Need help finding TDC mark on crank flange

Bubblehead · #1 01-03-2009, 09:01 AM

I've just installed new cylinders on my wide deck IO-360. I am checking dry valve lash and found TDC on the compression stroke on #1 cylinder, but I do not see a mark on the crank flange to get it precisely aligned at TDC. Can someone give me some guidance?

I would normally look on the starter gear assembly to find a TDC mark, but the prop and adapter and ring gear are off because I needed to replace the front crank seal.

Also, I noticed that my "LW-16471 SUPPORT ASSY., Starter ring gear" has TDC and 25 BTDC marks on the front and on the back but the marks are at different points! In other words, if the assy was mounted on the crank, the TDC mark on the front would position the crank differently than the TDC mark on the back! Is this normal? Which do I use? I marked it before removal with a match mark to the crank flange but I want to put it back together correctly, not just the same way it came apart!

Thanks,

John

Radomir · #2 01-03-2009, 09:12 AM

Use the ring gear for this.. there's no marking on crank flange....

If you're lining up with the engine case split line, use rear mark on ring gear.

If you're lining up with the small hole on the starter, use front mark on the ring gear.

Just to clarify -- both of these will line up with their respective references at same time.. so it doesnt' matter which one you use.. whichever is more convenient to you.. But they're in different places becasue they use different references...(case halves split line vs. hole in starter)

Mel · #3 01-03-2009, 09:23 AM

Radomir has it exactly right. the "stepped" insert on the flange keys the flywheel. Without the flywheel on there are no timing marks.

Kahuna · #4 01-03-2009, 10:18 AM

Also dont trust that your ring gear is on correctly. They can be put on out of position. Your first check is the compression stroke #1 tdc. Well does not have to be compression. Then verify the ring gear is on properly with the marks mentioned in the above posts.
I have seen many a builder pull hair out only to find the ring gear was not properly installed.
Best,

Bubblehead · #5 01-03-2009, 06:33 PM

Now it makes sense to me. I also found a little more info in the Lycoming Parts Catalog PC-306-14 pg 1-6. It shows the crank, flange and gear assy. By looking at the lengths of the bushings on the crank flange you can locate the #1 Cyl crankpin. There are 4 long bushings, one short bushing and the propellor indexing bushing. By looking at them you can figure out which long bushing corresponds to the #1 cyl crankpin.

I checked it today and sure enough, when I have #1 cylinder physically at TDC the bushing corresponding to #1 crankpin is lined up with the center line of #1 cylinder.

The different marks lining up with different references makes sense too. I found the little hole on the starter today and sure enough, the two sets of marks line up fine. Very interesting approach.

Thanks for the input guys!

nucleus · #6 01-03-2009, 11:30 PM

Quote:

Originally Posted by Radomir

Use the ring gear for this.. there's no marking on crank flange....

If you're lining up with the engine case split line, use rear mark on ring gear.

If you're lining up with the small hole on the starter, use front mark on the ring gear.

Yes, don't be like me and stand in front of the engine and align the mark on the front of the flywheel with the case split line. Seemed easy and intuitive at the time.

Hans

Noah · #7 01-04-2009, 06:28 AM

Quote:

Originally Posted by Kahuna

Also dont trust that your ring gear is on correctly. They can be put on out of position... I have seen many a builder pull hair out only to find the ring gear was not properly installed.

I must be missing something. I just replaced my ring gear, and there are NO marks on the ring gear when purchased. Therefore, there is no timing of the ring gear and no way to install it with incorrect timing. There ARE marks on the ring gear support (some people call this the flywheel) which must be transferred to the ring gear AFTER installation of the ring gear via acid-etch. This procedure is covered by Lycoming Service Instruction 1141. Given this SI, I don't understand your statement, Kahuna. Can you please clarify?

LifeofReiley · #8 01-04-2009, 07:17 AM

Quote:

Originally Posted by Noah

and there are NO marks on the ring gear when purchased. Therefore, there is no timing of the ring gear and no way to install it with incorrect timing. There ARE marks on the ring gear support (some people call this the flywheel) which must be transferred to the ring gear AFTER installation of the ring gear via acid-etch. This procedure is covered by Lycoming Service Instruction 1141. Given this SI, I don't understand your statement, Kahuna. Can you please clarify?

Yep... thats what I was thinking.

I think you are correct!

Webb · #9 01-04-2009, 07:57 AM

Quote:

Originally Posted by Kahuna

Also dont trust that your ring gear is on correctly. They can be put on out of position. Your first check is the compression stroke #1 tdc. Well does not have to be compression. Then verify the ring gear is on properly with the marks mentioned in the above posts.
I have seen many a builder pull hair out only to find the ring gear was not properly installed.
Best,

As simple as it sounds, pull 1 spark plug out of each cylinder so you can turn the engine easily by hand. Put your finger in the spark plug hole for cylinder #1. When you hit the compression stroke, it will blow your finger out of the hole.

I had the same issue. Called my old A&P and this is what he told me to do and it worked.

Kahuna · #10 01-04-2009, 08:35 AM

With all the experimental stuff running around its hard to know what everyone is doing now. However, your bushings in the crank flange of a lyco are of different diameters. A few thou off. The ring gear holes that receive these bushings are also sized exactly for a close fit. If you try and put the ring gear on out of phase it will protest. You can force it on and swell the ring gear holes, which is where the trouble comes if someone has done this before you. Once a ring gear hole has been swollen, its toast for life, although it still functions fine as a ring gear.

These bushings are sized differently to clock your prop.

For all Lycoming direct drive engine models, the top dead-center position
of number-one piston can be verified by observing that the mark indicating the #1 TDC position on the rear side of the starter ring gear is exactly aligned with the split line of the crankcase at the top of the engine. This of course assumes the ring gear is properly installed.

There is a O stamped on the fwd face of the ring gear. Im hesitant to say where it is supposed to be for fear of being WRONG. Someone smarter than me will chime in.
Best,