This page is about odd or unusual noises found in a BMW. This is by no
means finished and maybe can't ever be finished. More noises,
symptoms, tests and possible cures will be added as time and memory permit.
I have only found this one a half dozen times. On a /2, a very hard hitting
noise will be heard at idle. It goes away at higher rpm. It seems to come from
the engine and sounds like someone is hammering on the engine case. It is really
awful. Owners think it a rod noise. It even sounds worse.
I don't remember the first one of these that appeared, but I will never
forget a later one. The customer came in with this awful noise. He was just
devastated, as he had been to my competitor and had been told that it would cost
$500-$600 (in 1968) to rebuild the lower end with a new crank. He was told that
it was rod knock. He could buy a new bike for only $1200. I listened and ran my
hands around the engine a bit and asked if he would be happy if I could fix it
for $100. He was very happy. I asked if he would be happy if I could do it in
about an hour. Now he wasn't so happy, as the current labor rate was $7
Up under the gas tank is the top motor mount. Here is what happens.
The one in the photo had been "spread out" to remove it from the frame. As
you can imagine a bolt is run through the top part to draw it together on the
frame tube. The lower hole is bolted to a bracket mounted on the engine. The
bolt isn't really tightened up, as it has rubber spacers to allow for some
slight movement. The nut is a castle nut and a cotter key keeps it on. It all
works well, usually. Not shown, because I don't have one, is the
short sleeve that goes in between the two upper
parts, in the "spread out" place. It keeps them from getting
drawn together so much that they are no longer parallel.
In normal operation the engine shakes and this part takes some beating. In
time, the shaking can cause the circular part to take a different shape around
the frame tube. This "new" shape conforms better to round than when new. Now the
short sleeve is too long to allow the clamp to be drawn up closely enough to be
tight on the frame. Now the whole thing can rattle. As the engine rocks, while
at idle, this part hammers away at the frame. The short sleeve now is keeping
the clamp from being tightened up enough. The solution is very simple, remove
the short sleeve, shorten it a bit and reinstall. The pain is that the
tank must be removed to get to it.
Isn't that better than buying a new crankshaft? That is a worst case
situation. The mount can be only slightly loose and just barely rattle lightly. Then it is harder to find. Reach up under the tank, with the bike at idle, touch
both the clamp above and the frame. You should feel no movement between the two
parts. Go test yours, it only takes seconds.
Valve noise from the head fins
In 70 and 71 the fins on the heads radiated a lot of the valve noise. In 72,
BMW came out with heads with roll pins inserted through the fins. This dampened
the noise a lot, but some still remained. The rubber inserts or round plugs can
be purchased and inserted. One can even make up some type of rubber plug
to slide between the fins.
Both great sketches by Chris, thanks
This shows the recommended position of the inserts on 70-71 heads without the
roll pins. It really matters little about the exact placement. They
could also be inserted between every fin and staggered.
This placement will work on the 72 and later heads. This will only make the
noise slightly less. The largest change is to tighten up the blocks on the
rocker arm shafts. Do it first and only add in the plugs if you want a bit
Front drum brake squeal
The famous shifting clunk.
Electric starter noises.