Lycoming SB 388C "Wobble Test" Tool Info

Brantel · #1 08-11-2017, 07:38 PM

So I was almost convinced I had a valve showing signs of sticking that showed up on my trip home from Osh a few weeks ago.

I proceeded to educate myself on the Lycoming guidance around this subject and found out about SB 388C and the "Wobble Test".

Long story short, I did not have a sticky valve but I did go ahead and decide to follow the SB 388C and perform the wobble test on my exhaust valves.

I realized that the official Lycoming tools to perform this test were ridiculously expensive. I also found out that the Lycoming tool rental program is also pretty much a joke and there was no way I was laying out that kind of cash for that either.

So I decided to have the tool shown in SB 388C 3D printed out of glass filled nylon. Here is the result:

The parts came out looking great.

I used the tools tonight and I can tell you that the fixture to hold the dial indicator works great. The valve extension not so much (pretty much useless).

The design of the valve extension is very poor. I should not have followed the design as it is shown in SB 388C. There simply is not enough surface area actually clamping the valve stem for it to work well. Most examples I have seen where people have made this extension have not made them per the SB 388C. I threw the one I had printed in the toolbox and made another one out of a piece of CPVC, split with a band saw and clamped with a hose clamp. It worked great since it can grab more of the valve stem.

Then I found another design flaw with the tool described in SB 388C. The SB says "Tool must measure parallel to the rocker at the following height above the rocker box cover surface of the cylinder (machined O.D. surface of rocker box cover surface). Angle Valve Clearance – 0.750 ± .015 Parallel Valve Clearance – 1.190 ± .015"

Well the design for the tool provided in the SB 388C measures at 1.875" above the rocker box cover surface not 1.190".

Since this puts the measurement point at a larger radius from the center of the wobble of the valve in the guide, this makes any measurement taken using this tool larger than it would be if the measurement was taken at the recommended height above the rocker box cover surface.

I plan to account for the geometry differences to ensure I am in spec but this is pretty ridiculous on Lycoming's part....

Note: Unless you have the latest version of the Lycoming rocker arms, they are not interchangeable. Lycoming SI 1454A details how to ID them and where they go....

rockwoodrv9 · #2 08-11-2017, 09:05 PM

I am getting so tired of looking forward to seeing a post and pictures and not being able to see them.

Without photos, this site loses a LOT of appeal.

rightrudder · #3 08-11-2017, 09:15 PM

Quote:

Originally Posted by rockwoodrv9

I am getting so tired of looking forward to seeing a post and pictures and not being able to see them.

Without photos, this site loses a LOT of appeal.

Agreed. I so enjoyed doing a build thread on my RV-9A, and it's semi-worthless now without pictures. Depressing.

az_gila · #4 08-11-2017, 10:34 PM

It's interesting that the two numbers are different.

But, this is what 338C says on the last page about the DIY tools...

The tools described in this section locate the tip of the measuring instrument 2-1/2 inches from the top of the valve guide. This dimension should be maintained on all locally manufactured tools.

Since it's quite specific, I wonder if it is an actual geometry change to allow for the different measuring techniques.

Using the Lycoming ST-71 tool, the valve springs and even valve retainer keys are left in place since the tool does the spring compression. With the DIY tool the SB338C says that the valve springs must be completely removed.

Perhaps Lycoming is being cleverer than we give them credit for and the extra 0.69 inch (approx.) actually is designed in to normalize the measurements after the removal of the valve springs and associated hardware?

Brantel · #5 08-12-2017, 05:23 AM

Quote:

Originally Posted by rockwoodrv9

I am getting so tired of looking forward to seeing a post and pictures and not being able to see them.

Without photos, this site loses a LOT of appeal.

Does the pic in the first post not show for you?

I can take more pics today of the process if you would like to see it.

Malndi · #6 08-12-2017, 05:42 AM

I can't see the pic in the first post either.

Brantel · #7 08-12-2017, 06:44 AM

Sorry guys, I had no idea those pics were not showing up. Looks like they show for some and not others.

I guess that method for posting pics won't work so I will try something else.

It should show up now if you refresh your screen.

rockwoodrv9 · #8 08-12-2017, 08:56 AM

Thanks Brian. They are working now. The problem is the thousands of pictures that are not ever going to be available again. They show up on the posters screen, but not everyone else's screen.

The ability to print out tools and parts today will change so many things. The 3d printer should make specialty parts that used to cost a boatload of money available to so many more people. Those you did look great. Nice work.

BillL · #9 08-12-2017, 10:22 AM

Quote:

Originally Posted by Brantel

Sorry guys, I had no idea those pics were not showing up. Looks like they show for some and not others.

I guess that method for posting pics won't work so I will try something else.

It should show up now if you refresh your screen.

I had this happen once of my photo posts. I think it was a login issue with google photos it appeared I was logged in, but was not. I logged out, rebooted, logged in, and reposted. All was well. 1% issue for me anyway.

Brantel · #10 08-12-2017, 02:09 PM

Quote:

Originally Posted by az_gila

It's interesting that the two numbers are different.

But, this is what 338C says on the last page about the DIY tools...

The tools described in this section locate the tip of the measuring instrument 2-1/2 inches from the top of the valve guide. This dimension should be maintained on all locally manufactured tools.

Since it's quite specific, I wonder if it is an actual geometry change to allow for the different measuring techniques.

Using the Lycoming ST-71 tool, the valve springs and even valve retainer keys are left in place since the tool does the spring compression. With the DIY tool the SB338C says that the valve springs must be completely removed.

Perhaps Lycoming is being cleverer than we give them credit for and the extra 0.69 inch (approx.) actually is designed in to normalize the measurements after the removal of the valve springs and associated hardware?

I don't think so...

The numbers don't add up. If we assume that the ST-71 measurement is somehow restrictive due to the spring/retainer being in the way and the DIY tool is less restrictive and allows the valve to move more, the normalization would have to be in the other direction. The way it is, the DIY tool is further from the pivot point (the valve in the guide) which would would cause a larger reading on the same valve/guide vs the ST-71 tool which measures closer to the pivot point.

The bottom line is Lycoming should have included a different tolerance table for the DIY tool since the measurements with that tool are always going to be larger than those of the ST-71 on the same valve.