Connecting rod

[triumph406]

2 articles on making model airplane engine con-rods (not V8/leafblower/motorcycle rods)

http://www.go-cl.se/dye-rods.html

http://home.wxs.nl/~wakke007/fmv/fmv_1.htm

(read the whole article if you want to learn something about model airplane engines at the extreme end of their performance capabilities)

http://home.wxs.nl/~wakke007/fmv/fmv_4.htm

The page that has the con-rod information.

dc

[triumph406]

Originally Posted by sleeper142 The current set of rods I have are a 7000 series aluminum made from extruded bar stock, in which the grain of the metal is in the direction the rod is being stressed (lengthwise). I personally like this better than a forged rod, since the grain runs perpendicular to the direction of the forces in the rod. The down side however is that when machining the profile of a rod made of extruded bar stock, you will have exposed grain boundaries, creating possible stress risers. This can be avoided by shot peening and/or polishing, but nevertheless interesting food for thought. A forged rod will likely have less exposed grain boundary ends, fwiw. BME's are obviously working for the fuel cars, and my rods are obviously working for my alcohol car.

?? That's the whole point of forging a rod. Forging in a die forces the grain to travel in the shape of the rod. The die has a shape of the rod + machining allowance. If the material is the same, the forged rod will be better than a machined from solid rod, if used at it's limits

Shotpeening puts the surface of the part in compression, minimising or eliminating surface crack growth. it won't help with poor grain direction.

dc

[NC Cams]

The latest high strength alloy tricked up alloy is 7068. It has replaced 7075 for high strength aluminum alloy applications - it was a millitary secret but has since become commercially available.

WHen you select aluminum for an application, you need to consider the operating temperature. 2618 is a good piston alloy because it has superior HOT strength over other alloys (including 7075 and 7068 as I recall). On the other hand, the 70xx's have better room and moderately elevated temp properties as compared to 2618.

At room temp, the 2618 has lower tensile properties than 7075 or 7068. However as the temp gets higher and higher, the fall off in tensile strength is noticeably LESS with 2618 as compared to 70xx alloys - at high temps, the 70xx alloys are actually weaker. THus, when the pistons are in their high operating temp zone, they are actually stronger (less pliable) than their relatively cooler con rod cousins.

The morale here is to pick the ally based upon the properties it will demonstrate at the intended operating temperature and not to pick an alloy based solely upon a snapshot of the tensile properties at room temp - a situation it may not, in fact, ever see when the thing is actually in service.

[9lrac9]

I have early sets of four different makers of alum, rods and the shaving tests done on them produced the material number of 2219T8 it means the the material will stretch at high rpm and return to size. it's all about grain structure ie direction. toy stuff I don't think you could stress it to brake. enough horsepower you can brake anything. Look at NHRA pro/stock or pro/mod. Leave at 2G's your going to brake s--- every time. Your taking the fun out of the toys. Buy some 6AL4V and have fun forever...9lrac9

[nyleeloise]

Stutx bearcat engines of the late 20's used aluminum rods . I,m sure the quality of alum was not any where close to what we have today The Hornet model engine of thr 40's and 50's also used alum rod but I also have seen some of them break. Stutz nhad a problem maqking babbit stisk to alum ,most gys today use Packard steel rods . Nyle

[triumph406]

Originally Posted by 9lrac9 I have early sets of four different makers of alum, rods and the shaving tests done on them produced the material number of 2219T8 it means the the material will stretch at high rpm and return to size. it's all about grain structure ie direction. toy stuff I don't think you could stress it to brake. enough horsepower you can brake anything. Look at NHRA pro/stock or pro/mod. Leave at 2G's your going to brake s--- every time. Your taking the fun out of the toys. Buy some 6AL4V and have fun forever...9lrac9

Some 'toy' 2.5cc engines produce 1+hp at 30'000 rpm, 400+bhp/liter. Toy stuff indeed. Go to a model airplane meet (especially control line) and you'll be surprised just what 'toy' engines are capable of. They break just like real engines do.

[bluejets]

If you are running castor @ 25% , we found in pylon racing models that up to 6 thou clear on the big end worked. Stock engines with 2 to 3 thou blew every time.