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Lycoming SB 388C "Wobble Test" Tool Info

Brantel

Well Known Member
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". :eek:

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. :mad:

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

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....
 
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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.
 
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.
 
Perhaps it's in the design?

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 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.
 
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.
 
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.
 
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.
 
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.

emBivM5.jpg
 
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Here is the DYI version of the DIY valve extension that is in the Lycoming SB 388C. Never fear the fact that it is made out of a 1/2" piece of CPVC pipe. It only has to grip the valve stem and withstand the pressure of the dial indicator spring nothing more....

reGAFCj.jpg


Here is the DYI dial indicator holding fixture per the 388C. This one is 3D printed from glass filled nylon and works great. All it has to do is hold the dial indicator steady. It requires a washer to keep it pushed over against the rocker arm shaft boss. This is per the manual. The screws can be hand tightened to hold it all in alignment. The key is to ensure that the dial indicator is parallel to the rockers and at a right angle to the valve extension and the dial indicator is centered on the shaft of the extension. It is easier than it sounds to get it all lined up.

iRW7CLd.jpg


From the side:

PoLo5hq.jpg


Per the SB 388C, the dial indicator measuring point is 2.5" from the top of the valve guide.

ehjtsMM.jpg


The test is simple... push the valve in a little to get it off the seat then take your finger and press down on the valve stem right where it goes into the guide. Set the dial indicator to 0 then without rotating the valve (rotating screws up your 0 point unless you take the time to ensure that the extension is perfectly concentric with the valve stem) take your finger and press up in the valve stem right where it goes into the guide. Take the reading...repeat a few times for accuracy.
Easy Peasy!

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....
 
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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.

emBivM5.jpg


There may be another major difference.

The DIY tool instructs you to push the valve at least 1/2 inch into the cylinder.

With the ST-71 tool I don't think the valve is depressed that much. The dimensions depend on the clamping plate/valve spring retainer and on the counterbore in the valve stem fitting.

I'm going to do the test on my Tiger this week - I'll report back how much the valve is moved into the cylinder by the ST-71 tool.



UPDATE -

I checked mine with the ST-71 tool. It does put the valve 1/2 inch into the cylinder for measurement.

The Lycoming distance discrepancy between tools is strange...
 
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glad to see there is some info being developed here on the exhaust valve SB. it is one of the weak part of the lycoming. i call it the silent killer. great thread. :)

interested what values you have come up with so far.

my mechanic did my mid time engine in an hour. i had it opened up and ready to go. if there was some reaming of the guide that would of added to the time. if you are experiencing lead balls in your spark plugs then this inspection is a must at some time.
 
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To revive the conversation...
So, Brian, what did you find? Anything? You started this because you thought you might be seeing the beginnings of a sticking valve.

When cold, we're noticing one cylinder consistently taking a little longer (a few seconds) to come alive. At startup (only when cold) the egt is definitely behind the others, associated with a bit of rough running, only for a few seconds, then smooths out as that egt catches up... I was thinking sticking valve, but it is not sticky at all after we dug into it. I haven't made a wobble test fixture yet. Conclusions, comments?
 
To revive the conversation...
So, Brian, what did you find? Anything? You started this because you thought you might be seeing the beginnings of a sticking valve.

When cold, we're noticing one cylinder consistently taking a little longer (a few seconds) to come alive. At startup (only when cold) the egt is definitely behind the others, associated with a bit of rough running, only for a few seconds, then smooths out as that egt catches up... I was thinking sticking valve, but it is not sticky at all after we dug into it. I haven't made a wobble test fixture yet. Conclusions, comments?

Scott,

I found that my valves were not sticking. The issue I was chasing ended up being totally unrelated to the engine (post 17 of this thread is where the root cause of my issue was found). Mike Bullock is the expert on morning sickness due to his frequent valve sticking issues. Here is his thread on the subject and how he has dealt with it:

http://www.vansairforce.com/community/showthread.php?t=128673&highlight=bullock

As for the wobble tool that I built, The fixture works great but there is tons of conflicting info as well as just plane wrong info out there about where to measure the wobble and how it applies to the tolerance that Lycoming has published. The Lycoming documents are confusing and contradict themselves in several places. I ended up using the Lycoming tolerance and corrected it for the geometry of the DIY fixture and found all of my valves in tolerance.
 
Dragging up the past here, but I am doing some research for this job and page 11/13 of the SB says that the contact point of the dial indicator (measurement point) should be 1.19" relative to the gasket surface of the VC cover. How do we know (from the SB) that this is not the same as the ST-71 dial indicator location? I don't find any other reference dimensions. Sadly, the complicated dial indicator holder (DIY) is lacking any datum that can be used for this confirmation.

Also, note the split for the extension has to be 90 deg to the measurement direction, otherwise it might rock on the two contact points. I doubt the internal diameter will be precise enough to be a tight fit otherwise.

Does anyone have the distance from the center of the rocker shaft to the valve on a parallel valve? And . . anyone have dimensions for the plan view of the ST-71?
 
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