Lycoming SB 388C "Wobble Test" Tool Info

[Brantel]

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....



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.



From the side:



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



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....

[az_gila]

Originally Posted by Brantel

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.

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...

[turbo]

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.

[scard]

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?

[Brantel]

Originally Posted by scard

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/communit...hlight=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.

[BillL]

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?