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Old 08-12-2017, 02:44 PM
Brantel's Avatar
Brantel Brantel is offline
Join Date: Mar 2006
Location: Newport, TN
Posts: 6,767

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....
Brantel (Brian Chesteen),
RV-7/TU, #72823, N159SB
Lyc. O-360 carbed, HARTZELL BA CS Prop, Dual P-MAGs, Dual Garmin G3X Touch
Track N159SB (KK4LIF)
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Last edited by Brantel : 08-13-2017 at 06:06 AM.
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Old 08-12-2017, 02:55 PM
az_gila's Avatar
az_gila az_gila is offline
Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 9,103

Originally Posted by Brantel View Post
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.


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...
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ

Last edited by az_gila : 08-22-2017 at 10:10 AM.
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Old 08-13-2017, 05:59 AM
turbo's Avatar
turbo turbo is offline
Join Date: Jan 2007
Location: Stuart, FL /Hartford, CT/Virgin Gorda,BVI
Posts: 2,946

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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Last edited by turbo : 08-13-2017 at 06:09 AM.
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