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Post  Ken Cook on Sat Jun 07 2014, 13:36

I've been using the Norvel .061's due to the fact that they're not as loud as the ball bearing variants. I also have been getting some serious performance out of them. In fact, I've been running faster than some of the Fora's I've been competing against. The cases have been honed and bit more clearance has been placed in the journal. Piston skirts have been modified and lots of nitro. They've been working great but the longevity is incredibly shortened. I can't keep rods in these things and they just explode. My next step is to have some rods made out of better material. This is somewhat of a crazy idea for an inexpensive engine but I haven't had success with any other engine and this allows me to fly at our stunt field. I never tached one of these but I will tell you, they're hauling the mail. The best prop yet I settled on is the Cox black trimmed down to 3.5". To my surprise, I almost won 1/2A combat last weekend and I was the only plain bearing engine in use all others were Cyclon's and Profi's. I even used the heavy duty older rod which hand grenaded. This is the third rod in the past few months. Amazing little buggers. Ken
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Post  RknRusty on Sat Jun 07 2014, 13:47

Wow, it did let go. Maybe because the crank eventually starts to wobble in the honed journal a bit too much for the rod to tolerate. Do you reuse the crankcase?
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Post  Ken Cook on Sat Jun 07 2014, 19:51

I installed another rod and put it back together. I staked the piston over a V -block on top of my vise with a small ball peen and a 2 mm drill bit. This raised a bit of a burr on the piston. I was able to use some Mother's aluminum polish and lap the piston into a old cylinder to remove the raised burr. It worked well and I followed with a very soapy solution and old toothbrush cleanup prior to assembly. I had it out today and it was honkin away. I blew two plugs however. The new rod is raising the piston slightly higher in the bore than the old rod. I have to double the cylinder base gasket and add an additional head shim. Ken
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Post  RknRusty on Sat Jun 07 2014, 19:58

I might have to pick up an AME one of these days. I've only ever had the Big Migs, which I love.
I have a brand new MP Jet Factory Race Special BB .061, and plan to put it on a 1/2A Gladiator combat wing Mike Londke gave me. That ought to be a handful.

I never did inspect the crankshaft for embedded aluminum in that Big Mig I had that was bogging. Since I had another already outfitted with a 128 thread NV, I just swapped and put the other one in the drawer. i'm still curious about it though. Some time I'll open it up.
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Post  Surfer_kris on Sun Jun 08 2014, 03:55

Ooops, that didn't look good. What prop was that with, 3.5x4? It would be interesting to know what rpm's your are getting, the rod might give up above 30krpm?

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Post  Ken Cook on Sun Jun 08 2014, 05:30

Kris, the prop is a 3 pitch. Without a doubt, I'm running the engine above it's rated capacity. I have 2 others that are like this which have been bullet proof. I had a mid air flying this engine which essentially was a head on. I've broken a few cases when this has happened in the past but this time the prop took the hit. Usually, it becomes a prop change and carry on. This time however it jarred the wrist pin staking and when the plane landed the wrist pin now loose and floating, slid into the port at the bottom of the cylinder. I was able to get the engine apart and restake the pin, but it did it again a few flights later.

I almost wonder why Norvel didn't make the wrist pin slide in from the rear and be held in using a circlip. This problem is apparent with the Big Mig due to the porting holes in the cylinder not lining up with the wrist pin like they do with the AME.

What I'm discovering is that between the rods I've been acquiring I'm seeing a few thou differences between them. The piston is lightly hitting the compressed shims under the glow plug. Either that or possibly due to the compression of the base gasket from repeated removals and tightening, the piston is now rising slightly higher than the glow plug landing. The head is showing signs of what looks to be glass beaded (Generally over compression) and the glow plug shims look like they're chewed up on the inside. I've taken out a few plugs probably due to the element being struck by micro particles from the head shims. I will try some new gaskets and different head shims. Ken

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Post  roddie on Sun Jun 08 2014, 11:14

Ken, I am by no means an engine-guy... but I thought of two things somewhat related that you might consider. If your crank journal (like Rusty mentioned) has excessive clearance.. wouldn't that lengthen the stroke? Also.. have you checked the crank-pin for wear? A combination of these two factors I would think, have an ill effect on the con-rod.. as well as increasing the stroke's design- length. You could check the crank-pin's size/concentricity with a vernier caliper against another cranks' for comparison. You may find it to be undersize and/or egg-shaped. Two Cents
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Post  ian1954 on Sun Jun 08 2014, 17:06

This is a tricky one to diagnose as there are a few issues that come into play.

It first sight the rod on the left looks like one of the small diesels I have flooded followed by an exhuberant flick start.

The one on the right gives the appearance of stressed metal breaking on the down stroke.

I have always found it difficult to chose a material for conrods. There are pros and cons to different types of aluminium.

It is easy to think of aluminium as a "soft" metal but all depends on the alloy. Too soft (arguable what is soft) and the rubbing action around the pins will caused "galling" and distort the holes in the conrod. Usually this calls for bushing but that is a delicate process for an 061. It doesent usually result in breakage though, just poor running, as compression is effectively reduced.

The alloy 7075 T6 will have best durability without galling under a high load and is very strong, stronger than some mild steels, but it isn't very ductile. It's rather stiff and brittle when compared to lower aluminum alloys. 7075 can work harden, fatigue and fracture after millions of extension and compression cycles.

I believe Fox MFG uses 2024 T4 alloy.

I always used something called Dural. The main alloying constituents are copper, manganese, and magnesium. A commonly used modern equivalent of this alloy type is AA2024, which contains 4.4% copper, 1.5% magnesium, 0.6% manganese and 93.5% aluminium by weight. Typical yield strength is 450 MPa (65 ksi), with variations depending on the composition and temper. Another difficult to get item but easy to cast.

I have recently been using 6060-T6. Easy to machine and using it to replace missing conrods or worn, or broken in my rstorations). Then again - non of these are highly tuned or stressed.

However, I have noticed in some of my tuning experiments that catastrophy can occur when the engine gets hot. It starts and runs fine - then five minutes later - bang! I can stop and start the thing for ever but an extended run - bang!

I made the con rod three thou smaller with no difference in performance but a ****** to start. Needed quite a rich start and a while to warm up but then I ran this particular engine for twenty minutes. Does the con rod expand under heat?

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Post  Ken Cook on Sun Jun 08 2014, 18:42

I know I certainly need to back off of the nitro. I would say that this is probably and more than likely responsible for this happening. I've been able to get away with using it for a bit due to the low pitch prop and small diameter. This by no means is a complaint I'm well aware I'm running the engine beyond it's desired rpm characteristics. The only rods I have left are the round style rods. These apparently are the newer versions that Norvel offered. I myself liked the rectangular rods as they certainly appeared to be beefier. Unfortunately, that broke as well and at a faster rate over the round looking one. Ken
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Post  Davenz13 on Sun Jun 08 2014, 22:11

Hi Ken
That's quite a beefy looking piston. Once you have the rod issues sorted have you thought about making the piston a bit lighter. It looks like there is enough meat on it to peel some out from inside the skirt. At the revs your pulling any reduction in weight has got to make life easier for the rod. Dave
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