Husaberg/KTM hybrid engine

I'll admit to being a bit of a sucker for the old (Gen 1) Husaberg engines. As such, I seem to have collected a number of them in various states of disrepair. I have a '95 vintage FE501 currently fitted in a 2003 frame that I just put back together after fixing a broken transmission. I also have a '97 FC600 with a bad rod and timing chain. This engine is apart and not in a frame at the moment. My last curiosity is a complete '96 FC600 bike that I purchased second hand back in December hoping to scavenge the engine for my '03 frame (before the trans was repaired in the 501 engine). I got this bike running but only briefly. Sounded like there was a handful of rocks in the engine so I shut it off. Maybe another bad rod.

So, I began to research engine components and just about everything is still available through various sources. Taffy even had a few rod kits for the 600's, although they may be gone by now. The problem is future availability. I really enjoy these 600cc engines (I rode my 600-in-the-03-frame back-to-back with a 2010 FE570 and really preferred the 600). So much so that I plan to ride them until they're worn out and need subsequent rebuilds. This means that I want to rebuild them with parts that I'll still be able to get in a few years.

Enter the KTM RFS engines. The rod in my 600 rides on a 30mm big eng pin. Smaller even than the later 644 engines which were rumored to have main bearing problems due to the 32mm big eng pin allowing excessive crank spreading. Well, the KTM 450/525 rod rides on a 35mm pin, just like the later Berg 628 engines. The KTM rod also has the same little end diameter as a Husaberg rod so it can use a Husaberg piston. The RFS and Berg engines are so similar that it appears that a RFS head can be used on a Husaberg cylinder!

There are a number of issues to be overcome in order to use the KTM rod. First is the crank pin diameter and my plan is to machine the crank to accept the larger KTM pin. Second is the rod width at the big end. The Berg width is only 18mm and the KTM rod is 21.9mm. Again, my answer is to machine the crank to accept the extra bearing and rod width. That sounds stronger to me anyway. Third is the rod length. The KTM rod is 129mm and the Berg unit is 130mm. My hope is that this relatively minor difference can be made up in the piston. And, the Berg piston has a 23.02mm compression height so, the KTM rod begs for a piston with a 24mm compression height. I'm going to ignore the .02mm as this is quite small. Problem is that the KTM 525 EXC piston has a 23.5mm compression height and I fear that my compression ratio would be way too low. I suppose that I could offset-bore the crank pin hole by 1/2mm to make up for this difference but I'm nervous about rod clearance to the engine cases which will already require rod modifications since there's just no room for case trenching.

In keeping with my easy-to-source theme for future rebuilds, I'd prefer to avoid the custom piston route. So, does anyone know of a common piston for a 95mm bore that has a 24mm compression height and valve pockets for a Husaberg or KTM head?

The JBS piston library shows exactly what I need except that it's for a 102mm bore. At the moment, this doesn't help me although I'm hoping to build a big bore version in the second 600cc engine that I have yet to disassemble. That engine will probably benefit greately from a large-valve head of either KTM or Husaberg origin seeing as how it would displace 686cc with the 102mm bore.

Thoughts from the seasoned engine builders? Or am I completely bonkers?

Yeah, that's me with the exhaust. Been tempted to bolt it on just to make the bike look good wile it sits there with an ailing engine, but I've been too busy getting the 501 running.

I had a crazy idea later on today that I might pursue eventually. The achilles heel of the Husaberg engines, especially the old ones, as well as the KTM RFS engines is the small valves, especially when knocking the displacement out of the realm of sanity. How about adapting a KTM 690 head! I'll bet it's got decent sized valves. I wonder if that head could run with splash lubrication like the Husaberg does or if it needs pressurized oil. I could design a cylinder to adapt the head stud spacing to the case stud spacing...

Whoa! I've go to focus on the task at hand!

I've thought about milling the cylinder to drop the head to meet the piston but then I'm toying with cam chain length which requires the head to move either in very small increments that the tensioner can accommodate or larger 8mm increments which are equivalent to a cam chain link length.

I suppose that the answer may eventually be a custom piston for the stock bore size. I'll order several to cut the per-unit price down and keep them for future rebuilds. Then go with a JBS piston if I want to get crazy.

I dropped a line to Wossner and gave them the specifics on the piston that I'll need. $350 is expensive but at least it'll be a reliable source for future rebuilds. Think I'll go that route and just have them make me a high compression Husaberg piston with a 24mm compression height instead of the stock 23mm.

If this works, I'll take baby steps toward the extreme dream engine by building the next one with a 97mm bore. That'll get me a 621cc displacement and I'll do some mild work on a stock head.

Tentatively scheduled for a day in the machine shop on Father's day. Yep, my wife asked me what I wanted to do that day and I told her "I want to work on an engine"!

So, that's two weeks away and should be a good indication of how slow my projects go. Got too many of em - 6 bikes, a '69 Bronco, a '70 BMW 2002, and I promised my Dad that I'd dual-sport his 400EXC. What was I thinking?!?!

I'm going to be slowly posting progress for this little project. At the moment there's little happening. I took a "before" picture of the rod. Just your standard KTM 450/525 long stroke rod from Hot Rods:



The rod big end is too large to fit the 600 stroke without hitting the cases and there's no room to trench the cases for rod clearance so the rod is going to have to be machined down. It's almost 4mm wider than the Husaberg rod so my hope is that strength won't be an issue. I'm hoping that the rod is being machined right now. This is the only part of the machining that I'm not doing as I'm going to be working on manual machines and don't have a rotary table. I was tempted to just hit it on a sanding wheel but I'm worried about the heat from the wheel and I want to be more precise than that. So, I'm "borrowing" time on an NC mill at work. I will post an "after" shot once the rod is back in my hands.

Then, as mentioned above, Sunday is my day in the machine shop.

More updates to follow.

...and here we are after the machining. I don't have my notes handy but I seem to recall that we had to remove somewhere around .100" to .150" from the outside diameter of the big end.



Here's another shot showing where we programmed the CNC to begin the cut. Should provide ample clearance at all positions throughout the stroke.



I think it turned out well and I have high hopes.

Incidentally, here's what keeps me motivated.



Although this bike may not get the 600 cc engine I'm working on, as I'm finding the 501 currently living there to be very satisfactory for Sierra singletrack and the occasional commute to work, it was the chassis that the 600 came out of and was the bike I used for Vegas to Reno training. I've got the '96 vintage FC 600 chassis begging to get this engine as part of a restoration effort. It's not pretty so I don't have photos at the moment.

More updates coming next week after the Fathers' Day Machine Shop Extravaganza.

Tom

Oh, I just realized, that bike would really pop with a set of new blue 'berg wheels, especially if I got my custom stator cover blue anodized! Hmmm...

And if anyone's wondering, the #325 was our number in Vegas to Reno in 2011. We didn't race the bike in the picture. Used a 2008 FE550 I had and what you see is the rear plastic from that bike.

Here's Rodger, one of my two riding partners for that race and the guy who did the machining on the rod today, probably somewhere around pit 6.



Tom

Good point Bergini. I'll take it into consideration.

Results from the weekend in the machine shop:

Pressed the crank apart and that crank pin was seriously hashed! It took just a little over 15 tons to get it apart...although I was pressing the pin all the way through both crank webs.



The rod and bearing actually looked pretty good despite the pounding that must have been going on.



Nice shiny new RFS rod next to the dingy old 'Berg rod. The larger big end should really spread the load.



The difference in rod width. Again, the RFS rod is bigger and hopefully stronger.



The KTM pin on the left and the 'Berg pin on the right. I had to shorten the KTM pin some .150" or so to match the Berg pin. Also, the Berg pin had no oil holes in it. That was expected. The KTM pin does have oil holes in it, which was also expected. One oil hole must match up with an oil hole in one of the crank webs of the KTM and the other aligns with the big end bearing. Plus, the KTM pin was plugged and not open to the crank case so the oil pressure would feed the big end bearing. I opened the pin so the hole goes all the way through and I will orient the center oil hole so that any oil the might build up inside the center of the pin will sling out into the rod bearing.



4 jaw chuck. Indicating in the center of the rod pin hole. I was happy to get both webs within .0005" of the old rod pin center.



Ideally I would have oriented the crank web axle away from the boring bar so I could choke up on the bar and make it nice and rigid. However, there just wasn't room to do this and I didn't have any long-reach chuck jaws.



I think it turned out alright though. If the measuring tools I was using were accurate enough, one of the crank halfs is dead on the size I want for a .0045" press fit on the pin and the other is .0003" under.

There's still more work to do as I have yet to machine the .077" to .080" clearance for the wider rod into each crank web. It will be interesting to see how this little project turns out because, although I'm getting much more surface area in the press fit due to the larger diameter pin, there's much less press fit length because everything is narrower in the Husaberg engine than in the KTM engine. This is good because it gets the main bearings closer to the load from the rod but there's less structure available to counteract crank spreading. I wasn't originally going to weld the crank but now I'm starting to have second thoughts.

That's all for now...and the real bummer is that I may not have time to work on this any further until the middle of next month.

In the meantime I suppose I can weigh each of the components of the assembly and start trying to figure out a balance factor. If needed, I'm prepared to machine up a Titanium wrist pin to lighten up the reciprocating end. Oh yeah, and there's that whole piston problem...

Stay tuned.

Well, the craigslist ad that listed the engine for sale said that it was a '98 vintage but I'm having serious doubts. I was beginning to believe that it was a '97 until your statement about the '97's having the porkchop crank. Now I'm thinking that it might be a '96. I'll know for sure when I take apart the other 600 I have which is supposed to be a '96 vintage. Does your '97 have an oil pump? This one certainly doesn't. I think they added the oil pump somewhere in there - '97 or '98. My '95 501 has the circular crank. They're definitely easier to chuck up and machine.

Well, progress will at least be really slow until next month although I'm working on securing a little more time at the CNC machine to clearance for the wider rod. I'll probably also measure the weights for the piston I'm going to try (it's a stock KTM 525 piston that measures .5mm (.020") too short for my application but it's worth a try anyway), the matching wristpin, wristpin retainers, and each end of the connecting rod.

So, I suppose there will be progress but I'm certainly not going to use the press I have access to at work. It doesn't press straight and is responsible for wrecking a $3000 aircraft part recently. I'm going to wait to press it all together until I'm back in my Uncle's machine shop where I was doing the work yesterday...and that's in Reno while I live in Sacramento.

So, this engine is pre-'97 and, if I'm not mistaken, the first year of the 600 was...'94? So I guess this engine could be between '94 and '96?

So, I took another look at the idea of machining the cylinder to make up the difference in piston heights. Does anyone know how much you can shorten a cylinder before the cam chain tensioning system is significantly affected? I know that 8mm increments is suggested because of the length of a cam chain link. I've heard of the existence of a 1/2 link which indicates that 4mm increments are possible. But, what about .5mm? That's only .020". Maybe with brand new cam sprockets, cam chain, and tensioner slider this could work?

Thoughts from the peanut gallery?

Tom