how to avoid cross-threading

[22camels]

I am looking for tips on how to avoid cross-threading, in particular when mounting the rear derailleur back onto the permanent hanger of a steel frame.

It's not that I've ever cross-threaded anything (as far as I remember), just that I understand the consequences of cross-threading that particular thread are a bit serious and I'd rather avoid it (there may well be some sort of fix but the damage to the frame would be permanent).

Basically I want to try unscrewing the rear mech and tying it to the chainstays for travelling by plane - viewtopic.php?f=16&t=100505 - I know there are other ways to protect the mech such as gear protectors but I'd like to try taking it off. However, having never unscrewed the mech before and put it back on, it makes me nervous.

So far, I have

- screw it on very slowly and stop if you feel resistance

- turn the thread back a bit until it clicks and then forwards

- I guess it helps to minimise chain tension by putting it in smallest front cog and rear sprocket, or even taking it off the front cog.

- put some grease or lube on the bolt - is this useful? which one? shouldn't it already have grease on?

What I really want to know, is, can the risk of cross-threading this bolt be eliminated completely by taking appropriate precautions or is there always a small risk? Put differently, do you have to be a bit of an idiot to cross-thread it?

I have also been told that the permanent hangers on the steel frame are somewhat harder to cross-thread than the replaceable hangers (which I guess are alloy), is this true?

Thanks!

[Heltor Chasca]

That's what I was taught. Go anti-clockwise until it 'drops' or clicks into place then gently tighten up. Assuming you can keep control of a herd of nearly 2 dozen camels I think you'll be fine.

[TonyR]

That's what I was taught. Go anti-clockwise until it 'drops' or clicks into place then gently tighten up. Assuming you can keep control of a herd of nearly 2 dozen camels I think you'll be fine.

+1 and that's true for starting all bolts and screws off. When gently tightening it have a good grip on the derailleur, try to hold it square to the hanger and allow it to gently move as you start the thread off to allow it to properly engage. It should go very easily and if it starts to get tight don't force it because its probably cross threaded. So back off and start again.

[Neilo]

You are doing the right things.
Practice and experience, there is no short cut. You get a feel for it. Don't force it.
Even after 25 years of proessional spannering I still sometimes cross thread though.
Keeping the threads clean helps, keeping them greased, and thus avoiding corrosion in the thread in the frame and on the bolt. Finer pitch threads cross more easily than coarse.
A small tube of copper grease from a motor factors or an Iron monger will last you years. I have had a 500g tune for over 25 years, and I have used it on my cars as well. I have another unused tube as well. I'm 45, so I'll have to leave it to someone in my will.

Neil

[bainbridge]

+1 for gently turning in reverse until you find the thread.

I wouldn't grease the thread for this particular bolt because you don't want it working loose once put back together. I'm unsure if the spindle bolts (or qr) cover this bolt once fitted, but I wouldn't grease it.

[gregoryoftours]

Yes all the stuff you've put really.
It can be a little hard to feel if the thread is taking at the beginning when putting a rear mech on because the mount bolt usually turns in the mech with some stiff friction masking somewhat the feedback from the thread.
What makes it most hard is chain tension, particularly as you have to screw the mech on with it swung right back so that the b tension screw clears the tab on the hanger. You do need to be careful that you have cleared the tab, as if not it can send the mech off and cross thread if the b tension screw is caught. You could even note the position of the b tension screw and back it right off out of the way. Also drop the chain off the chainrings. Takes a bit longer but if you have a quick link I'd split the chain and get it out of the way completely, makes the whole process a lot easier. You might as well practice before you go, but I don't think you'll have a problem as long as you don't go at it like a bull in a china shop.

[Brucey]

if you use a long series allen key, with the long end in the bolt, I think this greatly reduces the chances of cross-threading the bolt, because you can see very easily if the length of the allen key is lying parallel to the hub axle or not. Also by turning the bolt using the short end of the key, you cannot easily apply enough torque to damage anything, either.

BTW I'm assuming that you have modern rear mech, in which the bolt is retained by a circlip; with some older mechs the bolt can drop out of the mech in transit, because there is nothing holding the bolt in the mech.

I agree that doing what you are planning to do is a wise precaution; bent rear mechs are quite commonplace otherwise.

cheers

[reohn2]

if you use a long series allen key, with the long end in the bolt, I think this greatly reduces the chances of cross-threading the bolt, because you can see very easily if the length of the allen key is lying parallel to the hub axle or not. Also by turning the bolt using the short end of the key, you cannot easily apply enough torque to damage anything, either.

And

turning the thread back a bit until it clicks and then forwards

And lubricating the threads with coppergrease.

Whenever I/we've flown with the bike/tandem I always remove the rear mech and chain.The mech is than taped up between the chainstays with the cable still attached,the chain is put in a plastic bag and slipped into a short length of pipe lagging,and taped to one of the tubes or between the forks near the top.On assembly I've never yet needed to do any adjusting of the gears,everything worked perfectly .

I agree that doing what you are planning to do is a wise precaution; bent rear mechs are quite commonplace otherwise.

It's always seemed the sensible option to me,I try to leave as little to chance a possible.
Cover all the bases and there's far less likelihood of problems at the other end .

[22camels]

Thanks a lot! Loads of really good advice here.

[hamster]

One other point when fitting the rear mech is to have no tension in the chain - either do it with the wheel out, or drop the chain off the front rings. That way you can get it decently aligned as you make that crucial first turn.

[22camels]

I did a practice run and everything seems fine, it was easy with the wheel off and chain off the front ring, as there was no chain tension.

By the way, how important is it to tighten this bolt to the right torque value? The manual for my mech (Deore XT 771) says 8-10 Nm, but this seems a bit high? What is the risk of leaving it too loose?

[cycleruk]

By the way, how important is it to tighten this bolt to the right torque value? The manual for my mech (Deore XT 771) says 8-10 Nm, but this seems a bit high? What is the risk of leaving it too loose?

I don't have a torque wrench that will measure that low and I just do it by feel. Do it up reasonably tight and there won't be a problem.
It's highly unlikely to come loose and you will soon notice if it does. ( not indexing correctly.)
I bet that 99% of mechanics don't torque bolts up to the recommendations.

[[XAP]Bob]

One other point when fitting the rear mech is to have no tension in the chain - either do it with the wheel out, or drop the chain off the front rings. That way you can get it decently aligned as you make that crucial first turn.

With all the chain tension comments I was thinking the same - or just open the quick link, pop the chain in a bottle so it doesn't smash about on the frame and it's easy

[Vorpal]

I think the hardest part about putting a rear mech back on is making sure the little tension screw clears the dropout tab. If the tension screw gets caught on the dropout tab, it may cause the rear mech to go wonky & either cross-thread, or deform one of the parts.

[22camels]

Thanks for the tips all. I took off the mech twice (flying UK->Spain and back) and zip tied it to the frame. No trouble putting it back on at the other end each time (with the wheels off and no tension it was easy to make sure the b-tension screw stayed the right side of the tab). It seemed like an effective way to protect it. Perhaps cross threading is still a small risk though especially if I get complacent about it.