Exhaust Valve Question

[Thomas Lukasczyk]

Hi all

I did the Exhaust-Valve wobble test (Lycoming SB388c) yesterday and all clearances turned out to be within the specified numbers.
However, both rear-cylinders almost reached the maximum allowable clearance at a reading of 0.025 and 0.028. According to the paperwork, this engine has
only 400 hours on the meter. Are these values plausible for a "low" time engine or do they necessarily indicate a "high-time" engine? How does the valve-guide wear over time?
Is it just getting worse as your hours build up or does most of the wear occur during break-in?
Obviously there is a minimum clearance to determine carbon build up. So what would be the wobble-test reading on a brandnew cylinder?
Thank you very much for your experience

regards

Thomas, RV-4
http://www.rv-4.de/

[az_gila]

...plausible, unfortuneately.

My Tiger factory rebuilt O-360 engine - with new cylinders and the "new material" guides - failed the wobble test on two cylinders at 600 hours.

The variation seems to be very engine/airframe dependant, and I have not really heard a good explanation of exactly why excess wear occurs, as opposed to failing on the low end, which would be a symptom of pending sticky valves.

I would just perform the test at 50 or 100 hour intervals and track the readings.

If you are using the real Lycoming fixture, as opposed the the Aircraft Spruce substitute, the test is quite non invasive since the fixture pushes the valve down as it is is tightened.

This is what the Lycoming fixture looks like -



Most will have been modifed to use a dail indicator gauge instead of the specified feeler gauges.

Lycoming has some info around on operation to reduce valve guide wear...

Originally Posted by Thomas Lukasczyk

Hi all

I did the Exhaust-Valve wobble test (Lycoming SB388c) yesterday and all clearances turned out to be within the specified numbers.
However, both rear-cylinders almost reached the maximum allowable clearance at a reading of 0.025 and 0.028. According to the paperwork, this engine has
only 400 hours on the meter. Are these values plausible for a "low" time engine or do they necessarily indicate a "high-time" engine? How does the valve-guide wear over time?
Is it just getting worse as your hours build up or does most of the wear occur during break-in?
Obviously there is a minimum clearance to determine carbon build up. So what would be the wobble-test reading on a brandnew cylinder?
Thank you very much for your experience

regards

Thomas, RV-4
http://www.rv-4.de/

[aerhed]

I'm open to dispute here, but I think a lot of the variation is in the fit of the guide when new. As for wear, it will be an accellerating thing. The greater the clearance, the greater the angle of valve to rocker foot, the greater the sideload on the guide/stem, the faster the wear (until failure). Didn't some of the Tiger guys do a lot of work on measuring oil flow around the 360 valve train? Maybe a pro engine guy can chime in but I know Nitralloy guides are ball-honed and I think the rest are still reamed when new. There's always a little variation when you push a 1/2" reamer through a guide, they don't come out exactly the same.

[az_gila]

Originally Posted by aerhed

I'm open to dispute here, but I think a lot of the variation is in the fit of the guide when new. As for wear, it will be an accellerating thing. The greater the clearance, the greater the angle of valve to rocker foot, the greater the sideload on the guide/stem, the faster the wear (until failure). Didn't some of the Tiger guys do a lot of work on measuring oil flow around the 360 valve train? Maybe a pro engine guy can chime in but I know Nitralloy guides are ball-honed and I think the rest are still reamed when new. There's always a little variation when you push a 1/2" reamer through a guide, they don't come out exactly the same.

...Tiger guys doing oil flow analysis - Bill Scott and Bill Marvel, IIRC.

My cylinders were factory new, I even checked with Lycoming - they do not "reuse" cylinders in any of their overhauls. They should have started off within limits.

The wobble check is designed to be a progressive "see how it goes check", so accelerating wear theoretically can be detected - however for the new guide material Lycoming specifies a first check at 1000 hrs and then every 400 hrs.

The problems could be airframe-engine dependant as I mentioned before.

http://www.lycoming.textron.com/supp...dfs/SB388C.pdf

http://www.lycoming.textron.com/supp...fs/SI1485A.pdf

[RV3 Pilot]

Don't forget to check the rocker arms. If the contact surfaces were not machined correctly they can sideload the valves and that will accelerater valve guide wear. Happened to a friend of mines "freshly rebuilt" engine.
Tom