Timing the 390

[rockitdoc]

I have researched, and contacted Lycoming, re this and am still a bit confused. Have my engine at approx top dead center on the compression stroke based on looking into the spark plug hole and guessing whether I am at the top of the stroke. But, I would like to know for sure where in the stroke I am as there seems to be quite a bit of leeway as I rotate the prop between the time the piston stops moving on the up stroke and the time it starts moving again downward. In other words, lots of degrees of rotation between the place where the piston stops moving and starts moving again as the prop is rotated. I have two sets of timing marks on my starter ring gear. One set on the aft edge which I have shown in the photo lined up with the center of the engine case based on where I think the piston as the top of it's stroke. But, there are marks on the forward edge of the ring gear, as well. These don't seem to line up with anything. I have read that the marks (TP-1) on the rear should line up with the center of the case, which in my case, they kind of are. But, what about the marks on the forward edge? I read the TP-1 on the front should line up with a hole in the starter case. But, I have no such hole and as can be seen from the photo, they don't line up with anything on the starter.

I'm used to timing auto engines with a fixed pointer on the engine block closely aligned with the timing marks on the crank pulley or starter ring gear and would like to get my 390 timed as precisely as possible. And, lining up the TP-1 mark with the center of the engine case seems, well, a bit imprecise and clumsy.

 

[rmarshall234]

Extend a straight Edge from the split line of the case and line it up with the TC1 mark on the aft side of the flywheel. Make sure you are on the compression stroke of #1 cylinder. You can ignore the marks on the front side.

 

[jrs14855]

You should approach the desired mark in the direction of rotation. This compensates for the backlash in the gears. I have a 6" machinists scale that is very thin. Bent as necessary to go from case split to marks on aft side of flywheel.

 

[Toobuilder]

Since you are using a piston stop, then the technique is to ALSO use a pendulum style pointer on the prop hub. Find the number where the piston hits the stop with the prop going CW (record that number), then spin the engine CCW until piston stop contact and recorded THAT number. Do some grade school math and split the difference between the two and that’s TDC

…or just use the timing marks on the ring gear carrier:

On the BACK of the ring gear, line up the projected split line of the case with the numbers (laser or straight edge)

OR

On the FRONT of the ring gear carrier, line up the numbers with a pin/drill/wire inserted in the starter housing timing hole.

Magneto Timing - KITPLANES

Checking your magneto timing is part of every yearly condition inspection, but beyond that it is something you should be able to do whenever you suspect that your magnetos’ performance is not up to snuff. If your plane seems a bit down on power or your mag tests start giving you drops of more...

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[rmarshall234]

Speaking of piston stops and pendulum style pointers…
I use this method nowadays.
Others have said the longer lever arm doesn’t equate to greater accuracy but didn’t provide a convincing argument.

 

[climberrn]

Depends what ignition you are timing also. With Mags, it makes no difference if its compression stroke since the ignition fires in pairs.

What ignition are you timing?

 

[Full Throttle]

"Depends what ignition you are timing also. With Mags, it makes no difference if its compression stroke since the ignition fires in pairs."

I think you meant waste spark ignition. Not Mags.

 

[jliltd]

"Depends what ignition you are timing also. With Mags, it makes no difference if its compression stroke since the ignition fires in pairs."

I think you meant waste spark ignition. Not Mags.

THIS /\ You musta been referring to PMags instead of conventional Mags?

 

[jliltd]

I would suggest using a piston stop with a digital protractor. Looks like you have a tricycle gear so it should be easy enough to get the readings in each direction to split the difference for TDC. Were it a taildragger I would suggest lifting the tail to level so the crank and prop disc aren't tilted allowing the angle to be measured more accurately.

 

[jliltd]

Since you are using a piston stop, ....

I didn't see mention of a piston stop in the original post.

On the FRONT of the ring gear carrier, line up the numbers with a pin/drill/wire inserted in the starter housing timing hole.

As mentioned and as shown in the photo his starter doesn't have a timing hole. I wonder what brand of aircraft starter doesn't have a timing hole?

 

[rockitdoc]

Depends what ignition you are timing also. With Mags, it makes no difference if its compression stroke since the ignition fires in pairs.

What ignition are you timing?

P mags.

 

[Ironflight]

Wow, people can make some simple stuff complicated! Assuming that you have a timing mark on the back of the ring gear, and assuming that the ring gear is properly clocked on the crankshaft, lining that mark up with the crankcase split line will set the crankshaft EXACTLY at TDC (unless the crank is twisted). Any variation in piston TDC at tat point is play that you can’t really ”adjust out”, and there is also some gear backlash between the crank and the ”ignition units” (whatever they might be). The play and backlash constitute the “+/-“ in your timing…these engines are not precise to the tenth of a degree…or even a degree. Heck, P-Mags are only accurately timed to the nearest 1.4 degree becasue of their encoder.

Set the rear timing mark to the split line, and everything else is noise.

 

[msstahl]

P mags.

If you run Pmags on your 390 it’s important to account for the 25 deg internal offset. With the 390 being a 20deg advance motor you will need to compensate when setting TDC on the mags.

 

[Toobuilder]

I didn't see mention of a piston stop in the original post.


As mentioned and as shown in the photo his starter doesn't have a timing hole. I wonder what brand of aircraft starter doesn't have a timing hole?

Yep, read the OP too fast. Saw the words ”piston” and “stop” and made the connection In my head. Also skipped right past the “no hole in the starter” part. I have also never seen that before.

 

[Toobuilder]

If you run Pmags on your 390 it’s important to account for the 25 deg internal offset. With the 390 being a 20deg advance motor you will need to compensate when setting TDC on the mags.

…and then don’t forget the added additional mechanical retard the Angle Valve users need…. Is 1.5 teeth you 390 guys use? And of course jumper in!

 

[GOFT]

…and then don’t forget the added additional mechanical retard the Angle Valve users need…. Is 1.5 teeth you 390 guys use? And of course jumper in!

Yes.