Technical question to the guru's.

[Bennie Vorster]

Just a quick technical question to the guru's on this sight. I am not an motor mach nor an engineer but due to the nature of my hobby I was forced into a jack of all trades and need some urgent info.

I am flying a Bush baby micro light aircraft that is fitted with an air cooled VW Beatle or kombi motor. It is a little bit modified. It is fitted with Gotech a fuel injection system. Have a better crank shaft fitted, bigger pistons, bigger valves exc.

Supposed to be a 2.3 motor now and with the Gotech system should be putting out between 120HP up to 180 HP. The sub assembly is imported and have an Z85 oil filter fitted to it. Now to speed it up a bit. After flying it for 41 hours, the crank broke and I had to get the motor redone. Since then every time we check the valve clearance the gaps tighten and the valves does not close properly. First time we checked after 10 hours, then after one hour and then again after three hours. Every time it was necessary to reset the valve gaps as it was to tight. The only thing I could think of was that the valves was eating into the seats so I removed the heads but could not find a problem with the valves or seats. The cam followers is the manual type but had been modified to move inside a brass bush. I did found how ever that one of the cam followers is much more tight than the rest and wondered if it might be busy to size.

Can any one please tell me why the gaps keeps on getting tighter all the time.

This is a picture of my Baby.

[fig]

This often means that the valves are stretching and it's just a matter of time before one of them breaks, which means catastrophic engine failure; which could spoil your day in a car; never mind an aircraft.

It's probably a good idea to rebuild the heads if you can't identify any other source of this problem. Or even better, buy a new Rotax engine.

[Bennie Vorster]

This often means that the valves are stretching and it's just a matter of time before one of them breaks, which means catastrophic engine failure; which could spoil your day in a car; never mind an aircraft.

It's probably a good idea to rebuild the heads if you can't identify any other source of this problem. Or even better, buy a new Rotax engine.

Eesh I thought I'll find some loyal VW fans on this sight now you also giving me the Rotex speech.

On a more serous note, what will cause a valve to stretch I actually ordered some high performance double valve springs to try and solve the problem. The valves is stainless valves. Now you are making me worried. Had the crank broke on me whiles flying and it wasn?t any fun so I would not like to go threw it again.

[fig]

Eesh I thought I'll find some loyal VW fans on this sight now you also giving me the Rotex speech.

LOL. Well if it was my own ass I wouldn't want to trust something cobbled together out of 40-year-old VW parts. If I blow an engine in my bus I just pull over. Dropping a valve on take-off may not be so easy to deal with.

On a more serous note, what will cause a valve to stretch:?

Old age and fatigue. It's usually just one valve. I've only experienced all the valves stretching after a major heat seizure.

[Bennie Vorster]

Old age and fatigue. It's usually just one valve. I've only experienced all the valves stretching after a major heat seizure.[/quote]


Engin hours = 63 hours since build.

ETG = 680 C to 720 C temp.
CHT = 260 deg C max. temp

And this hapens on all the valves, just one bank needs to be set more than the other bank's tappets.
Do you sugjest I replace the valves again.

[fig]

It doesn't sound like your valves have been subjected to serious abuse, but if you can't find another cause for the tightening valves then valve replacement is definitely indicated.

You can't really be too paranoid about safety while flying.

Maybe one of the real engine gurus will be able to give you some other ideas.

[Pine]

I'm no guru myself, but (except for the large fan up front), what other provision is made for the air flow and cooling of the heads, sleeves, oil cooler etc. Are those 'forget-me-not' plates still relevant in an aircraft, for instance? From the pic of the plane it seems like the heads are pretty much covered, or are there ducts on the back side to let warm air out?

[calooker]

Broken crank??? Let us have details on that one. I bet you it was a cast crank, forged cromolly is what you have in there at the moment, I hope.
As for valve train problems I think you need to look at your cam followers/brass bush, there have been many sorry stories about after market sets, and I would use a set of reground original German ones over any others that are selling out there. Valve gaps always need to be set with the motor ice cold or else it will affect settings as a hot motor expands slightly, what gaps are you working with?
By adding dual springs you are not going to resolve a valve train problem of this nature, only compound it, they are intended for use in high revving motors 8000 rpm +, planes don't do that kind of revving single hi rev springs would be good for 6000 rpm dual springs are only creating more stress in a low revving motor and are not needed, it is unlikely that the stainless steel valves would have stretched with such little run time, it could be that the followers bush have moved or are of poor quality, was the block new? If so why the bushes, were they correctly drilled for lubrication? I don't think the cam is a problem in this case but could show signs of damage due to the followers not turning in the bushes, as if and when they start to go flat the valve gaps tend to increase not decrease, but if the cam was not run in correctly it would have lead to cam lobe follower face damage which can prevent the follower from rotating in their bores and causing heaps of problems seen this before and setting the valve gaps was an endless story, I think your problem lies with the cam followers/bushes. One other thing you could look at an depending on what you have used are the push rods, are they chromolly or ally, chromolly has a diff. expansion rate than the ally stock ones so initial setting is critical as the settings are different.
Have a look at your oil it could tell you a few things remove the sump plate look for metallic residue, visually or with a magnet if you pick up any shiny metal particles you are looking at a rebuild, I would open up that motor and start again.
Please explain those temp readings you have, heat is one of the biggest enemy on a air-cooled motor, cooling is king on a big motor, give us more specific specs on the motor.
Btw brass bushes should have been phosphor bronze (sp?) for durability, maybe someone with more metals knowledge can explain that one.
If valves start to strech they normally show up on the #3 cylinder first, seen more valve/head/piston damage on that one than any of the other #'s.

[Chris]

Phospher bronze or ally bronze as I know it, is more durable and more resistan against wear and heat.It is a bit harder than brass and and can handle a lot more stress because brass is more likely to wear very quick.Brass also accumelates brass,i.o.w.,the brass that comes off during friction will will accumelate in one place and make a "paste" that will cause more wear that will make more paste...and so on.The best way to test if bushes are brass or ally bronze is to use a magnet.Brass will not take to magnet,but ally bronze does,very lightly.If that motor has brass guides in,change them as soon as possible.

[IMPI]

I would check those heads for movement of the valve seat then check the valve stems lengths as there is definetely movement taking place
Worn lobes wrong lifters etc will increase valve clearance as wear removes metal. the valve working against the seat will move to the outside of the engine (figuratively speaking) and decrease valve clearance thus leading to your problem.
I know that there is some builders who install hydraulic lifters in aircraft engines could this be the case? if so then your method of adjustment may be wrong. bushing of lifter bores is sometimes done to fit type one lifters into type four cases but usually brass is not used.
did you take photos of the inside of the engine?
is it type one or type four?
The valve need only stretch 0.16mm to close the gap so it will be extremely difficult to know what the condition is.

[Tony Z]

I have been thinking about this for a short while.... the absolute only answer that I can come up with is that when your engine was rebuilt, the guy who rebuilt it tossed away the pushrods and installed the stock aluminum pushrods. Pull them out and check. Wish stronger valve springs, these will bend. Also if the "rocker geometry" is not set correctly, the pushrods will push the rocker at a slight angle. This coupled with bent rods may lead to a setup that changes length as the pushrods rotate.

While writing that, I also had an idea. Maybe there is nothing wrong, except (sorry to say it) for your untrained hands.
If everything is correct, and you are adjusting your valve clearances while the engine is warm, you are on a road to no-where. The engine has to be completely cold when you adjust your valves. Engine should stand for a minimum of 2 hours before adjusting the valves, preferable overnight. Another thing that may be compounding this, if you have cro-moly pushrods, these are normally set to zero clearance with the engine cold. You might be setting this to the factory speck, then when the engine is warm and the gap has increased like it will with cro-mo pushrods (the reaon for a zero gap when cold). Then you may run it, let it cool off a bit longer than the time when you reset the gap and find that your gap has become less, maybe even lifting the valves.

Also, check your head torque.
When the engine was rebuilt, was it built with copper washers betw the head and barrel??? This could also add to the problem.

I personally think you are just resetting the clearances when the engine is too warm.

Let us know what you find or if you have any more info....

[Bennie Vorster]

Old age and fatigue. It's usually just one valve. I've only experienced all the valves stretching after a major heat seizure.

Engin hours = 63 hours since build.

ETG (Exhaust temp Gage)= 680 C to 720 C temp.
CHT = (Cilinder head temp Gage)260 deg C max. temp

And this hapens on all the valves, just one bank needs to be set more than the other bank's tappets.
Do you sugjest I replace the valves again. [/quote]

ETG (Exhaust temp Gage)= 680 C to 720 C temp.
CHT = (Cilinder head temp Gage)260 deg C max. temp

I do indeed set the clerance with the motor when it is hot, maybe that is my problem. The gaps is set for 0.02 mm

The motor is supose to cruise at 3400 rpm to 3600 rpm and at take off it runs between 3900 rpm to 4200 rpm.

I will find out more aboute the build of the motor and what changes have been made to it. All I know is that it is not a standard crank that is fitted, but I will defenatly have the local guys to have a look at all your ideas, thank's so far.

[man_of_steel]

When the fan stops, the pilot starts sweating,

[Bennie Vorster]

Please I need your input on this Idea. I have a theorie for the cause of my problems and the solutions. I do not know if the solution is the answer though.

Please tell me what you think. We have cut open a normal as the man on the street know it, an old time mechanical cam follower and found a little surprise inside it.

[Bennie Vorster]

Nou hoe gemak nou manne?