Battle of the Little Big End.

I'm struggling with the dimensions of the 'Suffolk' big end. My crankcase has an internal diameter of 1.250". I want the stroke to be 0.750". The crankshaft is double ended, so the big end has to be split.

With a crankpin of 0.250" diameter I have 0.250" between the crankpin and the crankcase to clear the diagonal across the big end.

I have worked out that I can just fit in a proper big end if I use 10BA screws use a 3/16" crankpin.

This will be a 'square' four stroke with 3/4" bore and stroke, glow ignition and a heavy flywheel (probably about 1/2lb). It won't be super fast, but obviously it will have more stresses than a 2-stroke.

I'm not planning to drive anything with this, I just want it to run!

So the questions are:

• Will a 3/16 crankpin be strong enough? (medium tensile steel)
• Will 10BA big end studs be OK? (I think Seal uses 10BA)
• Will I be pushing my luck to squeeze in a bronze bearing shell or should I add an extra 1/32" to the pin instead?
• What would be a good material for the con rod if I don't use a bronze bearing?

Thanks for any suggestions (except build something in a more sensible size...)

Neil

Neil

Engine design is something I have never tried to do although I have looked inside many full sized ones. On your question though, the size of the crankpin, big end, bolts and the other bits have to be a compromise, no, thats the wrong word, they have to be in harmony and optimised so that in theory every part is as strong as it can be without one part being the weak link.

The sort of things I would look to do to get the largest pin diameter is have the big end cap joint line at an angle to the main axis, that way the extra bulk of the bolts does not increase the width of the con rod as much. There will be an increase in overall length but the crankcase in your picture appears to have some extra clearance at the bottom. Stress on the bolts will much less so if you can arrange some positive cap location (stepped face?) then the bolts could be a smaller size.

If you do have a bush you could make it with a really thin wall (its going to be fully supported anyway, or you could make the con rod out of aluminium and run straight on that (used in lawnmower engines so it must work)

I am definitely no experts and the above are just my thoughts

Ian P

You should be OK with HE15 for the rod without a bearing, plenty of model 2-strokes built that way.

I think I would be tempted to use High Tensile cap head screws rather than studs and nuts in either M1.6 or M2. you should need less room for the head than you do to turn a nut.

If you scale the Nemett Bobcat down to a 5mm pin then that will take M2 and only be about 12mm wide though you could get it a bit smaller.

J

Edited By JasonB on 08/04/2013 07:51:59

Thanks for the ideas.

I'm wary of going to 75% because with a double ended crankshaft if something jams the output end, the flywheel will keep going and ...

Resistance to flexing of a bar varies as the cube of the diameter (I hope), assuming this will be more relevant than shear mechanical strength the cubes of the various sizes are:

3/16" : 5mm : 5.5mm : 6mm : 1/4"

108 : 125 : 166 : 216 : 250

As 7/32" is slightly bigger than 5.5mm and I have a 7/32" reamer, I think that the extra gain in strength would be worth it, if only for my peace of mind!

The suggestions of using M2 cap screws combined with solid alloy crank sound like they will do the trick.

I have a big aluminium alloy toothed belt pulley of a motorcycle, about 8" in diameter. I suspect this is HE15 of something similarly tough. If it takes a good finish I'll use a bit of that.

I know cast iron in a plain mild steel liner is recommended for diesel. What about glow engines? The Kinglet plan has no materials suggestions on it at all. I have plenty of 1" diameter meehanite (following a visit to College Engineering), is a cast iron line, iron piston and iron rings overkill?

Neil

My firefly is EN8 liner though EN1A would do with a CI piston, good for 8500rpm and no rings.

I'd be surprised if the pully is HE15, more like HE30 which is not as strong. M-Machine sell small section HE15 by the inch.

J

Don't know the grade, but a good high strength alloy can be got from bike pedal cranks, might make a good con rod. Ian S C

Michael,

Nice lateral thinking. I'll play around with configurations on the computer, but the capscrew suggestion might mean that I don't need to depart from a traditional split.

Ian,

Good suggestion - I've already been eyeing up a motorcycle fork brace (rescued from the tip) and an ugly mountain bike handlebar stem (since replaced twice by more elegant solutionss!)

Neil