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I have a bike with a really old shift cable and housing. There is significant friction between the housing and cable.

I will replace the cable, but hesitating to replace the housing because I will have to make many cuts to new housing and do not have a housing cutter tool or a dremel. If I can reuse the old housing effectively, that would prevent me from making bad cuts.

Instead, I'd like to use WD-40 to flush any dirt/gunk that may be in the housing now.

My question is: After I do this, will the residue from WD-40 gunk up inside the cable? If so, what alternatives can I use to flush the housing?

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    Nope. Dunno if the WD40 will do any good, but it should do no harm (so long as you keep it off of tires and brakes). Jun 9 '20 at 22:56
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    Worth a try - if it does gunk up cables, brake cleaner will sort them out. Better to replace housings. If you buy cable and housing from LBS and take the existing lengths into them, they will probably cut to length for nothing. IMHO, its worth buying cutters - they are not that expensive and you don't need workshop quality for the occasional use at home.
    – mattnz
    Jun 9 '20 at 23:27
  • Cuts aren't that hard - you need basic hand tools and patience.
    – Criggie
    Jun 10 '20 at 0:16
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    @mattnz IME a Dremel with a cut-off wheel is much better than a cable cutter for cutting cable housings. Just be sure to wear eye protection, and have something like a 1/16" drill bit handy to clean out the center opening. Cheap cutters especially are prone to crushing the housing more than cutting it. Jun 10 '20 at 1:20
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    If you use a cable cutter, the trick is to follow up with an ice pick or awl, swirled around in the opening, to bend open the end of the coiled wire. Jun 10 '20 at 12:43
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I will have to make many cuts to new housing and do not have a housing cutter tool or a dremel. If I can reuse the old housing effectively, that would prevent me from making bad cuts.

Before I was able to afford/invest in a more suitable (i.e. less crushing) cutting tool I used to make do with common side-snip pliers; then doing my best to reform any squashed portion of the coiled housing so as to not obstruct the opening at the the end.

On top of this you also try to keep the face of the housing end as square as possible to the cable so it sits as neatly and unobtrusively against the barrel adjuster or cable stop at either end; minimising bend or creating a friction point.

Cut as short as lengths off as possible of course and use old waste lengths to do some practice cuts, knowing what you want to achieve.

It's doable; just a little bit more fiddly and irksome. I've been there.

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    Buying the proper tool is always the best. But, good quality, sharp, side cutters work alright. Crappy, blunt, side-cutters make it near impossible to do it correctly.
    – BramMooij
    Jun 13 '20 at 6:58
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It will probably be at least somewhat effective at breaking up and flushing out anything in there. The potential disadvantage is once the solvent component dries, the oil residue WD-40 leaves is thick enough that it itself can cause future contaminants to stick around. How big of a deal that is depends on how dirty your riding conditions are.

A solvent without an oil component (or at least a non-negligible one) wouldn't have that problem, i.e. one of the citrus ones.

While not universal, a lot of shops will be nice and help a DIY'er out by not charging any extra labor if you take them the old pieces of housing and ask to buy new bulk pieces cut to that length. This is especially true of compressionless housing, which doesn't really take any time to cut to size.

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    The best option is to flush with oxalic acid to dissolve the rust, then lubricate with a Teflon lube. Jun 10 '20 at 0:28
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    New housing and new cables are the best solution, especially 'with a really old shift cable and housing.'
    – Carel
    Jun 10 '20 at 8:28
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    Gasoline followed by denatured alcohol or isopropyl alcohol are common for cleaning bike chains. You could apply the same technique to clean the cable housing. I like the advice of going to your local shop with the old housing and buying new housing cut to match the old.
    – mhck
    Jun 10 '20 at 11:36
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New housing is the best way to go. It's not particularly expensive, so adds little cost to the project. The new ferrules and fresh housing, which comes pre-greased, yield such benefit (and peace of mind) to the job. It's the right way to do that particular maintenance. The cutting and prepping the ends of the housing can be successfully accomplished in so many ways--the most common and best ways are outlined in others' answers here--with common household tools. It's difficult to accept any rationale for making do with old housing over new cabling.

If you must proceed on that route, inspect the old housing for damage such as bends or cracked plastic. This is especially common at the areas around the ferrules. If such damage is noted, it should be repaired by trimming off the damaged section and replacing the ferrule. We're back to square one now, eh? The ferrules themselves can be a common source of excess friction as the ends where the internal cabling comes through become distorted and develop sharp edges as the hole becomes reamed out by the internal cable. Any burrs should filed off and make sure the hole is such that it doesn't interfere with the passing of the new internal cable. Again, replacement of the ferrules is best practice, yielding exceptional benefit for very little cost.

External cabling that is to be reused is best flushed with a Teflon (PTFE) containing spray such as TriFlow or specialist sprays by Liquid Wrench or specialist WD-40 (not the regular stuff which is more solvent than lube and dries out quickly actually leaving less lubrication than before). The TriFlow or specialist sprays above come with a little application straw which can be held on the end of the housing (removing the ferrule helps get more spray through the housings lumen than if left on. Flush using short bursts and continue until the spray dripping out the other end is not discolored by dirt and rust particulates. Note any areas of excess friction when you pass the new internal cable through the housing and evaluate if it needs replacing. There should be no resistance as the internal cable slides through the housing. It should slide smoothly through the length of housing. You may encounter a hitch when the end of cable gets to a ferrule, but once through, the housing length should slide smoothly back and forth on the cable. This is a good time to flush your shifters with the Teflon spray as well. I hit the derailleur's linkages too. Wipe off any excess spray after giving it several minutes to work in and it's carrier solvent to evaporate.

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