Technical 126 Crankshaft RPM Limit

[Bleeding Knuckles]

Greetings All, Question for the great and good on here - What is the expected RPM limit (Breaking point) of the standard OEM 126 crankshaft??
and what would the figure be if the said crankshaft had been fully balanced?? The well known 126 Blitz article quotes 6500RPM for a standard crankshaft, but is it fully balanced?? Also a well known German engine builder quotes (for a lot of money) an engine that will produce 70BHP @ 7000RPM and 80BHP @ 8000RPM :worship::worship: What about the 126 BIS crankshaft?????
able to take a higher level than a standard OEM Crank???? If so to what????
Has anyone had a crank break in use???

Ian.

 

[the hobbler]

Evening Ian;
I have always been led to believe that 6,000 rpm should be considered the "normal" maximum revs. Over that speed, the whole crank-line assembly is required to be balanced. Additionally, if over 6,000 rpm is going to regularly exceeded the core-plugs in the crank-shaft need to be "spot-tigged" (PRIOR to getting the crank assembly balanced)---if this is not done, there is risk that the core-plugs can be blown out, with the resulting loss of oil-pressure causing a fairly terminal end to the engine!
The other problem with revving the engine regularly over 6,000 rpm is that the crank-shaft flexes at high revs, with resulting excessive main-bearing wear. The use of STEEL backed main bearings does help to reduce this flexing.
I agree Ian, there are people out there who advertise that their engines produce peak power at quite high revs (up to 7,000 rpm), but how long these engines last is debateable. There is one RACE engine builder who reckons that he gets over 80bhp, and that his engine revs to 10,000--but the engine is VERY special, and includes a 'twin-cam' head and fuel-injection and, I suspect, cost a LOT of money---NOT suitable for road use!!

 

[Paolo66]

Check out dangelomotori.it Ian. They do some lovely engines and parts including balanced steel crankshafts

 

[Toshi 975]

Once upon a time on a forum far far away a wise old man in the ways of getting more zoom in the engine room defined a simple formula for the engine tuning. Carb, cam, cylinder head & exhaust. As a footnote he added that an additional bonus would be to fis a Bis crankshaft as they have better specification at high revs and are stronger.
Could be a fairy tale and the ramblings of an old mechanic who has lost some of his spanners but it made sense to me (still got a few spanners left)
The same crankshaft designed in the mid 50's has been used in the entire 500/126 range right up to the year 2000. That is apart from that fitted to the slightly more powerful water cooled 126 Bis which will fit the air cooled engines as well.
Some wizards in the ways of engineering have told me that by concentrating the mass centre of the counterbalance of the crank closer to the axis of the crank will make it more stable at higher revs.
Not for me to judge but the Bis crank profile seems to match some of racing cranks that sell for four figure sums.
The master wizard from that distant forum developed a 700cc engine, turbo charged with inter cooler that showed 100hp+ on a rolling road. He snapped one crankshaft and bent another one. Sadly he bypassed the Bis crank to see what it was capable of and has made his own steel crank with a centre main bearing.