Engine Rebuild- Crankshaft Bearings

Measuring Journals and Bearings

I shall start off by saying that replacing these was possibly a bad idea. It’s not easy and they probably last forever. I had been in two minds about it- ultimately measuring the specified clearance accurately to 100th (or even 1000th!) of a millimetre leads to some variation and some questioning of your technique and the quality of your tools.

I started by polishing the crankshaft journals with some 1000g wet and dry paper and then some metal polish. They looked in pretty good condition. Then I started taking my measurements with a micrometer. The crank journals are wide so the measurements should be taken at two axial points to check for taper, and also in two radial axes to check for out of round. This makes for a lot of measurements- I made a table on a piece of paper and took three measurements at each point and averaged them.

I was unable to find any specification about what constitutes significant “taper” or “out-of-round” but all my measurements were within 0.02mm so I took these to be within normal limits for those two potential abnormalities.

After multiple rounds of measuring, checking the surfaces of the journal and micrometer were clean, more measurements, some head-scratching, more measurements, I came to the conclusion that my journals diameters were as follows:

Front crankshaft journal; 59.97mm

Rear crankshaft journal; 59.97mm

However I would get quite a few readings around 59.96 and even the occasional reading around 59.94. I am unsure if these are spurious, or due to slight irregularities in the surface of the journal. It should be mentioned here that the factory workshop manual and Clymer both talk about these measurements to 3 decimal places. I think that measuring to this accuracy is a pipe dream with a micrometer. Maybe with the crankshaft between centres on a big lathe and a dial test indicator, but the my lathe is way too small to accommodate this crankshaft. However with the occasional reading coming in at 59.96, if I had to guess the diameter to 3 decimal places I’d say somewhere around 59.968

Then on to measure the IDs of the bearing shells. I used a bore-gauge for this- again I don’t really think it’s realistic to get three decimal place accuracy. I measured in 2 axes and averaged them.

Front bearing- 60.035 (average of 60.03 and 60.04 in two axes)

Rear bearing- 60.03

Checking Specification

The manuals have a bewildering table of values for the crank journal diameter with a confusing “colour coding” system. And they don’t specify the internal diameter of the bearing shells anywhere. Basically the specification here is the clearance

Crank journal to bearing clearance; 0.035-0.065mm

Using my above measurements my clearances were:

Front; 0.067mm

Rear; 0.065mm

Basically at or just above the limit. So I decided to replace them.

Colour System

To explain the colour system- the bearings come from BMW with a colour painted on their edge; the table specifies which colour bearing will bring which diameter journal into the above range. This is further confused by the fact that the manuals talk about RED (loosest or stock) and BLUE (slightly tighter) but turns out they also offer YELLOW (tighter still) and GREEN (tightest). The yellow and green bearings are not mentioned in Clymer or the factory manual. I found the following information online which is likely accurate but I haven’t seen an official source;

Red 59.98-59.99

Blue 59.971-59.98

Yellow 59.961-59.970

Green 59.951-59.960

You’ll note that the 4 sizes cover a range of 0.05mm and the clearance spec is a range of 0.03mm so there is clearly going to be some acceptable overlap with these bearing selections.

A final note on the colour system- these four coloured ranges are also available in at least two other oversize diameters for reground crankshafts. That is; the same 0.05mm range but set down 0.25 and 0.5mm. This gets complicated and most people don’t recommend regrinding a crank journal unless you can re-heat-treat. And even then they seem to be asking for failure.

Choosing my bearings

You’ll note that my journal measurement of just under 59.97 puts me right between two colours- blue and yellow. I wasn’t sure which one would work best so I ordered them both. They are expensive, but also had to come from Germany and I didn’t want to have to wait around if I decided later that I actually needed the other colour. When they arrived I measured their IDs- which lead to another confusing observation- they measure bigger when not installed. The yellow were 60.065 and the blue 60.095. Obviously those numbers wouldn’t get my crankshaft anywhere near the specified clearance. In the end I though probably both would work with the yellow on the tight side of spec. I didn’t want to be too tight which could happen if I had underestimated the journal diameter so in the end I went with the blue bearing shells.

Removing and Installing Bearing Shells

Removing and installing the shells requires two things; heat and some kind of pressing tool. I made a tool for pressing by turning down some discs of aluminium on the lathe and using heavy threaded rod as shown. (Note- I started with a thinner threaded rod in low grade steel and it broke so I moved to the 16mm stainless steel rod you can see in the crankcase photos).

To remove the front bearing you first have to knock out the retaining pin from the inside using a small punch. Don’t loose the pin! I then heated the bearing carrier to over 100c in the kitchen oven. This was relatively easy and the bearing pushed out with my tool. The rear bearing gave me a lot more trouble. The crank case needs to be heated up hot which is not that easy, even using a propane torch. Eventually with some fiddling the tool worked and the bearing came out. I would recommend trying to remove and install the rear bearing in one heat cycle (ie install straight away) given the hassle of heating up the crankcase.

To install the rear bearing, line up the holes in the bearing so they are at 12 and 6 and the line of the bearing joint is at about 2 o’clock. Mark the bearing and the crankcase with a sharpie so you have the alignment and then put the bearing in the freezer. When the case is hot and the bearing cold it should press in using the tool. The alignment of the rear bearing in the crankcase is pretty much centred in the case- it’s almost flush on both sides, so i just pushed until the bearing bottomed out on the other side of the tool which was flush against the case between the thrust bearing studs.

Installing the front bearing is a little easier- heat the carrier and cool the bearing. Alignment is again with oil holes at 12 and 6 and the joint is at 11 o’clockish. Unlike the rear bearing, the front one sits closer to the back than the front. At the rear of the carrier the bearing should be flush with the surface and at the front there is a step of about 1-2mm. It’s very hard to measure this as there is no way to fit a depth micrometer in there. I just pushed it in til flush with the back surface of the carrier.

Once the front bearing is in place some more holes need to be drilled for oil ways. The two existing 3.2mm holes in the carrier can be drilled through with 3.2mm bit through the bearing as shown.

Finally, the securing pin needs to be re-installed. To do this drill through the old pin hole with a 3.9mm drill bit. I actually stopped just short of the coming through and finished with a 3.2. Basically you don’t want the pin dropping into the bearing surface. Then an h4 hand reamer was used to bore the hole to 0.5-1.0 short of the bearing surface. The pin is then tapped in and the entry hole peened off with a punch to stop it coming back out. Be sure to carefully deburr and dress the holes you have just made in the bearing surface! I used a rounded file gently and some 600g paper.