BMW Boxer engine build

BMW Boxer engine rebuild (pdf)

Our first job was to clean the parts prior to reassembly. Well over 100,000 miles of use had left its mark. Oil and grime virtually obscured the original alloy crankcase, the cylinder barrels were in a terrible state and the pushrod tubes were covered in rust.

Internally, apart from that errant valve head, the condition wasn't too bad. The barrels, as we mentioned previously, had almost no wear and, as they cannot be rebored, a set of standard size new pistons was obtained.

In addition, all bearings, oil seals and gaskets were replaced as a matter of course, as were the big-end shells and, where stated, bolts. Every single suspect part was replaced. We ideally would have blast-cleaned the whole crankcase but that would have meant stripping and rebuilding the gearbox to ensure the finish matched.

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This seemed pointless (there was nothing wrong with the gearbox), so the engine was soaked in a degreasing bath and then sprayed with Smoothrite. It came up as good as new.

    Replacement of the front main bearing was the first job of the rebuild. This is done with the housing out of the crankcase and the new bearing, a standard BMW part, is exactly the right one for the crankshaft. They come in a variety of tolerances and the crankshaft must be measured critically with a micrometer to ensure that the new bearing is a perfect fit. That's why this should be done professionally.

    The bearing housing is again heated just enough to allow the new part to drop in (pic six) without force being required. The second special tool is used here (seen in pic six).

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Once everything is cool, there follows a really tricky bit. As the new bearing does not have the necessary holes, the oil passages need to be drilled out (pic seven) along with a hole for the locking pin. If the bearing was already drilled it would be almost impossible to align the two holes.

    Three 3mm holes are needed, two oilways and one for the locking pin. Each hole is blasted clean with the air compressor. The locking pin hole must be drilled to exactly the right depth (pic eight) – too little and the pin stands proud, too much and it drops through.

    The first 3mm hole Bob drilled goes straight through. He then uses a 3.75mm drill to take the pin. A metal blanking plate (pic nine) is placed inside the housing to stop the drill, which has a little peg on the end to help locate it (pic ten) going too far.

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    A hand turn with a countersunk bit (pic 11) will remove any burrs protruding into housing. Now drift home the locating pin (or dowel) and centre-punch it (pic 12) to make it tight. This pin will prevent the bearing turning in the housing.

    (I have awful memories of the rear main bearing on my BMW R51/3 spinning in its non-removable housing over 30 years ago and having to be changed every 10,000 miles. Later models had a removable housing.) Also in the housing is an oil-pressure relief valve. This should not normally need any attention.

    Over now to our gleaming, re-painted crankcase (pic 13). Fitting the rear main bearing requires the same procedure, inserting it from the rear (clutch) end (pic 14). The special tools are needed but there is no locking pin and the oilways do not need drilling. Even so it is not a job for the inexperienced amateur.

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   Turning the crankcase on end, the sump oil strainer can be bolted up. First the 10mm bolts on the oil pick-up housing are coated with Loctite Lock 'N Seal (pic 15) and a new gasket fitted to the strainer. The assembly is bolted to the crankcase (pies 16 and 17).

    Inside the crankcase the bearing thrust-washers are the same front and rear. Various thicknesses of thrust washer are used, so if a replacement is needed make sure you obtain the correct washer. The part number is stamped on the washer. A little grease is applied to keep the front one in place on the locating pin (pic 20).

    Fitting the crankshaft feels like the job's almost done – but it isn't! Bob decided to extract more power from this engine and had (at enormous expense) the conrods and crankshaft web (pic 21) polished to improve oil flow over the surfaces. He also had the crankshaft balanced and lightened and the edges chamfered to give a smoother flow.

    The crank and the cases are thoroughly blasted with compressed air before the crankshaft is fitted. The crankshaft rear is then coated with Optomoly Paste PL (pic 22) to protect the bearings until oil begins to circulate when the engine is started.

    The crank is inserted into the crankcase from the front (pic 23) and the rear shaft drops straight into the rear main bearing without the need for heat. However, before the crankshaft was located, the front of the case was heated (pic 24) to help the front bearing housing to fall into place (plc 25).

    A little oil was put on the bearing before it was placed. over the crankshaft at the front and secured by its 13mm nuts.

    Next is the camshaft (pic 26). Again, a little heat is applied to allow the camshaft to drop in before being bolted up (pic 27) with the original bolts and new spring washers.

    Heat is also needed before fitting the main sprocket – use a hot plate to ensure an even spread of gentle heat (pic 28). Once warm it can be placed over the crankshaft end, locating its position on a Woodruff key. Unless it is damaged the key will not need replacing. The sprocket has been marked with chalk to make it easier to align it with the key. As it meets resistance it is driven home with a mandrill (pic 29).

    With both sprockets in place the cam-chain tensioners can be fitted – once the sprockets are in the correct position to ensure the valves open and close at the right times. Chalk marks are used to show exactly where they should be (pic 30). Then a circlip is snapped into position.

    The chain can now be fitted (pic 33) and, as we are working on the bench with the sprockets facing upwards, the chain can be fed over the sprockets, taking care not to move either top or bottom sprocket and louse up the timing. Chalk has been rubbed into the timing marks to make it easier to see.

    An old connecting link is inserted halfway into the two halves of the chain from the top (pic 34) to hold it together whilst the real link is then inserted from the underside, forcing the old link out. To make this easier, rotate the chain to position the link at 10 o'clock and allow more space under the chain.

    It is now a piece of cake to click the spring link home (pic 35) having packed the hole below with rag in case you drop it! Make sure the rounded end of the link faces the direction of travel.

    Now the crankcase can be placed upright and the hydraulic tensioner primed with engine oil (pic 36). While adding the oil, pump the tensioner blade to expel the air.

    Fit the slave tensioner, the one without the hydraulic system. A new blade should always be fitted – the photograph shows why (pic 37). It is held in place by a 13mm nut and bolt, again tightened to the recommended torque (pic 38).

    Make a final check on the alignment of the sprockets (pic 39) before retrieving the front crankshaft bearing from the hot plate and dropping it into place on the end of the crankshaft (pic 40) and then driving it home with a drift (pic 41). 

Click on the view original BMW article link to view images and read more. 

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