How to Choose an Engine Case Screw

By Mike Rosso

Originally printed in issue #69 of Still….Keeping Track

Here's a tip on choosing the correct screw length I learned from an old KTM engineer.

To make it as short as possible the engineer gave me a challenge when he scooped up all of the engine case screws that I had neatly arranged and laid out in the correct position on a shop rag as he was watching me reassemble an engine. I put them into the cases and he asked if I was finished and then he swapped two of them. Then he told me that was good but not perfect and then told me how to know the next time. He said there is a standard engineering practice of knowing the position by following the rule of minimum thread engagement when putting a steel screw (or bolt) into a steel thread (steel frame, steel case, etc.,) that you must have a minimum thread engagement of the diameter of the screw or bolt. In other words a 6mm steel screw must engage 6mm of the case or threaded hole, so when you put the screw in you will have 6mm between the head of the screw and the case/hole. If it's an 8mm bolt then there will be 8mm between the head of the bolt and so forth as Mr. John Penton would say.

If you're putting a 6mm steel screw into an aluminum thread as in this case (pun intended) then you need to have two times (2 X) the thread engagement or 12mm and you will see all of the case screws will have 12mm between the heads of the screws even at the dowel pin locations. You can pretty much grab a 12mm wrench and put it at every screw once you place them into the case holes and see the distance the screws stick out.

And for magnesium threads like the KTM magnesium engine cases with steel screws, thread engagement is THREE to ONE or a 6mm screw will need a minimum of 18mm of thread engagement and stick out a minimum of 18mm.

Follow this rule and you won't strip or damage a thread:

  • Steel screw to steel thread = 1 to 1 engagement
  • Steel screw to aluminum thread = 2 to 1 engagement
  • Steel screw to magnesium thread = 3 to 1 engagement

If you keep that in mind (even on non-metric applications) you'll spot when you have the correct or incorrect screw in the right or wrong place or at least know that you screwed up somewhere. Also helps if you're trying to figure out what length fastener to use if it is missing.