Rotax 914 head bolt failure

[Area-51]

48 minutes ago, RFguy said:

the manufacturer's advice. LOL. leads to over and under tensioning. LOL. My measurements and calculations showed that the mfr method leads ot tension errors of up to 50 %. likely +/-30%.... anyway.....  yes, initial stretch temp , temperature dependent.

 

There's a bit of a compromise between having enough tension with a crankcase temp of say 30 deg C, and having too much at crankcase temp of say , 90deg C. So,  still a good idea to take it easy until oil temp up to operating temp (which means crankcase temp , also, pretty much) . If the cold tension is too high, safer to TO with low crankcase temp. If cold tension too low, the opposite...

 

I don't want to pollute the forum, this is about 914 heads ,just wondered what the stud diameters were. I'll look it up. Looks like 8mm. 

 

I would be confident the Rotax engineers got their stud sizes correct. the rest is left to tolerances and install .....

 

Stud shank dia' = 0.280" for the early 9XX with through stud config.

 

Calculating stretch on studs will be arbitrary at best without knowing the exact alloy formula employed. You may be able to extrapolate this data from other european manufacturer sources from around the same late 90's early 00's period to get a less arbitrary figure.

Material yield is often the reason soft alloy components are tensioned when cold and harder when hot. Stuff needs to expend without reaching yield and retain clamping pressure when cold again; Nev made some mention of this above being the loss of clamping force cause; i.e the stud stretches to the point the nut losses tension and rolls off its thread. The M8 cap screw though i am not convinced; it is only about 2.5" long and will be a different alloy to the stretch studs. The studs and mains cap screws appear to be fitted at the factory with Loctite DriLoc 200; the head nuts have no thread locker applied.