The story starts with me being a bit unhappy about the engine, which I built lately. In essence, I've built the perfect race-engine, not much grunt down low, but oh-boy does it rev. (And make stupid power in the progress.) Unfortunately the specs said "everyday-engine" and it was rather bad at that compared to what I had before. So I inquired about combining the heavy crank with my BT/XVS/V-star rotating assembly and the universal answer was: don't. Prior to asking those questions, I had thrown some crude maths at it. (I will get into the intricacies of how and why this isn't very precise and you should do it differently in a subsequent post.) And worked out that the balance factor will go up, not realising that the new conrods have got lighter small-ends and that would limit the effects quite substantially.
stock XV1100
XV1100 crank with (heavier) XV1100 conrods and BT1100 pistons
XV1100 crank with light pistons and with the correct conrods as well
(The last calculation was done after I had the engine open and was able to extract crank and conrods.)
And with that being said, I went through my stash of XV1100 cranks, dug out the one that was in the engine before and cleaned and stripped it.
Pulling the engine apart revealed a nice surprise, the XV700 heads are working flawlessly with the tightened squishband as can be witnessed by the clean areas in the head where turbulence is happening.
A quick check of the clearances between crank and conrod revealed Yamaha hadn't changed their tolerance codes and I made a spot on landing in the middle of the tolerance field with 0.040mm clearance, when combining BT-conrods with a XV1100 crankshaft.
And there we have the unholy union, called bumblebee. Also my assembly lube is a bit stiff.
Then followed the usual steps of reassembling the engine.
Just to prove: These are actual BT-pistons
Put the cylinders on
No base gasket to improve squish, only rtv-silicone.
And then after assembling the rest of the bike, I gave the exhaust a quick wipedown - the smoke in the video below is from the oil burning off.
https://www.youtube.com/watch?v=Zdz8l_eMlBI
So what's it like: It's basically an XV1100 engine again, but with notably less vibrations. Actually hardly any vibrations at idle at all, which is almost a bit irritating. It has got low-down torque for miles and obviously it doesn't like to rev as hard or rev-up as fast as with the lighter BT crank. And incidentially it proved a point: a roughly 3 percent change in crank balance factor can be felt, if you compare them back to back. And also of course: bumblebees can fly.
(The full version is available on my blog under: https://greasygreg.blogspot.com/2020/04 ... lebee.html )
