft as it is proud of the inner rotor and your measured gap will be wrong and much too large.

The allowable range is (0.020 – 0.070 mm). I found the clearances to be:

Outer Rotor End Play: Between (0.064 – 0.076 mm)
Inner Rotor End Play: Greater than (0.076 mm)

Outer Rotor Thickness and Diameter

I pulled the outer rotor out of the pump body with my fingers. I used a vernier caliper to measure the outer rotor thickness and diameter.

WARNING:
You maybe tempted to remove the outer rotor with a magnet. I would avoid that as it will magnetize the rotor and it will attract and hold any steel fragments circulating in the oil which attract the metal bits to the face and lobes and increase the wear on the rotor. Over time as the rotor gets heat cycled, it will loose it’s magnetism, but I think it’s good practice to avoid magnetizing it at all.

The allowable range for thickness is (13.955 – 13.985 mm). I found the average thickness to be (13.953 mm). The allowable range for diameter is (57.075 – 57.100 mm). I found the average diameter to be (57.083 mm ).

Inner Rotor Thickness

I pulled the inner rotor off the end of the cam shaft with my fingers. I used a vernier caliper to measure the inner rotor thickness.

The allowable range is (13.955 – 13.985 mm). I took three measurements and found the average thickness to be (13.956 mm).

NOTE:
This inner rotor, (part# 11 41 1 335 194), has flats that mate with flats on the end of the cam shaft. Earlier rotors up to 09/1978 have a slot with Woodruff key that fits into a slot in the cam shaft. There are two different slotted inner rotors with different Woodruff key dimensions: the 2.5×3.7 mm Woodruff key rotor is (part# 11 41 1 253 294) and the 3×5 mm Woodruff key rotor is (part# 11 41 1 258 971). I believe the smaller Woodruff key version was used on early /5 series engines. The clearances and size of these three different rotors are the same.

Pump Body Depth

I used a vernier caliper to measure the pump body depth as shown in the picture below.

The allowable range for the depth is (14.010 – 14.025 mm). I took three measurements and got an average depth of (14.143 mm).

Visual Inspection of Rotors

I used a strong light and inspected the inner rotor. There are grooves at the ends of the lobes. The inner and outer faces of the inner rotor also showed circular grooves caused by metal bits going through the pump. This is likely since shortly after I got the bike, I found the oil filter high pressure check valve was broken which allowed unfiltered oil to circulate through the engine. And I also found scores in the connecting rod main bearings from metal bits that circulated in the oil.

Condition Assessment

Here is a table of the oil pump components allowable range of measurements and what I got when I measured my components.

Measurement	Range	Actual	Status
Outer Rotor OD	57.075-57.100 mm	57.07, 57.08, 57.10: AVE 57.083 mm	GOOD
Outer Rotor-to-Body Clearance	0.100-0.171 mm	Between 0.152 – 0.178 mm	GOOD
Outer Rotor Thickness	13.955-13.985 mm	13.95, 13.96, 13.95: AVE 13.953	MARGINAL
Outer Rotor End Float	0.025-0.070 mm	Between 0.064-0.074 mm	MARGINAL
Inner Rotor Thickness	13.955-13.985 mm	13.96, 13.96, 13.95: AVE 13.956	MARGINAL
Inner Rotor End Float	0.025-0.070 mm	> 0.074 mm	BAD
Outer-to-Inner Lobe Clearance	0.120-0.200 mm	Between 0.152-0.178 mm	GOOD
Pump Body Depth	14.010-14.025 mm	14.12, 14.16, 14.15: AVE 14.143	GOOD

The end gap of the outer rotor is almost at the limit and the end gap of the inner rotor is past the limit.

Combined with the scoring on the lobes of the inner rotor and on the faces of the two rotors, I will replace the oil pump rotors and the cover.  I’m glad I took the time to measure and inspect the oil pump parts since oil pressure failure is usually found out after serious damage to the engine has already occurred.

Revisions

2019-11-20  Edits, Typos.