Forum bmwbike.com

Recently I had that strange sound from the engine, sort of 'dingeling'. At closer inspection I also found that with every stroke of the piston, there was some puff coming out beside the spark plug.
So I thought 'that's easy' and tried to tighten the plug - but no way, so I took it off and had the impression, it had grown pretty fat in the meantime. But it wasn't the plug alone, its thread was nicely surrounded by another thread having come off the cylinder head.
Looking into the opening for the spark plug I found a little ring bouncing up and down on the piston - the remaining part of that additional thread and also my dingeling.
So obviously that additional inset for a ruined spark plug thread was at the end of its life and unfortunately the cylinder head with it.
After some searching I found a nearly new one at our local wizzard's stock. I think, he is repairing and restaurating BMWs since the company started, but still alive and kicking. Anyway, I put the new one on - of course I could not get it oil-tight with the sealing alone, so had to use some glue.
What I noticed was, that with the new head I had about 5mm more room to go for adjusting the valves (yes, 5 not 0.5), so quite some tolerance on the height of the heads it seems.
After having fixed that I went along happily for about a day or two, when there was that 'dangelang' and not much thrust left. As I was fed up with nuts and bolts, I took it to a repair shop. They suspected a worn out bearing for the crank shaft, but after a bit of inspection it was only one of the push rods, which had lost about 5mm from its top (but not the ones I mentioned before).
See, what comes next - not my lucky year so far.
Anybody similar experiences?

Hi Rolf,
Your original head could have been repaired by any competent engineering shop. It sounds very much as if the spark plug hole was fitted with a "helicoil", probably without the head being removed from the engine. Alloy threads are delicate, and if the plug/ thread interface becomes coroded or the thread exceptionally dry it sometimes picks up the alloy thread and destroys it as the plug is wound out. This was a common problem in VW aircooled angines. It is not uncommon to find old BMW heads with bronze thread inserts that cure the problem permanently - that goes for the exhaust thread too.

The 5mm extra gap at the tappet may be due to the valves sitting higher on their new seats in comparason with the old seats, which might well have been cut back excessively at some time in their past, but 5mm sounds a lot. Did you fit new valves?

When you say the pushrod had lost 5mm from its top, can we assume the hardened cap had come adrift?

We await future developments with interest!

For the additional thread: no helicoil, it was a big piece inserted, like the one you mentioned as repair. This one having come off did not leave any chance for repair, because it would have meant taking so much away, that the valve seats would come off in the next step.

For the push rod: yes it was the hardened top - I hope nothing is left somewhere inside the engine.

For cylinder head height: the new one came with valves inside - and I hope the proper ones. I measured 31mm on the outlet as diameter and 34 for inlet. The manual describes it sometimes the other way round.
More I did not check, because I did not dare to take them out.

Presently I still have heavy noise and poor thrust, wonder why?