Stuck exhaust valve GO-300 C175

Dutchskylark

Dutchskylark

Filing Flight Plan
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Dutchpilot
Good day everyone,

I’m in the possession of a 1962 C175C with a GO-300 Continental Engine. Unfortunately we suffered several stuck exhaust valves from different cylinders on the Engine. During servicing of the valve and its Guide we found a lot of black glassy residue in the Guide which made the Valve getting stuck.

some facts:
We just for several years Shell 98 mogas
Shell W100+ Engine oil
Engine hours is about 200

Question: is there somebody who has experience with this problem and has an answer how this can happen?
 
This is a good read and mentions Continentals:

www.avweb.com

Dealing with Stuck Valves

If your engine seems rough when first started, it might be giving you an early warning of a stuck valve. Failure to heed this warning and correct the situation promptly could cost you an engine teardown, or even result in a catastrophic engine failure and a forced landing. Here's the lowdown on...
www.avweb.com www.avweb.com
 
Assure correct position of Rocker Arms.

The Exhaust has an oil drip hole at the tip.

Sometimes Intake also.
 
The GO-300 seems to have it‘s unique issues.

Some hi- rpm helicopter engines would run a tap

down the Exhaust Guide to provide a path for oil

to cool and lube.
 
The small Continentals are famous for that. If the rebuilder reams the new valve guides to the minimum ID, it leaves the clearance small enough for valve stem carbon buildup to seize the valve in the guide. We had issues with the O-200s that way.
 
I‘ve also heard more clearance allows deposits and

also reduces heat transfer.

Is there a “ Sweet Spot”?
 
Dan Thomas said:
The small Continentals are famous for that. If the rebuilder reams the new valve guides to the minimum ID, it leaves the clearance small enough for valve stem carbon buildup to seize the valve in the guide. We had issues with the O-200s that way.
Click to expand...
Are you saying that we have to ream the guide to a higher width? Off course within the manufacturer limits
 
The GO-300 Engine has a higher RPM (max 3200). Can that be an issue in combination with the single grade oil en mogas?

Is it better to mix AVGAS with Mogas to bring down the lead ratio?
 
Dutchskylark said:
Are you saying that we have to ream the guide to a higher width? Off course within the manufacturer limits
Click to expand...
The overhauler should have been paying attention to those limits. They might have mixed up the clearances between the intake and exhaust valves. I don't have a GO-300 manual, and can't find a free on online, but here's what the O-200 manual says for valve stems and guides:

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So the exhaust valve needs a clearance in the guide of .003" to .0045". The intake needs only .001" to .003". The exhaust need more clearance because it gets far hotter.

If we take the exhaust valve stem max diameter of .4335 and subtract that from the minimum exhaust valve guide bore of .4370, we get .0035" clearance, pretty close to the minimum required. If one used the other exhaust valve, the 629404, that has a max stem diameter of .4340, our clearance comes out right at the minimum. Maybe a little tight for 100LL, with its four times the lead content of the old 80, around which the engine was designed.

Maybe.

When I learned to fly, it was mostly in a nearly new (six years old) Cessna 172 that had the O-300 in it. We had a valve stick in it while on a cross-country to another city where the school had its headquarters and shop. Ran fine at shutdown, stuck on startup. They gave us a 1968 Cessna Cardinal to fly home, only five years old! Smooth Lycoming, but underpowered.
 
455 Bravo Uniform said:
Do you know the role of lead in your valve stem to valve guide issues?
Click to expand...
Too much lead is no good, just like too much aspirin is no good. 100LL has four times the lead of the old 80 that these things were designed for. Lead has a bigger lubricating effect on the valve seats than on the stems and guides.
 
Dan Thomas said:
Too much lead is no good, just like too much aspirin is no good. 100LL has four times the lead of the old 80 that these things were designed for. Lead has a bigger lubricating effect on the valve seats than on the stems and guides.
Click to expand...

Does running leaner or richer have a greater effect? My concern modifying fuel would be octane reduction that could lead to detonation related damage that is worse than a sticking valve (unless mogas is acceptable). The fix might be in the red knob rather than fueling. No?
 
Thank you for your repl,

We have the original Engine Manuel in the Hangar and used it to check for the clearances so no problems in this.
Also we followed the Lycoming 1425 procedure as that is almost the same for the Continental Engine.

AVGAS 100 LL:

In Europe (Netherlands) the ratio of lead is 0,56grams per Liter!!
I think this to much for the 80-85 AKI Engine. Some people here advise to mix it with Mogas to bring the lead ratio down(1:3). I don’t know of this will help but I don’t like the idea to mix this type of fuels. Some users who are using or had used the Shell 98 stopped using it because of all kind of problems.
Some owners don’t mix the fuel but using 1 time Avgas en 3 times Mogas 98 and so o.
btw: our STC demands that the min AKI should be 98 with no Alcohol!! Which is the Shell98 here in the Netherlands.

what is your opinion Dan Thomas or anyone?
 
455 Bravo Uniform said:
Does running leaner or richer have a greater effect? My concern modifying fuel would be octane reduction that could lead to detonation related damage that is worse than a sticking valve (unless mogas is acceptable). The fix might be in the red knob rather than fueling. No?
Click to expand...
AVGAS has 100 AKI at lean power and 130 at rich power
 
455 Bravo Uniform said:
Does running leaner or richer have a greater effect? My concern modifying fuel would be octane reduction that could lead to detonation related damage that is worse than a sticking valve (unless mogas is acceptable). The fix might be in the red knob rather than fueling. No?
Click to expand...
I blended 100LL and mogas for the flight school airplanes that had an 80 octane minimum spec for the engines. No problems. 75/25 mix, 75% mogas. The mogas was 87 octane, already higher than required according to Petersen's STCs. Those STCs only apply to engines certified for 80 minimum.

But it was a hassle. Had to order both fuels, two trucks, correct amounts, all that, and the savings at the time weren't really worth it for us. The bulk plant had to drive a truck to the refinery several hours away, rinse the tank as required by law for avgas transport, so wasting some fuel. It was expensive for them for just one load. I gave it up after a few years. Another factor at the time was that Lycoming would not warranty their factory remans, which we bought, if we ran mogas in them. I think they have changed their minds on that.
 
I had been talking to my Belgium technician (Eng overhaul shop)
Acc to his opinion is that the reason for this problem is heat!
Important is to let the Eng cool down slowly after landing (idle power for 3 min at least) before Eng shut down.

The subject of mixing fuel is a good option too. If you only using Mogas than the Valve stem and Cylinder will be running at a higher temperature. Avgas has a considerably higher lead ratio than where the Engine was developed for. So the mix ratio of 1:3 is a good option. The second option is to flying 1 out of 3 with AVGAS and the 2nd & 3rd Mogas.

AVGAS here is very expensive here in my country. Average €3,50 per ltr (3,80 USD) and Mogas (Shell98 at the fueling station) is about €2,20 per ltr.(2,4 USD).
So in one hand you save a lot of money flying only Mogas but the maintenance costs will rise.

You don’t want to be on an outstation with a sticky valve is it!!
 
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