Leaky exhaust valves

[mike.smith]

Went for a flight the other day and before going out I manually rotated the prop.
I noticed considerably less compression on 2 (#4 and #3 I believe) of the cylinders, also, I can hear a hissing in the exhausts during compression.
Leaky exhaust valves? Certainly showing all the symptoms…

You don't have the Nickasil cylinders, do you? If you say yes then I'll tell you what may be up. If you say no then disregard :-)

I have the Nicom cylinders from Hummel, they are essentially Nikasils (nickel silicon carbide) but I believe the NiComs have chrome in them as well or so I've read somewhere.
I did ask Scott about them when the whole AeroVee Nikasyl issue happened and he said he never had issues with his brand, he still puts them on his engines as far as I can see now.

But yes, the thought has crossed my mind... you had problems with yours didn't you?

Yes. The walls of the cylinders were too thin to stand up to normal temperatures and torque values. The tops of the cylinders, where they contact the heads, were rolling over and mushrooming. So the heads were getting closer to the engine block each time I reset the torque, and that would decrease the gap between the valves and the rocker arms until they were in constant contact, making the valves leaky (they would never fully close). That also burned my first set of valves, but at the time I didn't understand what was happening, so I put the heads back on the bad cylinders after replacing the valves.

The real clues were twofold: 1. Every time I checked the torque on the heads the nuts had to move to get the torque to specs (on my cast iron heads the torque has stayed put, and the nuts have never needed tightening), and 2. regular oil analysis showed high levels of things like nickel (the nickasil) and iron (piston rings) and aluminum (the cylinder walls under the nickasil).

Here's my Kitlog entry.
http://www.mykitlog.com/users/display_l ... 380&row=38

Absolutely no issues since changing to cast iron cylinders.

[rizzz]

Yes. The walls of the cylinders were too thin to stand up to normal temperatures and torque values. The tops of the cylinders, where they contact the heads, were rolling over and mushrooming. So the heads were getting closer to the engine block each time I reset the torque, and that would decrease the gap between the valves and the rocker arms until they were in constant contact, making the valves leaky (they would never fully close). That also burned my first set of valves, but at the time I didn't understand what was happening, so I put the heads back on the bad cylinders after replacing the valves.

The real clues were twofold: 1. Every time I checked the torque on the heads the nuts had to move to get the torque to specs (on my cast iron heads the torque has stayed put, and the nuts have never needed tightening), and 2. regular oil analysis showed high levels of things like nickel (the nickasil) and iron (piston rings) and aluminum (the cylinder walls under the nickasil).

Here's my Kitlog entry.
http://www.mykitlog.com/users/display_l ... 380&row=38

Absolutely no issues since changing to cast iron cylinders.

Hmmmm looks bad,
I'll be heading to e airport tomorrow to find out what's going on, we have a borescope in our local SAAA chapter so I will use that first before pulling anything.

It's a shame about the quality of those nickasils currently on the market. Most are cheap chinese knock-offs.
I'm not sure where Hummel sources their NiComs though but I'm hoping for the best. They look quite different from the AeroVee and GP ones.

If I had the money I'd by a set of the original LN Engineering "Nickies" (http://lnengineering.com/products/airco ... ?limit=all), everyone on the VW forums raves about those, but at almost $3500/set they darn expensive, too expensive!

[radfordc]

Is there any value to the Nickasils other than the weight difference?

[rizzz]

Is there any value to the Nickasils other than the weight difference?

It is claimed they cool better.

[N190YX]

Note you can only fly a TCM engine (and this should be considered for any aircraft engine) with leakage past an exhaust valve is after a boroscope inspection shows a normal exhaust valve.

Excuse me, can you please provide a citation for this? Saying you can "only" fly the engine implies that the procedure is mandatory.

If your source is the "Service Bulletin" that was referenced in the previous post, its worth noting that Service Bulletins are not mandatory (with a few narrow exceptions - including when they are incorporated into an AD and incorporated into a Users/Owners/Maintenance manual).

Thanks,

--Noel
P.S. I'm not trying to be a jerk; but the claim goes perfectly to my earlier point about well-meaning facts and careful explanations.

I wrote about exhaust valve leakage based on my experience of owning an IO-470 TCM Continental engined airplane for 37 years (still flying it) and doing my own maintenance (under the supervision of an A&P IA), and having had a VW engine swallow an exhaust valve head that broke off the valve stem (the engine seized immediately with metal throughout the engine, the valve head caused a hole in the piston), and on following what I consider to be reasonable guidance from TCM regarding leakage past an exhaust valve. Hey, we can't pull over to the side of a road when our engine fails, and to read that, well, you can survive most engine failures in flight - If you think taking an unnecessary chance on an engine failure in flight is OK, you should not be flying, you don't have the judgement. Blunt statement, but we pay higher insurance rates due to pilots doing risky things. Please think about this.

[SonexN76ET]

One possible way to protect your valves from leaking early due to carbon fouling is to use Decalin or TCP fuel additives. I checked with Sonex before I started using it and while they do not explicitly recommend it, they did say people had reported good results with it. (Just be careful to read the directions as you put very little in to mix with ten gallons of 100LL, I am talking just a tablespoon or so per the directions. One guy mistakenly poured a whole bottle in his Sonex's fuel tank and had some bad results with reduced power output). On a SportCruiser with a Rotax 912, it was required to use the Decalin fuel additive when using 100LL to prevent fouling). So far, I have noticed no ill effects using the Decalin over the past fifteen hours.

http://www.aircraftspruce.com/catalog/appages/Decalin.php?recfer=1914

Sonex had also recommended that I ensure that I was not running too rich and warned that a too rich mixture can lead to valves and sparkplugs fouling. I now lean my mixture after landing when taxiing back to the hangar. I do not lean my mixture when taxiing to the runway for takeoff because of the off chance that I would fail to go to full rich for takeoff which could cause the engine to fail on takeoff due to a too lean of mixture. I lean in flight to stay in the upper range of the green for my CHTs and EGTs.

The other thing you can do is make sure your cooling baffles are adequate and do not leave any gaps that the air can escape through. You want pressurized air going through the cooling fins and ducts on your engine. High pressure on the top of the engine flowing through the cooling ducts and fins on the cylinders and heads and into the supertins on the bottom and into the low pressure below the engine and out into the slipstream. One thing I did not initially pay attention to is that gaps at the front of the engine in your baffles can cause a significant amount of air bypassing the engine. Another place that allows air to escape is in the corners of the baffles at the rear of the engine. The reason head cooling is important and related to valve fouling is that some people run their mixture excessively rich in an effort to keep their CHT's in the green. That excessively rich mixture used to offset poor head cooling causes the valves and sparkplugs to foul with carbon and lead.

I also wanted to make a comment on the proposition in this thread to look at different cylinder heads. On the surface the CB Panchito 44 heads look promising. I do not think I would like to be the first one to try them in flight though. One configuration issue I saw was the intake ports are milled significantly more inboard than the conventional heads. This would require probably using a spacer like the one Sonex uses with the MOFOCO heads but having them machined to create a smoother joint with the Aerovee intake elbow or having custom intakes made.

I agree with N190YX that you should not fly with any known deficiencies. Always double check everything and if something needs fixing then fix it before you fly.

On that note, pulling the heads on an Aerovee is very easy and very simple. It takes me about 15 minutes per head. My local machine shop will do a valve job on the pair of heads for around $150. Then when you reinstall them all you need is new exhaust gaskets, new push rod tube seals (I highly recommend the white silicone ones. They do not leak!) and new copper cylinder head gaskets. It might take an hour per side to reinstall them. You will have to adjust your valves again when you reinstall the head and then again at 5 hours, torque the heads and adjust valves at 10 hours, and 25 hours, just like when you first built your engine. You will also have higher CHTs as the valve job breaks itself in over the next few hours. I am tempted to get a second set of heads and just change them out whenever I suspect a fouled valve (with a valve job done when I pull the heads and then have them ready to go for the next time). Aerovee heads are very inexpensive compared to certified heads or Rotax heads.

Anyway, this is just my two cents on my experience so far. Please put safety first when you fly. These planes are lots of fun, but they need to be treated with respect.

Jake

[mike.smith]

One possible way to protect your valves from leaking early due to carbon fouling is to use Decalin or TCP fuel additives. I checked with Sonex before I started using it and while they do not explicitly recommend it, they did say people had reported good results with it. (Just be careful to read the directions as you put very little in to mix with ten gallons of 100LL, I am talking just a tablespoon or so per the directions.

I have run TCP for 220 hours on my AeroVee engine and it works great. But the instructions are 30ml for every 10 gallons. That's quite a bit more than a tablespoon. They sell a syringe for the TCP that is marked in ml; that's what I use.

[mike.smith]

It's a shame about the quality of those nickasils currently on the market. Most are cheap chinese knock-offs.
I'm not sure where Hummel sources their NiComs though but I'm hoping for the best. They look quite different from the AeroVee and GP ones.

I think if the cylinder walls were thicker then they would have worked fine.

[mike.smith]

Is there any value to the Nickasils other than the weight difference?

It is claimed they cool better.

I found that the Nickasil cylinders were VERY good at cooling. When I changed to the cast iron cylinders my temps were higher, but after break-in and a few minor modifications even the cast irons have run quite cool even in the dead of summer. (My cooling modifications are on my Kitlog site under the "Engine" heading.)

[rizzz]

OK, just got back from the airport, despite trying the many suggestions, in the end the heads had to come off as nothing seemed to improve the situation.

Here's what they look like, not pretty.
Right:

Left:


The cylinders look good though but the piston faces have the same burnt look.

Took them to my VW guy, his reaction:
Holy $#&!!" these have seen some heat, how lean are you running this thing???!!!

My response to that was, what do you mean? I always thought I was running too rich?