Here is some trivia that may come in handy  for TSIO 550 owners:

When my engine came back from prop strike overhaul, the dipstick had a problem.  When inserted and rotated to the locked position, it would pop back to the unlocked position every time...????...Ouch! - can't  run the engine like that!!!

The overhaul shop couldnt understand the problem however  the TCM factory mentioned that there was a convoluted washer under the gasket that could be the culpret.
When I removed the gasket, I couldnt see any washer but when using a magnafying glass, I could see a tiny line around the inside of the cap. If this was the washer, how to fix???

Since it appeared that the dipstick assembly was spotwelded together, I decided to drill  four evenly spaced 3/32 inch holes thru the top of the cap and then apply pressure thru the  holes, evenly around the spring, hoping to force it down into position.
But, as I completed drilling the first hole, there was a loud ":twang" as the spring reversed itself.  Problem solved!!!!!

I hope this trivia might save another builder a lot of time and frustration.  Regards,  Bill Hogarty

P.S.  Rebuilding N499R now 75% complete