BMW N20 thrust bearing

Those of you who are familiar with our little business up here in The Pennines will know that for several years the N47 diesel engine has monopolized our time. Slack, or jumped, or even snapped timing chains, broken chain guides, blocked oil pick up pipes, bearing damage, valve damage…it has been our bread and butter.

Recently however we have been more aware of BMW’s N20 turbo-charged 4 cylinder DOHC unit, as it is seeming to fail earlier and in our experience more comprehensively than the N47. On average 1-series and 3-series N47 cars have come to us with timing chain issues at around 100k miles, with 5-series cars about 40k miles later (we put this down to the type of mileage done by the different cars). The N20’s, which are fitted to much of the BMW range since 2011, are coming to us between 40k and 70k miles and with comprehensively more damage.

Our most recent example is an F20 125i M-Sport with 70k miles on the clock. Originally the car was booked in for being lumpy, having a ‘rumbly’ noise and hunting at idle. When we first examined the car it was clear that the chain was rattling pretty terribly, and that there was a familiar clatter from the turbo wastegate.

We removed the engine for ease, and stripped down to the timing chain and sump. The guides were still intact but the chain was very slack. The oil however was clean, there was no sump debris or swarf in the oil or filter. We initially decided not to strip further at the bottom end. Luckily, a day later we were training our apprentice and using a previous N20 job as an example. We went to our 125i engine on the stand and demonstrated to the apprentice the difference between axial and radial movement on the crankshaft. It was then that we noticed the crank had around 6-8mm axial movement…far more than we would expect.

We stripped the oil pump and the bearing caps and removed the shells. The radial shells had wear to the outer layer, but not as dramatic as we had seen previously. The thrust bearing was a different story though. Designed to restrict and control the fore and aft movement of the crank when the crank is thrust by clutch (or converter) operation, these shells are ‘winged’ with a grooved flange. Our thrust bearing was completely worn through the flange on one side (see images below), but more importantly only the top half was flanged. The lower portion was a regular shell, despite the block having the groove for a flange present. This is the second time we have seen this.

BMW sell a replacement bearing shell set for this engine which has the centre bearing in the form of two flanged shells. However, on at least two of the cars we have seen, it appears that is not how they left the factory.

Our 125i crank is terminally damaged with a 1mm deep and approx. 6mm wide groove in the thrust face. The repair costs are substantial.

It is our intention to continue to monitor N20 bearings as and when we see them, and if this pattern is repeated it may be necessary to advise our customers to change bearings before serious damage is caused.

 

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