Uncas

My Feedback: (1)

CP,

I have my new gaskets and the engine is torn down again.

The crank does not spin freely as you say it should. I have the prop on it, piston is removed and depending on how I spin it, it will turn 1/2 to 1 turn then stops. When I spin it with my finger, I feel some resistance - it is not free spinning as I think you say it should. It will not spin free for a few seconds. My guess is that now I should work the crank... What do you (or Larry) recommend? What grit sandpaper? Does it matter how I sand it?

Also, the piston and cylinder look fine. I think you were thinking I might have a major defect in he piston or cylinder wall?

Larry Driskill

[quote]ORIGINAL: Uncas

If you have a lathe or drill press put the shaft in the chuck and spin it at high speed. While it is spinning use a 1/4" wide piece of 400 or 600 wet/dry sand paper with light oil on it to work the center portion of the crank. Leave the last 3/16" of each end of the shaft surface untouched. You will not remove much material from the shaft doing this, but if you keep it oiled it will also be unlikely you will remove too much. Use two hands to handle the paper and pretend to be a shinning a shoe. I needed the second hand to run the Kodak.

Then mix some Bon Ami cleaner with light oil to make a thin paste. Put the paste in the case and on the shaft and spin the shaft with an electric drill for 30 seconds. Use a liberal amount of oil and move the shaft back and forth a bit while you are spinning it. Clean the shaft and case really well and check the fit. You may have to try two or three 30 second sessions to get it free enough. During this maneuver it is fairly easy to go too far. So don't work the process very long between cleaning and checking. On the final cleaning be very through. Use lots of soap and water and a small bottle brush in the case, if you have one. Work the shaft over with soap and an old (or borrowed) tooth brush.

All this is sometimes good for 1000 RPM.

combatpigg

My Feedback: (3)

Uncas, About the piston / cylinder, all I was saying is that if these parts don't look good, then don't waste any time on the engine. With a prime in the exhaust port, the cylinder should stay pumped up.

As far as working the crank goes, if you don't do a immaculate job of clean up with at least brake cleaner as you do your trial fits......bad news.

Figure out the best way to chuck the crank up in a drill press, lathe, etc. and use fine emery cloth to wear done the crank journal...except don't touch the extreme ends of the journal.
Leave the outer 1/8-3/16" of each end untouched. Just take the sheen off and then trial fit. If you have 1000 grit, you can put a quick finished surface back on the part you wore down, even briefly wipe the extreme ends with a rubbing compound. None of this is an exact science unless this is all you do every day for a living, so don't be in a hurry. Good luck!

combatpigg

My Feedback: (3)

OK, now I'm going to tell you about the so-called wrong way to do it, but it works for me. Mix up a slurry of dry tooth paste and oil and coat the crank with it. Put the crank into the case and spin it for a few seconds with a drill motor. Break it down, clean it up and do the flick test. Make sure you get all the gunk out. Once you are happy with the fit, break the engine completely down and sonic clean it in solvent for a lengthy period. You can tape a jar full of solvent to an orbital sander or use a denture cleaner.

Uncas

My Feedback: (1)

LOL,

I am not sure about the crank but I may try that to better whiten my teeth.

I have used 400 grit sandpaper and stuck the crank in my battery powered drill that has a very flat base and had at it. I cleaned up the shaft and did a turn test. I did this a couple times until I got a noticable improvement. I probably could do better but I do not want to overdo it. The engine is back together and tomorrow I will test it out. The new backplate gasket felt more plyable when I tightened it - new gaskets all the way around.

combatpigg

My Feedback: (3)

I hope it pays off. Friction plays a bigger role in lost power with these small engines than a lot of people realize. Even with big engines it is a key factor.
Pulling up just shy of the perfect fit is wise, you'll never know where perfect really is until you go past it. Let the tach be the judge next time you run it.

hkbii

Howd Gents. I have never seen a 074 so I dont know how the sleve fits in the case . I got a fox .15 once that acted the same way. after I tore it down I found that the liner was 180 deg off. the ports did not line up. I turend it around and it ran just fine. HKBII.

combatpigg

My Feedback: (3)

It is possible to reverse the liner on a Norvel, but you would really have to be a fried potato not to notice, because Norvel cylinder / liner is a one piece unit..[same idea as COX].....not a drop in sleeve.

Larry Driskill

Fried potato . . . is that a regional term from the Great North West, or a Piggism?

combatpigg

My Feedback: (3)

Larry, I think it was one of the nicer sayings that ever came out of my drill sergeants' mouth.

AndyW

My Feedback: (1)

Hear, hear, in spades. Particularly about the Cox props. IMO, the APCs, while allowing lots of revs, are less efficient.

A BIG fan of the .074, I've had a number sent to me by folks who were having troubles. Some had galling on the crankshaft. Some had bent rods. Some had wobbly carbs.

What shot Norvel in the foot is that early, 1/2A, .06 and .074 engines were set up extremely tight. As such, with lots of oil, they'd still turn over. As long as you snapped them over smartly. Hesitate, (even with lots of oil) and they'd lock up. The anodized cylinder has pores and these hold oil. This is good and provides an excellent piston seal. However, too tight and the pores lock onto the nickle plated piston with a vengeance. On top of that, any prime over two or three drops and the raw fuel washes the oil out and then you REALLY get a lock up. The solution, when breaking in and even at the field, with a lock up, is to apply heat to the cylinder. The cylinder gets the heat, the piston is cooler, the former part expands more than the latter and now you can flip that puppy, no problem. This really should ALWAYS be done when running a tight Norvel on the bench or in the field.

The preceding issues would cause galling of the crank and bending of rods as the uninitiated would grind away with their starter.

Now, as with the remaining Norvels on the market, the LiteMachines versions, they have given you a very mild fit to assure that you won't ever lock one up. I preferred a BIT of tightness that you ran in but this, better than that.

I can't imagine why, with Norvel's precision machining, you would ever have to work the crank to fit better. Aluminum expands more than steel. If that crank slides in at all, it's got clearance. When that crankcase warms up to ANY degree, it's going to get even more clearance. It may run faster with treatment but no way will that explain a 13K engine.

If you have binding, then you may have got a prop drive washer that was too thin. This happened a lot on one series of .06s that I had the #@%&$* pleasure of troubleshooting. With just the crank in the case it would spin freely. With a prop mounted, it would bind up. With excess chamfer at the inner, bottom edge of the prop hub and a too thin washer, that washer would distort when the prop nut was tightened. If it locked up completely, this would alert you to look for a problem. If it would bind just a bit that you didn't notice easily, then yes, you'd have friction and loss of power. In particular as the engine warmed up and the aluminum case grew longer than the crank did. The solution is to get a thicker washer or double up on the thin ones. One I worked on needed three washers.

I'd love to look at your engine, my curiosity is killing me but for now, for some 4 to 5 weeks I'm out of town, far from the shop.

Having said all that, your mentioning of the backplate reminded me that in a very few, the crank pin would grind on the backplate. That may explain the binding you're finding at one spot.

And if my tone is a little firm here, some serious painkillers are making me more obnoxious than usual. (if that's possible). [:@]

combatpigg

My Feedback: (3)

Andy, please let me know where I can buy a couple dozen of the vastly superior 7x3.5 Cox props.

Uncas

My Feedback: (1)

Testing complete for the day. Still no improvement.

I picked up the 7x3 at the LHS. RPM hit 15,500 - 15,720 all out. Thrust is still the same around 18 - 19 oz. I will have to double check though my zero on the scale was off a little. Pitch really affects these little engines! I also installed a second prop washer. The engine is noticably more responsive with the 7x3.

combatpigg

My Feedback: (3)

I don't want to over state the obvious, but you're running a good glow head, with a good seal...maybe 1 or 2 shims and at least 15% nitro with castor in the mix?
Also, the throttle opening is wide open?
Some small amount of end play with a plain bearing engine is needed, when the engine is running, the prop pulls the crank away from the back plate.
I'll bet the completely assembled bottom end [backplate and all] still has a catch in it.
All this guess work is based on your say so that the engine passes a leak down test. Don't be confused by a piston that is stuck at TDC versus the boyancy of a piston pushing against a compressed volume of air. The piston should feel "springy" near TDC for a few seconds or more.

AndyW

My Feedback: (1)

You make a good point, CP. I have just a few in grey and ONE in black, should last a lifetime. Just a point we were trying to make and I guess we've beat it to death. Just wish the folks who make props would take a look to see what we're talking about.

Uncas,

This is getting curiouser and curiouser. I have my MiniSport with me with the glow .074 installed along with flight gear. If you're willing, I could have a look at it, install it and give it a try. The answer is there, somehwere. That engine ought to be one of the best ever in power to weight and also throttle performance. Let me know.

Uncas

My Feedback: (1)

CP,

I am running Norvell 35% fuel.

I thought I didn't have a spare glow plug but it appears I bought a spare one earlier! Dumb! I will change it out. The existing one looks very good. (If this is it I may have to make up some bizzare problem just to save face)

I am using one shim in the glow head. Would 2 improve things?

Uhhm, I will have to repeat this leak down test - you are correct, it gets tight up at the top and it was kinda hard to tell. Maybe I need to compare with my .061.

Andy,
If this problem beats me I will send it to you... Thanks for the offer.

SST

My Feedback: (2)

Hey CP...I have 12 each gray & black! Sold off a couple dozen surplus grays recently, averaged about $8.00 each! It's not polite to drool...

combatpigg

My Feedback: (3)

SST, That would last me a couple of years. I flew a lot of .074 combat a few years ago with 36 inch span, 18 oz planes, towing 24 foot streamers and compared APC and COX props on numerous occasions. I never saw any glaring differences, both planes had the same amount of speed and vertical. That kind of flying, as well as racing between equally skilled flyers is the most apples to apples way to judge equipment that I know of. There's also a couple of guys [who chime in here from time to time] who fly a fair amount of .074 3D whos' opinions about the prop of choice would be interesting to find out.

AndyW

My Feedback: (1)

CP,

Your right in that this little .074 is a bit of an oddball, with few props specifically made for it. A 6 X 3, too small, and a 7 X 4 about right but how many brands? When I did direct comparisons of 8 X 4s on a dieselized version of the .074, I had some six brands, APC, Cox, Grish, Graupner and even Taipan. If you ever saw a Taipan, you know that they succeeded in taking the blade's true pitch just about all the way to the hub. So all props were stock and exactly the same. No half sizes as in 7 X 3 1/2.

In any case, in THIS application, the Cox prop was superior to the others in front of a tach and in the air. And surprisingly, the APC was near the bottom. An 8 X 4 on that engine runs the prop far slower than it was designed for. APCs obviously are designed to spin fast. And in this case, they ran below their optimum efficiency.

What distinguishes Cox props from most others is that they carry the flat bottom all the way to the hub. I modified the APC and a Grish by grinding the bottom of the blades flat all the way to the hub and both delivered a free, extra 500 RPM. Not a lot to notice in flight on a sport airplane but maybe some advantage in go fast competition?

But here's a thought. Those skinny little tips on the APC, surely they aren't producing a lot of thrust, are they? Despite the high speed of the tips? Is a 7 X 4 really a 6.5 X 4 with an extended tip intended to minimize turbulence and drag? Has anyone tried rounding off the tips to see what happens?

Uncas,

One other thing occurred to me last night. As we know, the wrist pin on a Norvel .074 is held in place by staking the piston at four points on each side. Sometimes the pin drifts through and scores the liner. Have a look, this may be your problem.

Uncas

My Feedback: (1)

Took the plane to the field tonite for testing and brought another to fly. Replaced the glow plug without much of a difference in performance (I am kinda relieved that was not the problem). So I am now getting 15,800 rpm tops with a 7 x 3, but I would not run it that lean in flight. I noticed when I was test running, when I optimized the rpm and leaned it way out that fuel would start coming out of the glow plug, when I richened the mixture then it would stop, I presume from high temperature.

Since I was at the field and the engine was runing fairly well I thought I would give it a test flight. I tuned it down a little to run around 15,450 rpm max. This flight was way better than my first flight with the apc 6x2. No doubt the engine is tuned better, since I know precisely where to set the rpms, but maybe I have gained a little ground with my efforts and perhaps the 7x3 works a little harder. Regardles of what my $3.98 pull scale says the thrust seems a bit more than 18 - 19 oz. My initial flight with the .074 I could just barely loop the plane, reminded of my first R/C plane, a GWS Tiger Moth. Now, I can hold it in a hover and I have some pull out. It loops effortlessly. The plane empty weighs 16 oz.

combatpigg

My Feedback: (3)

Uncas, nice to hear that you are having fun now. It sounds like the compression seal at the head is flawed, though. This is where the dunk test takes the guess work out of troubleshooting.
Running too lean is no reason for there to be a head leak, unless you've melted out the glow heads' insulator!

AndyW

My Feedback: (1)

Geez, a leaking glow plug. No wonder. Or is that two in a row.

combatpigg

My Feedback: (3)

Just a speck of grit, or a deformed head shim is enough to cause a problem. Lack of tightness, [depending on your definition of tightness] can be a cause. After an over haul, or whenever the engine seems "off", double check the snugness of everything. Don't go for gorilla tight, just secured.

SST

My Feedback: (2)

Look for a burr or an imbedded piece of grit on the head where the glowplug button seats. This will destroy the seal between the glow plug and the head and drop compression. Dirt is the enemy!

Uncas

My Feedback: (1)

No no, this is the first time I have seen a leak and it occured right after I installed the new glo plug. I took off the head and cleaned things, and put it back together but it leaked again, only not as much. I could not get the little copper seal out to check the surface. Those little seals are really hard to get out!

Talking with my friend at work who is a mechanical engineer, he thinks that I should buff the shaft with 2000 grit sandpaper - which he will give me some. He asked me if I noticed how shiny it was before I sanded it and said that a roughed up surface will not allow proper oil flow especially at the high rpms. He also felt my method of sanding the shaft was very crude with the hand drill and that it is now probably not truly round, he felt CP's method of using the lathe was crude also but way better than the hand drill and that I should have used my drill press. And he chastised me for not bringing it in to work and letting a machinist work on it. He also said not to be so cheap and just buy a new crankshaft. I then kicked him in the knee and grabbed his throat . . . . ok - just kidding.

So, I will polish the shaft but I may buy a new too. I will look at the wrist pin and stop the fuel leaking out of the glow plug. With a little luck I will even get some flights in before the weather turns windy and cold. The plane btw, is a Mountain Models Mini-Flash (electric converted to glo) and is a good flyer - the best of my small planes.