KTM Engine Stories

[Daniel P. McEntee]

I have some engines getting worked on by Gary Ellis, and one of them is a 250 that presented an issue to him that he says he has not seen before. He was putting the cylinder back on to set the deck heighth,and when he rotated the engine to bring the piston to top dead center, the crown of the piston stuck out the top of the cylinder! Something to the tune of .180"!
   He called me to advise me of this, and that he wasn't sure what the problem was. He measured and checked everything he could against know good parts, and everything was checking out. Another phone call later and he thinks the rod is too long, may be a 400 rod. The bottom end came from a donor bike that Mike Winter told me about and picked up for me up near Chicago. It was a parts bike, missing wheels, forks pipe and top end. everything else that was there was in pretty good shape. The frame had been boogered up by the previous owner, trying to convert it to lay down shocks, but it wasn't too bad, and I was able to restore the frame back to original condition. Mike has the frame and it is a really early '74. It has the earliest serial number of a true 1974 250 that he has been able to find. I think the bike was just what I call a farm bike, and it's the same rod that was in the bike when I got it. The bottom end was pretty clean, in good shape and the rod bearing was pretty tight yet.
   Gary hasn't found any machine marks of anything that would indicate a milling of the center cases or anything else. I told him I would go along with just doing what was necessary to shim out the cylinder to get the deck height he wanted. I have a 73 250 Husky in my shed that belonged to my late younger brother. It was his bike from back in the day that he bought from a friend of ours, who was the original owner. To make a long story short, '72 and '73 Husky 250 engines can have one of three rod/piston combinations due to Husqvarna trying to wring everything they could from that engine to ride the wave of popularity they were experiencing from their exposure in "On Any Sunday", while they finished developing the mag engine series. This '73 250 Husky needed a rod before my brother could put it back together, and the only one he could find, was 8mm too long! My brother Jim was an excellent mechanic and machinist, so he just made himself a proper aluminum spacer to take up most of the gap and set his deck height with an extra base gasket. The bike runs just fine! It is from this experience that I will just go ahead with the same thought on the 250 Penton engine.
   My question after this long winded story, is that what have some of you other guys encountered in rebuilding these engines? I know some of you have had to mix and match parts to build up an engine when you are just starting with a basic core. and I'm interested if anyone else has some experience with odd things like this. Lots to be learned from what others have had to do to get an engine back together to keep a bike on the trail. I'm sure I have read some threads over the last several years where it was mentioned that a couple of extra base gaskets had to be used to get proper clearances. Does anyone know what the actual length dimension of a 400 rod is compared to a 250 rod, from hole center to hole center? Again, I'm just looking for stories of people's actual experiences with things like this, and what they did to address it.
  Thanks a lot, and Merry Christmas Everyone!
   Type at you later,
   Dan McEntee

[Kip Kern]

69mm vs 62mm center to center

[Daniel P. McEntee]

Thanks for the information Kip. 7mm difference. I don't think he's dealing with quite that much, it's actually about half that. The possibility is that a rod was changed somewhere along the line in the engine's life, but from what I could see of the condition of everything that I bought, I doubt that the original owner would have attempted that, and if any work was done by an outside mechanic at the time, would he have done that? At that time I would have thought 400 rods would be the more rare of the two, probably that way now also.
  Thanks a lot,
  Dan McEntee

[Kip Kern]

Around .250

[Richard Toghill]

Bit of a long shot but the piston could be machined wrong where the pin goes through. What piston are you using?

[454MRW]

Gary said the piston pin height match other stock pistons he had. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R 79 CR250R

[Daniel P. McEntee]

Piston pin hole has been checked against another known good piston. So has overall heighth of the cyinder. Crank is NOT a 400 crank. The engine core was in pretty good condition, and to me, when I got it, showed no signs of ever being apart. It came off a parts bike I got from the outskirts of the Chicago area. I'm thinking it was what I call a farm bike. It had been parted out to the point where the cylinder, head, forks and wheels were gone, and I got everything else. Other than someone trying to do a garage job of laying down the shocks, it was in fairly decent shape. I got the cylinder second hand but it's good and known to work with this vintage of engine. The big variable that would cause what is being seen is if the rod was long. And thanks to the dimensions that Kip provided, I don't think that is the issue.
   Again, I'm not looking for a cure, I think it will run just fine shimmed out to the proper deck height. I'm just curious if anyone else has run into a similar issue, especially dealing with mix and matched parts from several different bikes and engines.
   Thanks for the feedback,
   Dan McEntee

[G Ellis]

Here is what I think it is, a cheater 250. Take a 400 rod longer stroke, if you figure it out. It would be the same as boring the cylinder to 78mm. This puts it to about a 300cc give or take. I think it will have low end grunt. I think I am going to build a motor like this and see how it runs.

[Daniel P. McEntee]

But isn't displacement dictated by bore and stroke? The rod length has nothing to do with stroke distance, that is determined by the crank pin dimension from center of the crank, correct? This just puts the actual diplacement a little further away from the crank, in my estimation. I don't know how that affects how it runs. Like I mentioned, I have my little brother's '73 250 Husky that he went through a lot trying to find the correct rod for, and his only recourse at the time was a rod 8mm longer, and he shimmed that out with an aluminum spacer. I did some reassembly to the bike when I brought it home after he passed away, put fresh gas in it and it started right up. I have only riden the bike up and down the street a few times, but it sure seems like it runs like every other 250 Husky piston port I have ever riden.
   Since Kip came up with the rod length difference, and the amount needed to shim out the diffenrence is less than that, that makes it more interesting. Since there was a lot more human interaction in making things back then, I wonder if it's just an "OOOPPPS!" moment when the rod was made, and if so, maybe someone esle has run into the same thing. I'm wondering if it is just one part in question, or an accumulation of tolerances over the whole length?
   Just something to kick around and BS about. Anything learned from the conversation might be helpful elsewhere.
   Type at you later,
   Dan McEntee

[Daniel P. McEntee]

quote:Originally posted by G Ellis

Here is what I think it is, a cheater 250. Take a 400 rod longer stroke, if you figure it out. It would be the same as boring the cylinder to 78mm. This puts it to about a 300cc give or take. I think it will have low end grunt. I think I am going to build a motor like this and see how it runs.

I don't think I left the ignition on the engine when I brought it to you, but if you have something you can easily drop it into to try it out and satisfy your curiosity, help yourself! If not then the 250 may be the first one to go back together and see what happens. I have no experience with Penton 250s so it will be a first for me. We'll have to get a more experienced butt on the seat to get a good opinion.
  Type at you later,
   Dan McEntee

[derek martin]

If cylinder length, rod length and piston pin location match 250 spec then wouldn't it have to be a stroker crank producing this? Wasn't there a Penton ISDT bike that was some displacement between 250 and 400?

[Bob Seymour]

Connecting rod length has no effect on displacement size of any given engine. A good example of this would be the common modicication of utilizing a 5.7 or 6.0 rod in the Chevy small block 400ci. The small block Chevy 400ci engine came with a 5.65 rod that creates addition side wall loading to the piston during high performance usage . The compression pin highth in the piston changes accordingly to each rod. The longer the rod, the closer the center line of the wrist pin becomes to the top of the piston,but no change in displacement occurs.
 Now here comes my other thought. In a two stroke engine,particularly like what is being discussed, isnt the location of the porting critical to where the piston happens to be in its given travel up and down within the cylinder? If so then the top of piston to top of cylinder dimension is what is important. Could someone prior used a rod from perhaps another manufacturer that had a dimension close to the KTM rod and did what was mentioned earlier,just shim the cylinder up to make up the additional distance?

[454MRW]

Another benefit of a longer rod, (combined with the appropriate base gasket shim height), is the longer rod raises the torque peak and rpm potential of the engine by stopping the piston longer at tdc and bdc. Hot rodders and pulling trucks & tractors routinely build long rod engines. Mike
Here is a pic of Dans 73 Husky 250RT with the spacer plate at the base of the cylinder:
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Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R 79 CR250R

[Daniel P. McEntee]

Thanks for posting the picture, Mike. If you look carefully under the bottom cylinder fin, or if you save the picture to your computer and zoom in on that area, you can see the shim plate all nice and shiny.
   I have rebuilt engines before, mostly Japanese engines, and never gave any thought about properly checking the deck height. I just put a new base gasket in place, and went on and finished assembly. Everything always worked! Higher production numbers and QC practices of the original manufacturers kept all the tolerances in a range where i got lucky and everything worked!
  I know I have read some threads here on the list where people have mentioned having to use multiple base gaskets or some other method to get the proper deck height settings, but haven't been able to pull any of those up with a search. I'm concluding that if I ever do a complete rebuild on a KTM engine, this spec needs to be checked and properly set and most likely will vary from engine to engine.
   MERRY CHRISTMAS EVERYBODY!
   Dan McEntee