Norvel AME .061 problems

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.
Rusty

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?

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. 

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?

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