BMW's 1800cc 'Big Boxer' Is The Bike World's Biggest Ever Flat-Twin

Although this might look like it belongs in a bike from decades ago, what you’re looking at here is a brand new engine. It’s the BMW Motorrad ‘Big Boxer’, and it’s the most powerful flat-twin the company has ever made.

Not only that, but it’s also the largest boxer-twin the bike world has ever seen, displacing a whopping 1802cc.

It’s been used in a smattering of show bikes including the Concept R 18, but from the sounds of it, the Big Boxer is heading to production. We suspect the power output of 90bhp at 4750rpm and the peak torque figure of 116lb ft - developed from 2000 to 4000rpm - will remain the same once it does.

The bore and stroke is 107.mm x 100mm, and it’ll rev to 5750rpm. The idle speed meanwhile is 950rpm. According to BMW, the engine has “enormous pulling power,” and thanks to the mass of the flywheel, you can expect “exemplary running smoothness”. Lovely.

The engine in the R5/R51 of 1936 - 1941 plus the unit in the R51/2 of 1950 - 1951 provided BMW Motorrad’s engineers with inspiration for their new creation. Hence the way it looks, and hence the way it functions.

Just like its predecessors, the Big Boxer has its left and right camshafts positioned above the crankshaft and driven by a ‘sleeve-type chain’. This means the pushrods can be shorter, reducing moving mass.

Modern bike engines tend to use hydraulic valve clearance actuation, but the retro Big Boxer isn’t interested in such tech. Instead, the job is done in the same way as it is on most old two-valve boxer twins - by using a manual adjusting screw for each valve.

Based around a split aluminium engine housing, the air and oil-cooled Big Boxer tips the scales at 110.8kg - including the transmission and intake system. On the subject of the gearbox, it’s a constant mesh six-speeder.

There are a few areas where the new engine deviates from its ancestors. The forged steel crankshaft, for instance, needs an extra main bearing due to the sheer size of the cylinders. This reduces vibrations. The pistons are cast aluminium, and move in cylinder bores lined with NiCaSil.

So, that’s how it goes together, now all that’s left to do is have a good ogle.