Good morning all. Quick question -- I bent the shaft on an Evolution 46 -- The bend is forward of the prop and prop nut. Can I fly it or must I scrap it?


thanks

jOHn

John,


Don't fly with it, but don't scrap it either...

If you can undo the prop-nut, remove it and the prop, clamp the shaft so only the bent part of the shaft protrudes from the vice and use a soft-face mallet, tapping lightly to straighten the shaft.

Even if you are left with a very minor deviation, i.e. up to 0.1 mm/.0004" runout, it is OK.

If you cannot undo the prop-nut, you might have to replace this crankshaft.

It depends on how much it's vibrating. How do you know it's bent? If it's vibrating enough for you to know it's bent, I'd replace it. Don't try to straighten it, that is a hit or miss situation for an amateur.

Dr.1

The Evolution crankshafts are very cheap to replace. I bought one for a buddy's Evo .52 for something like $17. You may want to check your crankcase very closely for cracks. Those are under $20 to replace.

Thanks guys -- Dar, the spinner nut and the prop nut came off easily. I can clamp it in my lathe with a dial indicator and threaded sleeve. I am convinced I can straighten it to a couple of mils but do I really need to?

Blw Thanks I am not sure the engine is worth the expense

John,


I was referring to straightening the crankshaft without going into full disassembly, which may require new bearings too (although even those can be had for about $7 from Paul McIntosh, at [link=http://rc-bearings.com/]RC-Bearings[/link]).

Such runout (0.1 mm), even running at full-bore, will not cause any perceptible vibration.

If it is more than that, it might...

If you're certain the bend is only on the exposed threads and the prop tips track correctly then I'd leave it alone. Usually a crank bends in one (or both) of two places. The first place is where the threads begin and the second is the machined area in front of the front bearing where the prop driver sits. Either is easy to check without dismantling the engine (other than removing the prop driver) by first putting the machined area into the lathe chuck up against the front bearing. Hold the crankcase and turn the chuck by hand. Any bend will show up by a wobble at the back plate. The length of the engine magnifies any bend. If there's no wobble then lightly clamp the threaded part in the chuck and repeat.

I always straighten them either by hand pressure (you can put some bar stock through the inlet passage for extra leverage) or by using the cross slide to push it over. Be reasonably gentle at first until you get some idea of what force is needed to make an improvement.

Downunder -- You were exactly right in the kind of bends. I had both. I used your procedure for straightening them out. Now with the motor body clamped tne smooth portion of the shaft runs out 0.005" and the threaded portion 0.012". I haven't yet measured prop tip displacement variation but will do so before I run the engine. I guess in the final analysis vibration will be my best measurement.

I won't be able to test it for a while as I am going on vacation. I will report results as soon as test are complete.

Again, thank you all

jOHn