Exploring the #3 cylinder

Come with us now as we explore the mysteries of the cylinder!

As I mentioned earlier, I was a little non-plussed that the compression in the #3 cylinder had fallen to 71/80, when it was 79/80 or so in January.

I couldn't find any reason for it. I didn't hear any air escaping during test, although I don't have very good hearing and I was kind of busy holding the prop under pressure to keep it from spinning.

So I bought a boroscope, the one Kevin Horton recommended on Van's Air Force -- the Vividia VA-400, which I think is a pretty nice piece of equipment for $149.

Last evening I had a look. Here, go on a 12-minute tour if you can stay awake.



As I was playing with the thing, my pal, Mike Hilger -- you probably know him from SteinAir -- stopped by. Mike is an A&P and looked at things and said things looked OK. We did note some carbon buildup, but he suggested flying for another 10 hours and checking the compression again because it's not unusual for these sorts of things to disappear.

Fine. I buttoned things up.

I posted the video -- a much shakier video -- on Facebook and another A&P (at least I think he's an A&P) said "pull that jug and fix the the broken ring."

What broken ring?

I don't know what a broken ring looks like but apparently he sees something there that others don't.

So I'm looking for analysis from other people who know what to look for.

For the record, I've noticed no increase in oil consumption.

Checking oil

I flew back to visit my mother in Massachusetts last week, which put about 15 hours on the tachometer (I don't have a Hobbs meter) and put my oil change slightly over schedule at 55 hours.

So over the weekend I changed the oil and filter and tested out the new oil filter cutter I bought at Oshkosh.

It's a horribly messy process but eventually I cut the paper out of the filter to examine.

Here's what I saw. See those two flecks? There are about four of them total. I'm not convinced they didn't come off my workbench when I rolled out the paper on a paper towel and stared separating each fold to inspect with a magnifying glass and magnet.

The magnet didn't pick anything up.




I also pulled out the new compression tester and found the #3 cylinder at 71/80, which is pretty pathetic seeing as how it was 79/80 six months ago at the annual condition inspection.

However, it's possibly I'm doing this wrong. My friend, Brad,is going to bring is equipment over some evening this week and we'll try it again.

I can't say the engine has been running rough but I've never been convinced the #3 is entirely healthy, so we'll see. I did notice that as I dialed up the air regulator on the compression tester, the right gauge wasn't moving until the pressure at the regulator went to about 30 and then it snapped up. I wonder if that might be indicative of a stuck exhaust valve and the pressure popped it closed?

Fixing the leaky RV-7A tank

A few years ago I had a leaking rivet on the underside of the left fuel tank on the RV-7A and Doug Weiler of the Twin Cities Builder's Group suggested some ProSeal and a closed rivet. It worked great.

For the last couple of years, I've had two nagging rivets on the top of the tank which, when the tank is full and pressurized, emits just enough fuel (and it's not much) to put some streaks in my nicely polished plane.

This was one of the tasks to put off until annual and this week I started on the annual condition inspection, vowing that this one would be the most complete yet.

So I took the tank off . I didn't have to; it easily could be repaired without doing so, but the underside of a polished fuel tank and inboard wing gets beaten up, so this would also make it easier for me to polish it up on the underside while working at the workbench. I'll probably do the other side too ... polishing, I mean, there are no leaks on that side.

Chasing Oil

It took a few months longer than I'd hoped-- hey, it's cold in Minnesota and you can only breathe so many kerosene fumes from the heaters! -- but the annual condition inspection has been completed. The goal has been to slowly move the annual date to the middle of winter from its original June so that I can have the plane "grounded" when I'm not likely to fly anyway. Mission accomplished.

I didn't find anything serious during the inspection, but a few weeks ago, I did find more oil in the cowl than I care for.

Some comes from the oil screen plug, which has a particular torque value that always seemed too low. It's something like "finger tight and 135 degrees," or something.

But it's hard to find the source of the drips because the oil flies all over the place in the cowling once it's pressurized by ram air.

For example, this is the picture of things a few weeks ago when I first started exploring:



Looks to me like it's coming from that cover near the oil return, and therein lies the problem. It's pretty hard to figure out for sure where it's coming from.

But then Dan Baier had a suggestion. Clean up the engine and wipe off the oil, and then spray things with a foot powder. So I did. I waited for it to dry -- it provides a "white" cover -- then took the plane out to the ramp and ran up the engine for 10 minutes.

This is roughly the area:



Things look pretty good, right?

I should point out that the worst thing I ever did was what everyone suggested I should do: put a "whistler hole" in the oil breather tube. It spits out oil (and water) which gets everywhere. It's nearly impossible, it seems, to get the tube itself to drop it drips on the exhaust stack (where it would burn up) because of the engine mount which gets in the way.

I suspect there's also a problem -- well, not really a problem -- that with this engine, if I fill to capacity (about 8 quarts) with oil, it immediately pukes a bit of it overboard. I've heard that's the case with a lot of airplane engines.

I did find one area of concern, and it's the case. Here's a shot from the front.



You can probably see a little oil coming out near where that bolt and nut are. That would account for the occasional drip of oil I see in the front of the engine.

Now, the problem here is it's pretty impossible to get wrenches on either one of those locations, although I did and gave it a little tug; it was pretty snug.

A check of the rest of the engine did not reveal anything suspicious, so I put the cowling back on and took her flying for the first test flights post-inspection.

Afterward, I just looked in the back of the bottom cowling and saw a few drips of oil directly under the oil strainer and the usual mess around the breather tube, including drips of water. I hate that breather tube!

I also determined that what I thought was oil running down the gear leg when I first started the firewall-forward inspection, was probably avgas, coming out of a small drain hole I made in the filtered air box. I've always gotten fuel drilling in there -- possibly after shutdown or from overpriming. Even after 175 hours, I'm still learning things about what makes the engine happy with starting and I see this as a lessening problem.

But the oil drips cause me concern anyway because I don't really know the difference between routine and worrisome. Airplane engines are dirty things. Beautiful. Just dirty.

But at least mine doesn't have athlete's foot.

The case of the weepy rivet

I've been working on polishing the plane for the last few weeks and have been working on the most difficult part, especially for someone who gets dizzy when flat on my back looking up and back: the underside of the left wing.

I completed the outer half of the bottom of the wing last night and found this when I was ready to move on to the next section...

What with the accompanying wind-swept blue streaks, RV veterans will recognize that immediately -- as I did -- as a fuel tank leak around a rivet. It's not uncommon, but it's disappointing nonetheless, especially since I used a lot of ProSeal (the sealer) on that tank -- almost an entire can, or twice as much as what most people use.

The rivet is along a rib line which just happens to be below the fuel cap opening in the tank, which at least allows me to peer inside and see if anything looks out of whack.

Behold...

That brown spot on the top of the stiffner certainly leads me to believe that something is going on there, but I'm not yet sure what. It's not where the weeping rivet is. The weeping rivet is on a rib line. Also equally perplexing is the color around the rivet head -- brown. Avgas is blue.

So what's next? First, of course, I need to drain the tank and I'm going to try to fix this without taking the tank off the wing. Then I'll poke around at that brown crud to try to figure out what it is and then I'll poke at that rivet head that's actually weeping to see if the ProSeal has broken down there.

I'd like to think throwing some more ProSeal on there (after attempting to cut out, perhaps with an Exacto knife duct-taped to a dowel) would solve the problem. But it's possible the route the fuel is taking is actually coming from the other side of the rib, which is not accessible.

There is the possible fix of using LocTite around the manufactured head and hope it "wicks" up into the area and block fuel, but I would think this is somewhat problematic given gravity and all. I could apply a slight vacuum, but this is a dangerous task because fuel vapors could ignite at the vacuum cleaner I'd use. Not good.

Alternately, I could remove the tank, cut a large access hole in the back baffle of the fuel tank, get in there and slather all sorts of stuff, and then put a removal plate over that hole and seal it tight. Of course, that introduces new possible points of failure.

I could also just try drilling out that rivet and using a long bucking bar to buck a replacement.

We'll see.

Hey, at least it's gotten me out of polishing for a few days.