Rodrigo Alvarez

Tinkering as a way of life

K75 Restoration: Engine diagnosis

Last time I wrote I knew that the engine didn’t start, but lets focus on the good things; The electrical system seems ok, ignition and fuel pump look also alright, and I haven’t seen any obvious leak. Since then, I’ve also taken the time to check the rear spline and it seems it’s in top condition.

Normally, I stand by the rule that you should always purchase quality tools. However, not being a professional, there are tools I’m a bit hesitant to buy as they will only be used once. One of such tools is a compression gauge. Therefore, at around 35€ I purchased the cheapest one on Amazon, just to get a general impression of the state of the engine. The results were as follows:

Test (Average of 3 measurements)Cyl. 1   Cyl. 2   Cyl. 3   
Without adding oil75 PSI0 PSI150 PSI
Adding 5cc of oil to each cyl100 PSI0 PSI160 PSI

So, found the problem. Cylinder 1 is wounded & 2 is dead, however, cylinder 3 is in spec (140-160 PSI). While I’m here I might also check the valve clearances

The workshop manual states that inlet valves should have a clearance between 0,15 and 0,20 while exhaust valves should have a clearance between 0,25 and 0,30. I measured the following clearances:

ClearanceCylinder 1   Cylinder 2   Cylinder 3
Exhaust     0.20-0.250.25-0.300.15-0.20

As far as I understand I should change the shims on the exhausts valves of cylinders #1 and #3, but this doesn’t really make any sense if I have to install a new head. Technically I could remove the head with the bike still in the frame, but the engine is so filthy it could use a bit of cleaning. I convinced Álvaro Pérez and Javier Pérez (not brothers) to come by, and in a few hours we had removed the engine, gearbox and final drive from the frame.

Alvaro and Javi were invaluable to dismantle the bike and sort all the screws into ziploc bags

After a bit more disassembly later with other buddies that wanted to take part, the engine made it to a fourth floor I can use as a workshop. I started cleaning the engine as I didn’t want a rogue grain of sand damaging anything sensible. Stuck to the engine there was a mix of oil and sand that tinted black anything that it touched.

All screws were removed and placed in bags labeling their position and any gasket residue was quickly removed with scotch brite. Another few days and the head was disassembled, and voila, here is the reason for the low compression on Cylinder 2; a broken valve…

Filthy cylinder head and broken No.2 Exhaust valve (bottom middle valve)

Unfortunately a new set of valves will set me back around 650 euros bought from BMW, however I’m searching for alternatives.

Shout out to Alvaro and Javier who helped with the heavy lifting!