Wednesday 23 July 2014

E60: Engine Rebuild 2 - Mashed up piston re-grind - ghetto style...

Bits of swirl-flap doing the skinhead moon stomp inside your cylinder is not good. It batters the top of the combustion-chamber in the head and the piston-crown good style, though in most cases won't score the cylinder-wall bad enough to need honing or re-lining. I went and swapped in a re-con cylinder-head as mine needed too much work, but the piston was a bit more of a head-wreck.

Firstly it's quite difficult to find spurious or 2nd-hand single pistons. There are plenty of other pistons on eBay, so I guess it's just waiting for an M57N to come up, otherwise it's over to BMW themselves who will charge about £250 for the part. It doesn't just double the cost of the job, but doubles the work too - the plastic and metal under-trays need to come off, along with cross-members and engine-mounts, the gearbox needs supporting, then the sump comes off and the big-end shells, finally the piston is knocked out of the top of the block with a hammer and block of wood.

I was working on gravel and moving the car onto ramps was out of the question, so in the end I went cowboy style and reground the damaged piston-crown while it was still in the block. Using a die-grinder I leant into the cylinder and remove as much of the sharp metal protrusions as possible, getting it pretty flat for the most part, but the worst bits are at least rounded into small bumps, rather than spikes that could break off and do more damage. I can't imagine combustion is as good as before, but as long as it compresses then I doubt any difference will be noticeable.

PIston 4, second from bottom, took a few valve strikes, worse on the exhaust side and needed a little bit of grinding left and right. At the top of the piston in the pic it seems a few bits of swirl flap did some damage, however the lower side is totally clear. I seriously doubt any performance will be lost on this cylinder.
Piston 6 took the brunt of the swirl-flap and was entirely covered in sharp peaks. The top and bottom faces came flat again with a few hours spent grinding, but the left and right really needed some work. About 5 hours spent in total leaning right into the engine and holding a steady hand and still its like the surface of the moon, but better than all that work to remove the piston and all 6 cylinders appear to firing great after 50 miles.

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