As many of us already know the cylinder studs for the Tigre engines should be rolled and not cut.
It happened on a warm summer evening over northern Germany in 2007. The engine began to shake heavily, lots of oil was coming, as we like it only in the movies...
I was very happy to be close to a private field and within 2-3 minutes on the ground. The engine was still jumping around but I could hardly believe what I saw then. On cylinder 1 all four studs were broken and i could see piston 1 doing it´s work. All nuts had been secured with metal brackets.
After investigating the cause of the stud failure, we assumed that the screw quality has played an essential role. We found that the studs of our engine were cut and i decided to produce new ones from chromium-molybdenum-vanadium hardened and tempered steel: 1.7734 The studs were manufactured in Germany by a certified company : http://www.grohmann.de/
If these engines are overhauled several times, studs in and out, there is probably also a movement of the secured studs in the engine block.
To my knowledge there was a rule in the Spanish military to check the torque every fifty hours. A lot of work.
With our new overhauled 150 hp Tigre ( in 2008 ) we have tightened the new studs with 50 Nm. (Newton-meters) We checked every 25hrs and found some of the nuts with 30 to 35 Nm. It was getting better every check and after 100 hrs we changed to check every 50 hrs. The engine was fine with this more than 300hrs in the last 5years.
In order to produce the studs, special tools had to be made to roll the thread. To allow other Bücker friends to benefit, i ordered six sets of studs and nuts. I have still two sets left over if somebody is interested.
I payed 5756,- EUR for all six sets means 966,- EUR each set and will sell the set now for 600,- EUR. Expensive stuff.
If anyone is interested in the test results and images in more detail, i have more paperwork.
As always happy landings
Gottfried Pönnighaus
Click on the Download link, select the "Technical" section and download tigre_studs.zip to see Gottfried's documentation.