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Timing the Magnetos

rvator10

Well Known Member
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hello, trying to time the magnetos on my new YIO390-Exp engine, it appears to have the 20 deg mark down by the starter motor, i have never seen this before, on other older lycoming engines, it's 12:00 at the case split line. Is the flywheel installed wrong? If the timing mark is at the starter, there isn't any clear alignment marking. lost?:confused:
 
Look for a tiny hole on the starter that should align with the 20-degree mark on the flywheel. You will have to look past the flywheel right at the starter.

Vic
 
AND use the front markings with the starter hole and the rear (of the flywheel) markings with the case split...
 
FFZ?

If you are building at FFZ let me know can stop by and help out. Or make sure #1 is on compression, bring to TDC the look for timing marks by starter. Then back up prop 20 degrees that should be a good starting point
 
I use a small drill bit that fits tight in the hole to bring the point closer to the flywheel marks , for more accurate locating.
Don't forget the drill bit when you are done.
 
Make sure you are using the figures of the REAR of the Flywheel not figures on the front, and line that up with the crankcase.
 
Not tight enough that you need to hammer it in. In with fingers , out with figures, never had a problem.
Should be able to clamp vice grips and lightly hammer on the vise grips to tap it out.
 
I use a small drill bit that fits tight in the hole to bring the point closer to the flywheel marks , for more accurate locating.
Don't forget the drill bit when you are done.


That would be a 1/16" bit..... goes in and out by hand.
 
Thanks all, how stupid of me, the flywheel inside had timing marks at 12:00 for the case split line, I only thought of looking on the outside surface. The starter that shipped new with the engine from lyc didn't have any reference hole. All is good again :D
 
hello, trying to time the magnetos on my new YIO390-Exp engine, it appears to have the 20 deg mark down by the starter motor, i have never seen this before, on other older lycoming engines, it's 12:00 at the case split line. Is the flywheel installed wrong? If the timing mark is at the starter, there isn't any clear alignment marking. lost?:confused:

Using the timing marks on the rear side of the flywheel with the case split as a reference is always reliable.

The front governor crankcase has a large rectangular boss just aft of the topmost front perimeter bolt. It's a great place to bolt on a timing pointer. Drill a hole in the boss, and make up a pointer from two pieces of 0.125 aluminum plate. The tip of the pointer gets sanded to a right angle wedge on one side. Very accurate.
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Doesn't anybody use that aluminum piston stop that fits in the #1 spark plug hole to find Top Center?

12-18100.jpg
 
Baffles

Using the timing marks on the rear side of the flywheel with the case split as a reference is always reliable.

The front governor crankcase has a large rectangular boss just aft of the topmost front perimeter bolt. It's a great place to bolt on a timing pointer. Drill a hole in the boss, and make up a pointer from two pieces of 0.125 aluminum plate. The tip of the pointer gets sanded to a right angle wedge on one side. Very accurate.
.

Unfortunately this doesn't work too well for those of us with stock baffles because the forward baffle is between this boss and the flywheel and the baffle bracket uses that boss for mounting.

Skylor
 
I’ve never seen this type of tool. More info on it, how and on what engine it works, Where to get it etc. would be appreciated! I always like confirmation when I find TDC other ways.

Bring Piston up against the stop in one crank direction and mark flywheel angle from a known datum, such as a plumb bob or case appendage like Dan showed. Spin crank in opposite direction until piston hits stop and mark flywheel angle from datum. Measure the degrees between marks and the center directly between those two marks is the marking for TDC, when compared to the datum. To get meaningful accuracy, you need a wheel with 1 degree tick marks. A digital level could also be attached to the flywheel.

I have never found a need for this type of precision for ignition timing and have only used it for setting cam timing. At least when using OEM parts; Their markings tend to be within a degree or two of actual TDC.

Larry
 
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I’ve never seen this type of tool. More info on it, how and on what engine it works, Where to get it etc. would be appreciated! I always like confirmation when I find TDC other ways.
I bought mine at Aircraft Spruce. What you do is - with the top bank of spark plugs removed, put your thumb over the #1 spark plug hole and rotate the prop to insure you're on the #1 compression stroke. Then screw in the aluminum piston stop tool and gently turn the prop in the direction of rotation until the piston just touches the piston stop. Turn the dial on the "flower pot" until the pointer aligns with TC (top center). Then rotate the prop in the opposite direction until the piston again touches the piston stop. Note the number of degrees on the dial Mine is 112). Then rotate the prop back in the normal rotation direction half the distance noted in the last step (56 degrees) in my example. That's your top dead center.
 
Have you ever just used rubber bands to attach your digital precision level to a prop blade - then rotate the engine the proper number of degrees after you found TDC by previously described methods.
 
Piston Pin

I have a box of brand new piston pins left over from a business I had selling timing disk kits to A&P Schools. I'll send you one (no charge) if you mail me a prepaid mailing envelope.
 
I’ve found it’s easy to use a mirror and sight along the case parting line to the back of the flywheel timing marks. My installation doesn’t have a baffle blocking the view.
 
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Here's how I did it:

My flywheel (probably off an O-320) has both #1 TDC and 25 deg stamped on the front of the flywheel. I didn't have to use the case part line at all:
rsOUGA.jpg


A 1/16" drill bit fits perfectly into the hole on my Sky-Tec starter. Don't let the bit stick out too far or the teeth on the flywheel will break it off. You can't see it in this photo due to parallax, but at #1 TDC, the drill bit is aligned with the center of a tooth. I added a short piece of blue tape along the TDC mark to extend the line down onto the tooth:
ZuSBDG.jpg


I used double-sticky carpet tape and painter's tape to fasten the digital gauge to the prop extension. I just zeroed it out at #1 TDC and could read the mag settings directly:
4bpSXu.jpg
 
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