Sticking Exhaust Valve - Diagnosis and Fix! - Page 9

bullojm1 · #81 12-26-2019, 04:14 PM

Quote:

Originally Posted by stevengrasley

To the chap that wrote this original post: Bravo! I wish I had found it when I was in need. I just read it today for the first time. Excellent post and pics. Well done my friend.

Hi Steve,

Thanks! I have since moved on from the "rope trick" to using compressed air to keep the valve closed. As part of my annual inspection, I check each exhaust/intake valve for how freely they move int he guides. It adds less than an hours to the total inspection time and gives me great confidence in the fact I won't get a stuck valve at an inopportune time in the next year.

Dave12 · #82 12-26-2019, 05:21 PM

Does anyone know or remember the post a year or so ago that discussed using a flex hone to clean up guides? There were two sizes listed that would work. I thought I had that info saved, but can?t locate it. A search hasn?t turned up any info..

rocketbob · #83 12-26-2019, 06:27 PM

Quote:

Originally Posted by Dave12

Does anyone know or remember the post a year or so ago that discussed using a flex hone to clean up guides? There were two sizes listed that would work. I thought I had that info saved, but can?t locate it. A search hasn?t turned up any info..

http://www.vansairforce.com/communit....php?p=1359647

Dave12 · #84 12-26-2019, 07:37 PM

That?s it. Many thanks Bob.

NewbRVator · #85 12-26-2019, 07:47 PM

Lycoming O360 A1A exhaust valve size? 1/2?.
Thanks

thiggins · #86 03-21-2020, 07:14 AM

We had a Commanche 250 who's O-540 was notorious for stuck valves. So, we ran marvel mystery oil in the gas AND in the oil. It's old school I guess, but we had no issues with stuck valves, and the minty scent was very pleasant.

RV7 To Go · #87 03-21-2020, 12:14 PM

Quote:

Originally Posted by thiggins

We had a Commanche 250 who's O-540 was notorious for stuck valves. So, we ran marvel mystery oil in the gas AND in the oil. It's old school I guess, but we had no issues with stuck valves, and the minty scent was very pleasant.

What quantities did you run? Haven't had issues with stuck valves to date but have often wondered about using the MMO as a preventative measure.

thiggins · #88 03-22-2020, 04:37 PM

You know, it was so many years ago I can't remember, but I think there was a recommendation on the dang MMO bottle. But, I think I'd just pour a half quart in each side upon gas fillup (?)... Something like that. Our A&P actually recommended this to us at some point, so we went with it. He said it acts as a light oil, which is better able to penetrate and lubricate the top end. ( Your mileage may vary...) I'd snag a bottle and read about the stuff. We liked it, and it's really cheap insurance against the sticky valve thing, plus it does smell nice.

Red Mtn flyer · #89 03-23-2020, 06:47 AM

The early O-540A's in the 250 Comanche had 7/16" valve stems -- later models [and the fix] was to replace with 1/2" heads, which has long been standard. The issue was heat transfer - the smaller diameter stem heated up more, and resulted in 'coking' more than the 1/2". And that is still what causes this - 'cooking' gunk into the guides/ onto the stems. Full rich ground ops, cold cylinders, etc.: you get gunk. Can't be absolutely avoided, but can be reduced by operational methods.

lr172 · #90 03-23-2020, 07:05 AM

Quote:

Originally Posted by Red Mtn flyer

The early O-540A's in the 250 Comanche had 7/16" valve stems -- later models [and the fix] was to replace with 1/2" heads, which has long been standard. The issue was heat transfer - the smaller diameter stem heated up more, and resulted in 'coking' more than the 1/2". And that is still what causes this - 'cooking' gunk into the guides/ onto the stems. Full rich ground ops, cold cylinders, etc.: you get gunk. Can't be absolutely avoided, but can be reduced by operational methods.

The black stuff that fills the clearance between the stem and the guide is coaked oil. Coak deposits are black and very hard and are the result of heating oil past it's oxidation point. fuel mixture during ground ops has nothing to do with this, nor does running your engine cold. The only real preventative measure is to increase oil flow to the rocker box, as increased oil flow in that area helps to cool the exhaust valve stem. I suspect that running LOP in most high CHT environments would also help, but brings detonation risk with it. Ensuring good seating between the valve and valve seat also helps to insure a lower stem heat.

MMO in fuel can help reduce the combustion temp at which lead in the fuel is atomized (exits via exhaust versus deposited). However, coak is not a lead deposit. Have no idea if MMO in the oil helps, but doubt it.

Larry