Catastrophy: '72 R75/5 Blown Piston

I have been a well pleased owner of a '87 K100RS for the last ten years, but recently branched out into the airhead community when I purchased a 1972 R75/5. After putting about 300 miles on the 'new' machine I noticed a loss of power and stopped to investigate. I was able to get home (~20 miles) running on one cylinder there was a lot of oil dripping from the exhaust on the problem side. When i removed the valve cover I discovered a quarter sized hole in the piston. I found another article discussing the same issue(https://forums.bmwmoa.org/showthread.php?3962-Holy-Piston!&highlight=piston) regrettably ending without resolution.

My understanding (from the above thread) is that this is likely a timing/detonation or fuel quality issue. I had been running regular unleaded gas. I took the head and piston to a local shop for repair/replacement, but that was over a week ago and they have made zero progress sourcing parts.

I am seeking advice, theories concerning the cause, encouragement, and similar personal experiences. I am anticipating considerable debris in the oil pan and valve damage, in addition to replacing the piston. Maybe i should just go back to the ol' faithful flying brick?

Wally -

Welcome to the forum, but sorry to hear about your /5 experience. The stock compression ratio is 9.0:1 on the bike...that's actually relatively high. My R100/7 was 9:1.1 but recently I did a top end with an aftermarket kit and had to lower the compression down to a shade under 9.0:1. I'm running the highest octane I can get. I think you should have been doing that...I'm surprised you didn't hear some pinging from the engine under heavy load when you were riding it. Could be also that the timing was slightly advanced, making the situation worse. I would think that you're going to have bits and pieces of small particles floating around inside the engine. Might have to consider a complete teardown and inspection. Or look for a replacement engine. Not fun, I'm sure.

Welcome aboard Wally, sorry you've run into this problem.

My first question would be, where are you located?

Second question would be, what kind of "local repair shop", are you referring to?

Third... have you dropped the oil pan, to see what kind of mischief has occurred?

wallyduke said:

When i removed the valve cover I discovered a quarter sized hole in the piston.

Click to expand...

Hi Wally...…….So you ate a piston for sure.....I wonder if you mean that you removed the head???......If you took off the valve cover and saw the piston, you have a REAL PROBLEM!!!!

You have for sure:
Ruined a piston of course
Probably totaled the head and combustion chamber. The valves probably got some pieces.
The cylinder is going to need work as it is probably pretty scored after 20 miles of crud working on it....

Yes, remove the pan, clean the shrapnel, new cylinders, new pistons, one new head...……..$$$$$$$

Lots of work; but nothing that many of us haven't done. It's now called a basket case when you go to sell it...…...

I would encourage you to think of this as opportunity to learn. Patience should be your M.O. If you are not already a member of Airheads Beemer Club, spend the $25 to join up. When your first issue arrives, go to the back cover and look up the Airmarshal for your state or province and contact that person. You will then be put in touch with someone who can advise you on the ins and outs of these machines. If you live in Illinois or one of the neighboring states, I would be happy to assist you.

I am living in Traverse City, MI. I took the parts to a local motorcycle repair shop called "Cycle Moore" which has been in business here for a long time. The owner -'Grandpa'- assured me he could source the parts and informed me there would some major crud elsewhere in the engine to be removed. The cylinder was surprisingly unscathed (relatively). It had quite minor scoring on only the very outer 2-3 mm. I have since bveen referred to the local BMW wrench- Rick Pelton- who I will be calling today.

Thank you all for your replies.