05 December 2015

BMW Timing Chain Inspection

The last oil change I did on the R90/6, I noticed some very small, black, plastic looking stuff inside the filter. I assumed this was from the timing chain tensioner as that's the only plastic coated component that could possibly make it into the engine, the timing chain did also appear to rattle and slap a little when the bike was at idle. I figured I'd turn the inspection into an Airhead Beemer Club tech day as the dis-assembly process and subsequent re-assembly is a little more advanced than typical Airhead maintenance. I ordered a new duplex timing chain, crank sprocket, and tensioner (and all the other bits like gaskets, washers, and tensioner spring) from Germany and they haven't arrived yet, so for now it's just the inspection.

Here's the process:

Remove the exhaust headers, I leave the mufflers in place. Some guys say to remove the front wheel and forks to give yourself more room to work, but I also leave them on. Set the left piston to TDC (that's the position the crank will be in for the entire process), and then loosen the rocker arms to remove the spring pressure from the right cylinder head against the camshaft, this will prevent it from rotating the cam while you're working. Next REMOVE THE BATTERY GROUND and only after this do you remove the front cover to access the charging and ignition systems. Failure to do so can result in a fried system from the cover shorting against the diode board. After disconnecting all the electrical connections on the alternator, remove the three mounting bolts on the alternator, and then remove it. To remove the alternator rotor, you'll need a special hardened bolt that threads into the crank and pulls the rotor from the tapered crank nose. Then remove the camshaft nut and remove the advance and points plate. Diode board is also pretty straightforward.

I thought I could get away without removing the starter cover and just use needlenose pliers to remove the wire going to the starter solenoid, but I wound up removing it anyway. That red wire on the bottom goes through the timing cover and connects to the diode board, obviously I took this pic after the diode board was already removed.

While removing the socket head screws and nuts, I managed to strip the head on the last one I had to remove. After about an hour spent drilling it out, I was finally ready to remove the timing cover and inspect the timing chain. To remove the cover you need to use a heat gun to heat up the cover around the crank nose where it's an interference fit against the bearing below it. After about 3-5 minutes of heating, lightly tap on the cover and remove it, it should come out very easily, if not then you need more heat. Once removed, this is what you'll see.

My timing chain had plenty of slack in it. Besides that, the chain tensioner was worn and missing pieces of the plastic coating (what was getting trapped in my oil filter, as seen in the pic below). If you plan on removing the chain for replacement, first remove the spring for the tensioner by removing the nut pictured at about 3 o'clock in the picture above. Next, remove the tensioner by removing the securing circlip and sliding it off its pin. Timing chain is removed by large bolt cutters (or a dremel if you're so equipped). Be sure to plug up all the holes that go into the case with rags so no metal can make its way down (I do this even before removing the circlip). Ensure you're still at TDC (OT "Obertotpunkt" on the flywheel). You can do this by, obviously, checking the inspection hole, and making sure the timing marks on the sprockets line up. The one for the cam should be directly above the keyway for the woodruff key (both should be pointing at 12 o'clock) and the one for the crank should be at 6 o'clock, lining up with the mark on the cam (woodruff key slot should be positioned at 9 o'clock).

I'm not sure I'll be replacing the crank sprocket and bearing just yet, the teeth appear to be in pretty good shape. Once the new one I ordered arrives I'll have to compare the teeth, if any wear is noted I'll just replace it so I don't have to go back in and do this again.

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