Rodrigo Alvarez

Tinkering as a way of life

K75 Restoration: Engine Rebuild & Final drive maintenance

Taking advantage of the christmas holidays I’ve been working hard on the engine.

Having the head removed, it makes sense to check the state of other components, such as the piston rings, connecting rods and bore tolerances. I should replace the big-end conrod bearing shells, which are worn but have not reached their service limit, but as this engine is side mounted, they can be easily accessed from the side later on, so that’s a pass for now.

When I finish this post, this is what the engine should look like

I’ve removed the pistons and measured the piston ring gap (1.4mm) which hasn’t reached the service limit of 1.5mm but now that I’m here it makes sense to change them. Fortunately for the price of three sets of piston rings, I managed to get three full piston sets original from the factory. The rest of the parts are within spec.

Alvaro decided to join in this one as well, after watching me replace a piston, he then replaced the remaining ones.

Alvaro, the new master of everything piston-related

Installing piston in their cylinders is a pain if you don’t have a proper tool, I know this because I purchased the cheapest on Amazon, and it is NOT the proper tool. Alvaro, with less care, or should I say, more determination than me, managed to force them in. Unfortunately I don’t have any pictures of the process as we were both very busy wrestling the pistons in. We then installed new big end bolts, and then the connecting rods have been torqued to their 30n·m + 80º spec.

Head Assembly

New head gasket torqued at 30n·m and after 20 mins at 45n·m and the head is installed on the engine. Then the camshafts are installed, but taking care to loosen the gears (it can be done after, but it is easier before installing), as the chain will not fit with the gears tightened. I believe this was an oversight by the design engineers. The chain goes back in, taking care to leave piston 3 in Top Dead Center (There is also a pin in the crankshaft that should be pointing towards the cylinder head) and the camshaft gears have a little mark that should be pointing upwards towards the air intake. The remaining slack of the chain should be left were the tensioner is located. I also checked and adjusted the valve clearances as well.

Starting to look like the first image!

The rest of the work is just putting covers creating a seal with a gasket maker. I used Permatex ultra black but I believe there are a few others that are oil and heat resistant.

I’ve cleaned every single bolt and nut with oil and a scotch brite pad and a buffing wheel. The engine really looks like it belongs to another motorcycle

Engine ancillaries

The combined oil and water pump is notorious for a failing gasket and thus eating the pump shaft. Fortunately I arrived on time and the shaft is still in a good state. BMW has changed the slip ring seal to a new one (11411741870) but it is a part that as it used to fail very often, it needs to be purchased from the dealership directly. I was told that on assembly it is normal that the pump will offer more resistance than usual, and so it did. If I hadn’t been told I would’ve freaked out. A thin layer of Permatex black gasket maker, new o-rings and into the engine they go.

Oil – Water pump seal rebuild kit, a must if your shaft isn’t damaged yet

Alternator and starting motor have been both disassembled, cleaned, and given a pass with the buffing wheel to remove rust. They have been reassembled using grease, and of course, great care.

Before dismantling the clutch I should’ve marked the position of the pressure plates and housing. Fortunately after a bit of cleaning I could make out the original assembly markings. From what I’ve read online, Sachs, the clutch manufacturer, indicates the heaviest side of the part. The clutch being formed by three parts, the best way to avoid vibrations is to try and set them 120º apart. Once dismantled its a good idea to replace the O-ring (11211460467) that separates the clutch from the crankshaft. To remove it I was advised to jank the housing to expose the o-ring and then cut it. Once the friction plate was out, the thickness was measured revealing it still has half of its life to give. A new clutch plate has a thickness 5.3mm and its service limit is in 4.5. mine is around 4.9mm.

After a spline lubrication the clutch went back together. The clutch can be assembled without the recommended tools, but they do make life a lot easier. I’ve 3d printed the friction plate centering tool and it works fine. I can provide the file if you want to 3d print it.

The rest of ancillaries were de-greased and cleaned up to the last screw

Final Drive

After draining both transmission oils, the final drive was up for an inspection. A critical point of these motorcycles is the power transmission splines, they are between the following parts; engine-clutch-gearbox-drive shaft-bevel drive. Fortunately someone along the life of this bike, knew that this was a critical point and decided to pack the whole final drive with Moly grease. Maybe even too much, but it has allowed the parts to withstand the test (and abuse) of time.

Can too much grease be harmful?

All splines were cleaned and greased with Molykote G-N plus (a reasonable amount) which I find Is the closest thing to Honda Moly 60 you can purchase in Spain at a reasonable price.

I am ready to put the bike together!