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May 30th, 2013, 02:17 PM   #1
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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?
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May 30th, 2013, 05:36 PM   #2
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Re: Husaberg/KTM hybrid engine

Finally someone else as crazy as me

i have been wondering the same thing, will parts be available next time......i have a brand new rod installed with about 10 hours on it........

you may be on to something. as far as the compression ratio, instead of mahining the crank halves offset, just mahine off the bottom of the liner so it will fit down into the bottom cases further.....basically bring the head down to the piston this way.

another thing is that i would also be scared of the crank halves spinning on the big pin at high rpm after they are machined and installed and cause a vibration or worse? those are some pretty close tolerances......maybe it could be welded or pinned after being trued for insurance?

ill re read that later when i get a minute to make sure i have nothing more to add, defintaly interested in a back up plan though
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May 30th, 2013, 08:14 PM   #3
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Re: Husaberg/KTM hybrid engine

Originally Posted by thorgan
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.


The only problem i see here would be the head studs, everything else would be able to be overcomable, new liner, bore the cases to fit the new liner,machine it down and bolt the head for that cylinder on and/or weld the coolant passages at the bottom even if needed in case it hung over the edge of the cases at the bottom, the head studs would limit how big the actual liner could be punched out too, remember that the cylinders are nikasil too, if they werent you would have to make it way thicker etc.

I just finished building my engine and the rod/pin has about 10 hours on it....hopefuly i got a while before i have to go down that road again but ill be glad to offer ideas if I have them as you go......and of course watch along

rods really are the bad part about the old bergs, tooo bad its not a common part or there would probably be more around still, the bearings and everything else can be had somewhere.....

Wait i remember you!! it doesnt sound like your putting that exhaust system to good use yet either then huh?
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May 30th, 2013, 09:48 PM   #4
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Re: Husaberg/KTM hybrid engine

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.
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May 31st, 2013, 05:44 AM   #5
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Re: Husaberg/KTM hybrid engine

without question for the cost of replacing the head and making it work, a good machine shop could cut the seats oversize and press in new inserts and install the bigger valves in your existing head. it wouldnt be cheap but neither would buying a different head and making the parts to make it work.....in fact there is a guy in pinion hills who i am sure could do it, im pretty sure he is still around and up to the task.....he actually had one made already that had a matching cylinder head cover with billet rocker arms and oversized passages and valves, the whole works but it wasnt cheap and he wanted an old head back so he could oversize those valves, kind of like a reman starter or something except with husaberg heads, i could try to find his info if you likem, he also had a bunch of parts avaible for the older bikes too for reasonable prices.........
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May 31st, 2013, 09:57 PM   #6
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Re: Husaberg/KTM hybrid engine

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?!?!
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June 1st, 2013, 07:43 AM   #7
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Re: Husaberg/KTM hybrid engine

haha i gave my wife almost the same response when asked what i wanted for father day--->im going to the shop and want to be left alone
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June 13th, 2013, 02:50 PM   #8
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Re: Husaberg/KTM hybrid engine

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.
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June 13th, 2013, 03:55 PM   #9
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Re: Husaberg/KTM hybrid engine

This'll be good
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June 13th, 2013, 06:35 PM   #10
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Re: Husaberg/KTM hybrid engine

You got this thorgan
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