big bore balance factor

I've hinted at this project in the past and now it's becoming a reality. What insanity am I up to now? The engine in question is another 1996 Vintage FC600.

See my old thread for the other 600 I've done (http://husaberg.org/mechanical/16703-husaberg-ktm-hybrid-engine.html)

This one, however, is taking the insanity to the next level. As such it's out of the realm of what I'm able to accomplish in my garage. With the popularity of big bore KTM engines and the similarity between the KTM RFS and the original Husaberg jugs, I decided that I'd contact Travis at Thumper Racing to do the work. He bored the cylinder and engine cases and pressed in a cast iron sleeve honed to work with a Wossner Husaberg 550 flat top piston (100mm bore diameter). He also had Carillo make a custom rebuildable rod matching the stock length, width, etc. He even had a custom Titanium wrist pin that I suspect was for another project but went unused so I got it for a steal.

Then I purchased some ceramic bearings from Microblue including cam, crank, and a few other bearings while I also had them run the piston, rings, and rod kit (rod, big end bearing, and crank pin) through their process as well. I even had them process the cam follower rollers, pins and needles. I figure this'll be a really good test of the Microblue process given the lack of an oil pump in this engine and a few of these are common trouble spots.

Well, it's now time to put the engine together (now that I've saved up some funds from kicking the beer and firecrackers habit) and, once again, I'm in contact with Thumper Racing to do the work.

Going to have them install larger valves in the head along with a little runner porting, port the combustion chamber for a reasonable compression ratio, bore the carb, balance the crank, and perform most of the final assembly.

...aaaannd I finally get to the point of my post. In talking with Travis, he is recommending a 58% balance factor. Since this engine is destined to reside in a 1996 chassis, does this balance factor sound reasonable? I've heard of balance factors between 60% and 80% being used in the newer '00-'08 chassis and my hope is that the '96 frame is similar enough to warrant an equally similar BF. What say you, the collective engine building geniuses.

And now for a few pictures to whet some appetites...

wow, a 600 four stroke without oil pump !?? that's the craziest thing i've heard in a while. lol

Anywhere between 30 and 80 % but that depends on chassis geometry and stiffness and resonant frequency of the assembly, etc etc... hard to say if it's good before you try it out.

I'd try what travis reckons. Then if you get the chance at a later date maybe make an alteration if you think it needs one.

In both my bikes (With no balancer shaft. One with an 80mm stroke / 105mm bore and the other 80x100) 0.62 was nicer than 0.57.

Honestly though when the engine makes really good grunt balance factors are the last thing you think about while riding.

Wow.

Thanks Bushie. Hearing this from you gives me a lot more confidence in the 58% number Travis mentioned.

I also get what you're saying about grunt-y engines and not caring. In the other 600 engine I built (the one with the KTM rod) I had to remove a lot of material in the area of the crank pin to make room for the wider rod and, although I never determined the balance factor, I expect that it was much closer to 100%. However, it's got GREAT low end and pretty much nothing on top so I lug it most of the time and don't really care.

On the other hand, I will want this engine, which will end up at 660cc, to be able to rev comfortably on occasion. My plan is for it to be as versatile as possible so I can ride it anywhere from tight singletrack in the woods to wide open in the desert.

Fantastic project thorgan. In my bikes the main vibes are felt through The foot pegs under 3000rpm.

At higher rpm the engines are smoother than they were with the counterbalancers in.

The 100x80 engine has shown 10K on the data logger. A little bit crazy it lasted 100hrs lol Prox piston cracked then be big end went. Almost at the same time.

I went back to the stock cam to settle it down a bit and with less rpm has done another 150 hours on a 570 wossner piston.

Would love to see more pictures as you are able to post them.

Hmm...all I've got at the moment is a shot of the box of parts after returning from Thumper Racing. I'll get more when I have time. Gotta finish replacing the timing chain in my Toyota first. Too many projects!

more pics:

Cases all machined, clean, and ready to go.


Closeup of the work.


Big end pin. The surface in the middle is the Microblue coating.


New piston vs old. I expect that the new one is significantly heavier. Bigger diameter and much wider skirts.


KTM 560 SMR head gasket.


Rod big end bearing (came with the pin from Taffy and I sent them in for the Microblue coating), a transmission bearing (not sure which one yet), and the mains. These ball bearings are hybrids. The balls are ceramic and the races are steel. Very expensive!


Cam follower rollers, pins, and needles. All got the coating. We'll see how they last...probably for ever given how often I'm able to ride these days.


Bearing seals.


Cam bearings and, again, not sure which these larger 2 are but I know they go in the transmission somewhere. Travis will figure it out.


Another shot of the rod.


Cylinder - bored, sleeved, and honed by Thumper Racing and then coated by Microblue. Can you tell that I've jumped onto the Microblue bandwagon with both feet? Kinda crazy since I haven't got any experience with them at all.


Closeup of the sleeve work.


That's it for now. Gotta box it all up with the rest of the parts and get them up to Travis. Now to figure out what I'm forgetting. Oh, I know...the Titanium piston pin. I guess I'd better find that. Probably in a box somewhere.