Teeth and Degrees (TDC & BTDC)

[211º]

Information that will probably push something useful out of your brain...

149 teeth in 360 deg (M20E IO-360-A1A)
360°/149 teeth or 2.42 deg per tooth
I counted 10.5 teeth from TDC to 25º BTDC mark

10.5 x 2.42 = 25.4º

[takair]

Great, another thing that’s “not quite right about the world”, to drive me crazy. 

[A64Pilot]

Teeth on what, starter ring?

I don’t understand, what’s not right?

[Shadrach]

4 hours ago, A64Pilot said:

Teeth on what, starter ring?

I don’t understand, what’s not right?

He is suggesting that mathematically the timing mark on the starter ring is at 25.4° as opposed to the specified 25°.

[Hank]

4 hours ago, Shadrach said:

He is suggesting that mathematically  the timing mark on the starter ring is at 25.4° as opposed to the specified 25°.

In theory, there is no difference between theory and practice.

I'm an engineer, I deal in practice, breakdowns and repair. This is NOT a problem!

[A64Pilot]

How did he determine it’s exactly .5 of a tooth, eyeball?

I keep telling you guys these things weren’t designed or built in the digital world, besides you can’t time the thing that accurately anyway with the play in the gears etc. It’s going to drift some from thermal expansion of the aluminum case vs the steel gears

 

[Guest]

360*/ the number of teeth on ring gear is a thinking exercise in my shop for young apprentices who are trying to tell me how far off the timing is.

 

[jetdriven]

Use the time-rite and a piston stop and find TDC, then back it up to find 25 degrees.   The mark on the flywheel at the starter dot always corresponded to 25.  Within .2 or so which is the limit of accuracy.  

[211º]

Oh. I was not clear. The engineer in me was hoping and thinking that there would be something like 180 teeth in 360°.  (Or at least 150, not 149). 
 

Of course, last night as I was falling asleep I was thinking… maybe if this was a straight line and the tops and troughs were averaged over the straight line, it’d be a simpler number. 
 

 (Not a mechanical engineer)

[211º]

57 minutes ago, 211º said:

Oh. I was not clear. The engineer in me was hoping and thinking that there would be something like 180 teeth in 360°.  (Or at least 150, not 149). 

And not for any real reason… except if you could define something like this, why not make it seemingly reasonable.”

[Hank]

4 hours ago, A64Pilot said:

How did he determine it’s exactly .5 of a tooth, eyeball?

I keep telling you guys these things weren’t designed or built in the digital world, besides you can’t time the thing that accurately anyway with the play in the gears etc. It’s going to drift some from thermal expansion of the aluminum case vs the steel gears

There we go with theory and practice again.

4 hours ago, M20Doc said:

360*/ the number of teeth on ring gear is a thinking exercise in my shop for young apprentices who are trying to tell me how far off the timing is.

Yep, a great exercise for the students. They have to learn these things somewhere. 

1 hour ago, 211º said:

Oh. I was not clear. The engineer in me was hoping and thinking that there would be something like 180 teeth in 360°.  (Or at least 150, not 149). 
 

Of course, last night as I was falling asleep I was thinking… maybe if this was a straight line and the tops and troughs were averaged over the straight line, it’d be a simpler number. 
 

 (Not a mechanical engineer)

I am a Mechanical Engineer. Dealing with fractions and decimals is a simple, expected, daily occurrence. Think of it like this:  each gear is designed to a particular design, but when making the thousands of each gear required for all of the engines (not just on Mooneys!), there will be some.variation--this is allowed within certain tolerances.

I have molded plastic gears smaller than a quarter, and the teeth had tolerances of +/- 0.001" on thickness. So on these big gears, call it 0.005" or even 0.010" tolerance. On each of 149 teeth.

Give me some dimensions and the calculations are simple:  diameter of the gear at the root of the gear teeth and width of the gear tooth. It's simple geometry. Nothing made in the thousands will ever measure exactly the same.

[carusoam]

The cool thing…

The starter ring can have 148 teeth… and the O360 will still start pretty well….  
 

PP thinking out loud only… the extra tooth was missing before my ownership, and never showed up again…

It is probably a kick-back danger…  something to look for if you experience a kick-back during starting….

Best regards,

-a-