What is causing that knock?

Jo said:
Hi Piet,
I'm still lost. What I do not understand is the point that an OE rod would be too long if the crankpin moves 3mm upwards (in tdc). Sure you can shed some light on this ...
;)

Cheers,
Jo
Click to expand...
The 1200 piston is exactly 3mm shorter than the 750! Pretty simple. The clue is in "a stray 1200 piston laying around".;)

Of course this can all be achieved using bespoke pistons and rods, but I'm a big fan of using readily available resources.

piet
 
That is a lot of weight off the crankpin side.
The owner of the broken crank had the idea to go the opposite direction Piet. Same idea but short stroking for bigger bore 750 race motor.
 
Like Julian's 650 SF race motor. I also noticed the weight off the heavy side (reducing the balance factor). Innaresting. I've added weight back onto my SFC crank to regain 60% after I'd dropped it to 50% when I first twisted it.
 
Tippie said:
That is a lot of weight off the crankpin side.
The owner of the broken crank had the idea to go the opposite direction Piet. Same idea but short stroking for bigger bore 750 race motor.
Click to expand...
The longer stroke shifts quite a bit of weight outwards, I would have needed to add to the counterweight. The counterweight is only so heavy, I didn't want to muck around with tungsten plugs and the like. Like I said, I like to keep things easy...

IIrc, I went for 64%, now nice and creamy in my favoured rev ranges. Still gives off a buzz above 6000.

piet
 
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