This is the second time I've stripped threads off of a 170 crankshaft. What happened in both cases is the inner nut gets bad threads and starts going round, taking the crankshaft threads with it. There follows great havoc in its wake - getting a busted nut off means holding the engine with big pliers while forcing the nut off. If you're lucky, only the crankshaft thread damage is left, if you're unlucky the spinner/backplate/prop washer and related parts are busted too.

What to do? I'm thinking the nuts are the problem, they are just plain lousy. I'm not sure of the brands but I've busted both tru-turn and original YS nuts. It's possible to get the crankshaft threads back into shape or avoid the problem area by using a thicker prop washer but which nut/lock nut combo do people use?

I'm thinking the best bet is the Tru-turn double jam nut - the one that extends into the prop. Then you have a much wider set of threads meaning it's less likely to come loose and less likely to bust threads. Problem is, this ##¤%&Q! nut extends about 5mm into the prop hub itself and is a whopping 13mm wide, meaning you have to ream away much of the prop hub. Is this safe? Which tool is best for such a job? Are there other _good_ nut/jam nut combos I don't know about?

Jon,

I have been using the T/T 8mm x 1.0mm double jam nut for over 15 years, since the 140 FZ days without any trouble. I used to use the T/T aluminum spinner, but have since switch to the plastic cones about 6 years ago.

Once in awhile, you will find one that runs slightly tight, just try it on a different crank & will eventually loosen up. In addition, be careful not to over tighten.

Adrian

Same as Adrian on my case, running the TT for more than 15, never had that issue, has the engine drive washer eaten the spinner backplate? that happened to me once and discarded the backplate.

Sorry, but it looks like you are overnighting the nuts, unless your prop hub is worn as well as the spinner and the nut is reaching the end of the threads.

This ideas are based on the information you provided, maybe pictures of all the parts might help to figure this out.

Regards

Perhaps I am overtightening it, if nobody has similar problems. How about the TT adapter, does anybody ream their props with a hole that size? I'm just worried I'll weaken the hub and throw a blade.

Yes how did you know [X(]

It's really worn (TT light backplate), do you know why that happens, is it because the engine throws the prop?

Jon,

Here is the adapter that I use/d when I was using YS: [link]http://www.truturn.com/cgi-bin/store/agora.cgi?exact_match=on&p_id=tt0814a&ppinc=adapt[/link] Ran this on my 170 last and that was actually taken off my 140. Everyone at my field also runs this on their 170's.

Hi Jon,

Yes, it did happened to me in flight but my engine threw the spinner cone only and the engine stopped, what happens is, the engine torque is so brutal that the spinner backplate start to spin over the drive washer and it eats the back plate, when this happens the prop gets loose (as the spinner backplate gets thinner)and the condition gets worse as we keep using it, I found the reason, I did not removed the prop after the flying day, so it was slowly damaging the engine drive washer and spinner backplate, I started to remove the prop and put it back on the flying day, so I'm 100% sure is tight and the parts condition is good and never had the problem again.

So you might be reaching the end of the threads on the crankshaft due to the worn assembly and that's probably why the threads are stripping as your engine is throwing the prop.

Is it the glow version?

Best regards

Attach a piece of sand paper - belt sander grade - between the drive washer & the backplate, with the rough side against the backplate, & that should solve the problem.

Adrian

Thanks for all the great advice!

JAS: My TT nut is different, I'll give the one you recommend a try (save me reaming all those props!)

Thanks also for the sand paper tip, I'll try that as well. I have both the glow and CDi in two planes. The CDi seems a lot smoother and starts very softly so I'm hoping the ignition system actually solves the prop-throwing issue.

Jon, What you are describing is usually a sign that you are running a little lean. The engine is kicking back occasionally in the air and trying to throw the prop / spinner. I would richen the pump a little or run a few clicks further open on the needle.

Joe Dunnaway

Jon, are you running a lightened back plate? I've noticed that when I run one that I run into a problem of not being able to tightened down like you need. I switched to a regular back plate and that solved my problem.

Greg Grigsby

Yes both have TT lightened backplates - that's interesting Greg, do you think that is part of the problem? What makes those backplates different in that respect?

Oh yea... I never run the lightened backplate on the YS... it's just too 'light' for the constant knocking for me. Now on the electric stuff....