Magnesium engine cases?

[Mike Lenz]

The 72 and 73 cases are not marked Magnesium as the 74 and on ones are.  Are they mag or alum?

[Larry Perkins]

I am pretty sure all the KTM ones were mag

Larry P

[Mike Rosso]

Magnesium through the '70s and early '80s until the water cooled engines...for some reason water/coolant didn't get along with magnesium. Big problems to this day with KTM magnesium clutch covers with water pumps in them DAMHIK

One way you may be able to identify magnesium versus aluminum cases is by the length of thread engagement of the steel screw or bolt into the magnesium threads of the cases. If it's a magnesium case thread then the steel 6mm screws should thread into the mag thread 18mm, no less but a little more is okay. If the case is aluminum with aluminum threads then only twice the diameter of the screw/bolt engagement is required. 6mm steel screw = 12mm aluminum thread engagement. Steel screw/bolt into steel thread is 1 to 1 engagement. i.e., 6mm steel screw in to steel thread = 6mm  

Yes this sounds strange and there's a long story and tech tip I learned from KTM engineer and friend Josef Hattinger who was in charge of KTM 2-stroke Open Class engine development, including the LC4 4-strokes.

Short version I was getting ready with Sepp to do the first KTM dealer tech training for the soon to be released LC4 4-stroke and had completely disassembled the engine and laid out all of the components in an orderly fashion including all of the case screws, nuts and bolts mostly on red shop rags. I just put the case halves together and went to reach for one of the case screws when Sepp grabs ALL of the case screws and with a grin on his face hands them to me with scooped hands. I knew right away with his (smirky) grin it was a challenge to get the screws into the center case screw holes correctly and quickly. I put all of them in and Sepp asked if I was finished, I said yes nervously, and he exchanged two screw with each other and said "that was very good, now why did you do that? I could only say that "it just looked correct" for some unconscious reason??? That's when he said, "There's an engineering standard (worldwide) that applies to fastener for thread engagement. At KTM we have used that basic standard and for engine case screws it is (basically)

1 to 1 for steel screws into steel thread.

2 to 1 for steel screws into aluminum thread.

3 to 1 for steel screws into magnesium thread.

I grabbed a 12mm open end wrench and sure enough with the 6mm screws just sitting in the center case screw holes there was 12mm of distance between the head and the case seat at each 6mm case screw.

You can apply this to just about anything you're working on, for instance you have an engine completely disassembled as a box of parts and there are screws missing.

A few years later I did a KTM quarterly Underground Newsletter  (magazine) article on this subject. Funny side note later on, the KTM dealer tech training manager would call me into the training room as the dealer's tech were getting ready to assemble the tech engines. Most would put the case and clutch covers 6mm screws into the covers or center cases off to the side or in a similar pattern as the case and cover holes on a shop rag as I used to do. I would walk in be introduced and just stroll over to the work bench and pick up a cover or case half with the screws in it and ask "Tyler is this one of those bad clutch covers?" and turn it over and all of the screws would fall out of their pre-staged positions. The dealer team at that work station would gasp, then a few not so nice words were mumbled. The other dealers would kind of laugh quietly as this part of the reassembly was always a speed competition, though it wasn't supposed to be. Then the instructor would ask me to explain the 1 to 1, 2 to 1 and 3 to 1 thread engagement so if some stupid butthead like me ever does that or you need to find a missing bolt or you are building something from scratch and you want to know the correct length bolt or screw you should use....just remember the one to one, two to one, three to one basic minimum fastener rule. Sorry again for the long, long short story. The guys at the POG meetings have always known: "Never ask Mike what time it is, he'll end up telling you how a watch is made"

Mike

[Daniel P. McEntee]

Neat story Mike. The thread length thing is something I just always went by because it made sense. Steel is harder and less likely for threads to pull. Aluminum is softer when cast and double the length just for durability and strength. Magnesium is brittle and just gets more brittle with age, so the three times length. I remember the first time I drilled and tapped a hole in a mag case and wondered if it was made out of glass! I have wondered but never asked anyone with the knowledge as to just how much lighter magnesium is supposed to be than aluminum to make it worth all the extra measures it takes to work it? Even as far back as WW-2 they started to get away from magnesium in aircraft part for this reason and for the extra fire threat if it ever got hot enough to burn. The only relatively modern things I have seen made from magnesium are hand placed dock plates for loading docks, to make them as light as possible. Welding it is not as difficult as one might imagine and welds a lot like aluminum but with a green halo that glows around the puddle for some reason. The hard part about welding magnesium is just finding filler rod!! A weld shop told me once that they could order it at 50 bucks a pound and a ten pound minimum! A strange metal indeed.
   Type at you later,
  Dan McEntee

[Pat Oshaughnessy]

Im sure most POG members Know About  Salt . It will eat magnesium up!A pal took Mag. wheels to Bonneville & the salt ate the wheels up in 5 days! He gave me the wheels   & i ran them & got away with it! Lucky & dumb1[)]

[Toolsurfer]

Magnesium is about the 1/3 the weight of aluminum per volume. Aluminum is corrosive resistant and magnesium is not but corrodes slow. Aluminum is cheaper especially for die casting parts. Porsche and GM used this stuff early on in auto's . Cutting edge material for strength versus weight .

[firstturn]

Great post Mike Rosso.  I  remember leaning some of these  principals the hard way  with Go Kart Engines, Racing Outboards and of course Motorcycle engines.  It took a time visiting with an old Machinist at Mobil Oil to get me squared away.  Thanks.

Ron Carbaugh

[Mike Lenz]

Thanks Mike.  Very useful!