Help / Brain Cells Request Continental O-300 Problem

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Aviacs

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Photos to clarify description of my approach to the "Lycoming Rope Trick" (actual SB is in my files, forgive the popular term it has come to be known by).

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above - using a piece of bare brazing wire (bronze) through top plug hole, to help lasso the valve

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secure. run the knurled nut down on the handle to snug the cable loop.

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Valve removed. Push it down with a wooden dowel, or a piece of aluminum round, if tapping is necessary.
Guide it with the loop, and avoid banging on the cylinder wall.

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Tool shown above is a bronze scraper for the valve stem deposits that build just below the guide area & can split the guide if they grow. Inner face of the tool is milled to same diameter as valve stem. (For convenience, better/deeper fit near sparkplug hole, "sharper" end). Other than that, there is nothing special about the shape - it was a convenient, suggestive shaped piece from other cut-outs in a piece of cast 954 scrap. Mic the barrel or use verniers down at the stem to verify diameter as each end and middle of the sliding surface.

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Above shows the machine tool straight shank collet chuck described previously, on the left. To twist and tap valve on seat to break up light deposits and improve seal. This is for a valve that seats easily and clearly, but does not seal because of "crud". As TFF stated, if the valve does not seat easily because it is hanging up on one side, it is bent and should be replaced. Before the head comes off. :)

I had forgotten the shop made split collet on the right as a loaner, made from a reject from a batch of mortise machine reducer collets. (just a straight, reamed to size bore. In this case quickly bandsaw-split, de-burred, and closed with a cross bolt.) The knurl is a convenient artifact of the original reject blank for the original purpose. Exhaust valve barrel is larger than the intake, but the reduced section of the stem is the same size. So the same size collet, any style, will fit either.
 
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TFF

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Good stuff.

I have lapped exhaust valves in the cylinder. It’s called last ditch for an owner who panics about taking a cylinder off.

Getting a valve back in its guide is not hard really. Good or bad, a couple of magnets, mechanics fingers and both plugs out it’s not hard. Feeding enough rope in the cylinder to hold the valve closed while you pop the keepers off makes me think of clown cars and how much needs to go in. I got a special rope just for it so it do pick up strange dirts. I put about ten feet in there and then pull the prop until it feels cushy. Both valves up.

I would not run it anymore until you have done work.
 

Dana

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Good stuff.

I have lapped exhaust valves in the cylinder. It’s called last ditch for an owner who panics about taking a cylinder off.

Feeding enough rope in the cylinder to hold the valve closed while you pop the keepers off makes me think of clown cars and how much needs to go in. I got a special rope just for it so it do pick up strange dirts. I put about ten feet in there...
We (local A&P and I) just lapped the valves in my O-290. Had low (35-40) compression on 3 cylinders; borescope showed nothing amiss but we could hear air escaping through 3 intakes and 2 exhausts (the remaining cyl was ok). After lapping, cold compressions are all around 70 and no audible air leaks. 2 hours to lap in place sure beats taking 3 jugs off.

We used air pressure instead of rope to hold the valves closed while removing the keepers.
 

BBerson

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How do you insert and remove the valve seat lapping compound?
(never heard of seat lapping without cylinder removal)

Generally, lapping seats is a temporary repair and the three step seat angle grinding or seat cutting is required.
 

BillW

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Jan 28, 2021
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My 172 with 0-300 has STC for unleaded auto gasoline (non-ethanol )
It runs real clean, plugs don't lead up, oil changes last twice as long. Also has oil filter and cooler. I watch egt and cht and er on the rich side, because some cylinders can run hotter than others. Stock 7:1 compression ratio.
 

Victor Bravo

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STATUS REPORT:

First, thanks to all of you for the time and expertise. There are many people here with a lot more experience working on this stuff than I. I appreciate the brain cell donations, to compensate for a lack thereof in my own toolbox :)

I removed the #4 cylinder, with great effort and several very bad words shouted. The problem is that I had apparently loaned out my cylinder base wrenches and never saw them again. So I borrowed a set of cheap wrenches, which immediately got stripped (the wrenches, not the base nuts!). I managed to get the cylinder off using other non-appropriate tools (end wrench held with Vise-grips, galvanized pipe "cheater bar", etc.), and figured I'd get a better set of wrenches when it came time to put the jug back on. (I got good new ATS wrenches from Fresno Airparts).

When I got the jug off, the good news is it wasn't cracked or gouged, no horrifying grooves in the bore, etc. I can still see most of the 45 degree hone marks in the bore. So I don't need major repair or replacement. The exhaust valve had to be pounded out with a brass hammer and brass drift pin. But it cleaned up pretty well on a brass wire wheel. Pounding it out of the guide left just a little bit of very fine streaks lengthwise down the stem, and I am hoping I can clean that up with crocus cloth.

The bad news is lead fouling in the valve guide. A set of brass rifle bore brushes all the way up to .40 caliber didn't get the lead out of the guide.

So I spent a hundred and twelve !)#*$&^# dollars on the special McFarlane "cleaning reamer" (it's sitting in a box in my house, waiting for me to have the time to use it).

BECAUSE the compression in that cylinder wasn't bad at all and the bore looks rather good, the plan is to clean the carbon off of the piston and the piston grooves, put the existing rings back in carefully, and put the cylinder on with new base nuts (lubricated, and then with "pal nuts"), and run the engine to see if it goes back to it's previous status of running well.

If it does not immediately come back to having good compression, or if I have another cylinder that starts acting up any time soon, then all six cylinders will come off and I'll have a traditional "top" done on the engine, with commensurate damage to my bank account.

The OTHER bit of information that I want to share is that the consensus from three people I know (all of whom have more experience on this specific problem) is that the TCP additive will indeed reduce or perhaps eliminate this lead fouling problem, AND the consensus is that even more aggressive leaning on the ground and in the air will help. I already had been leaning it very aggressively on the ground, and on the last few flights I have done so in the air as well. The big issue is that the O-300 intake system is more than likely not giving me equal distribution to the cylinders, so even when leaned out in the air there is still some cylinder that is rich when others are lean.

One of my more knowledgeable friends made me promise to put in a 6 channel CHT so I can at least know what is happening to each one, and I will try to do that. I can't stomach the idea of a big expensive engine analyzer that costs as much as the airplane itself, so it may have to be a "scanner" instead of one of the big square screen display types.

Anyway, I'm definitely down for 2 or 3 weeks because I have a lot of other time commitments with our chapter airplane build, work, etc. and I can't just spend three or four days doing this all at once.

BillW... is the STC still available for the unleaded car gas? I know that it clearly says no alcohol, and I would probably be willing to find or distill alcohol-free car gas in order to stop this problem from coming back.
 
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TFF

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You lean one, you lean them all. Even if you don’t know, the cylinder will be leaner than full rich no matter. Yes a full engine analyzer is overkill mainly because the numbers will not be pretty and even. I know a friend complains constantly after he put one on a carbed engine. Nice to have for diagnostics though. Valves out, I would lap them. Hedge the chances of leakage down the road with them cleaned up.
 

Dana

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How do you insert and remove the valve seat lapping compound?
(never heard of seat lapping without cylinder removal)

Generally, lapping seats is a temporary repair and the three step seat angle grinding or seat cutting is required.
He puts a tiny dab of it on the end of a zip tie and goes through the top spark plug hole to apply it to the valve in a couple spots, then pulls it back against the seat and slowly spins it with an electric drill. I didn't watch the removal, but I know he used a long thin strip of a twisted paper towel to wipe it out. I'll have to ask the specifics when I see him tonight.

Apparently it's becoming a standard technique for valves that aren't burned or badly worn, and he's done it many times. Goes with the philosophy of disassembling the engine only as a last resort. Granted there is a small risk of some compound getting away and causing further damage, but it may be less than the risk of damaging something when removing and reinstalling the entire cylinder.
 

TFF

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I transfer as much lapping compound to the valve and clean multiple times. Popping the exhaust and intake tube gets a little more access. Once the engine fires, it’s going out the exhaust pretty quick, unless you put a pound of it in the cylinder. It’s a case of saving something that already might be junk, so to lap them on wing is a chance. Popping a cylinder off is dependent on how easy it is to get to. The actual cylinder, with tools, is a two minute job off and on. It’s the work, you do to it,that snowballs. Some engines only like one cylinder at a time, though to keep main bearings from slipping. Getting a wrist pin out can be a different story.
 

BBerson

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The exhaust valve guide needs the hard carbon (not lead, as far as I know) removed to base metal so the valve can contact metal to metal to transfer heat to avoid baking more carbon again.
The sticking is from worn guides that have too much clearance. The only long term repair is new guides.
Measure the the guide bore diameter after the carbon ream. Check book for max guide bore allowed.
 

Richard Roller

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Olathe, Ks.
He puts a tiny dab of it on the end of a zip tie and goes through the top spark plug hole to apply it to the valve in a couple spots, then pulls it back against the seat and slowly spins it with an electric drill. I didn't watch the removal, but I know he used a long thin strip of a twisted paper towel to wipe it out. I'll have to ask the specifics when I see him tonight.

Apparently it's becoming a standard technique for valves that aren't burned or badly worn, and he's done it many times. Goes with the philosophy of disassembling the engine only as a last resort. Granted there is a small risk of some compound getting away and causing further damage, but it may be less than the risk of damaging something when removing and reinstalling the entire cylinder.
I've done this for years on everything from A-65's to TIO-540's. You wipe out and flush out the best you can. What remains is not a problem. The amount of lapping compound is minimal.
 

BillW

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Cleaning out the valve lapping compound is easy on aircraft engines , the bottom spark plug hole is your source for light and also the drain. After wiping it out, aerosol spray it with WD40
 

proppastie

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So I spent a hundred and twelve !)#*$&^# dollars on the special McFarlane "cleaning reamer"
less than an overhaul

if I have another cylinder that starts acting
now you know how to fix it save yourself some money

6 channel CHT
easier to buy cheap Laser temp tool and point at exhaust or cylinder.......otherwise you will be chasing probe failures as much as cylinder failures

 

Dana

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it certainly can be done with those suction cup things by hand...
Not with the cylinder still on the engine. Gotta be tough even with it off, way down inside the cylinder.

Spinning it with the drill, you hear it grinding, then it smooths out and when it squeaks, it's done. I was surprised how easy and effective it was.

easier to buy cheap Laser temp tool and point at exhaust or cylinder.......otherwise you will be chasing probe failures as much as cylinder failures
A laser thermometer is fine on the ground, but the CHT will tell what's happening in flight, where it really matters.
 
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