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'84 CX650E that is evolving into a GL500
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I have in front of me the March 2022 issue of Motorcycle Mojo Magazine, open to pages 60 & 61 where Costa Mouzouris describes a (new to him) method of softening old, hard rubber parts. His online research found forum threads about using 3 parts rubbing alcohol and 1 part Oil of Wintergreen so he tried it on the airbox manifolds of an '86 GSX-R750 he was working on. The article includes pics of the manifold with a 4kg steel bar on it, barely deflecting the old, hard part before and pretty much folding it after soaking in the mixture for 2 days.

I am on my way out the door so I don't have time right now to research this (I'd really like to know why wintergreen is used instead of some other oil) but a quick search found the video below (that I will watch later) that looks like it investigates several alternatives
 

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I have used that exact formula to soften the intake rubbers, although it does very well to soften the boots there is to me a significant amount of swelling of the rubber. Once the part is allowed to air dry it will return to near original size. A side benefit is if you soak Honda intake manifolds, valve covers, other coated aluminum parts, the finish will peal off quite easily and the parts are ready for final polishing.
 

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CX500EC Eurosport 1982
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I used this too a couple of years ago on my Euro intake rubbers; think I might have ended up using 1:1 ratio mix but also the only wintergreen I could get was already diluted to some degree. Anyway, it worked, the rubber was supple but still old the the cracks were there. I have since added a couple of layers of shrink wrap over the whole intake run, the rubber and metal parts. I am pretty sure this has fixed the intake leak I had.
 

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'84 CX650E that is evolving into a GL500
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I finally got a chance to look at the video. Seems like he did his best to test what he tested but I'd have liked to see tests with alcohol and other oils that don't cost as much.

I haven't seen alcohol & wintergreen mentioned on the forum before. I've boiled parts in water and wintergreen in the past and they stayed soft for at least a few days until I installed them but in the case of the 650 intake manifolds they were hard again when I had them off a couple of years later.
I've heard good things about wintergreen and xylene but I haven't tried it myself.

Speaking of those 650 manifolds, the last time I had them off before I replaced the engine, carbs &c with ones from a GL500 I rubbed silicone grease into them (inside & out) and wiped off the excess before installing them the next day in the hope that it would help prevent them from hardening more and I was surprised to find them much softer when I took them off a couple of years later when replacing the engine.
Note that this is strictly anecdotal and there may have been more at play than the silicone like wintergreen residue from previous attempts and who knows what else.

If I remember when it is warmer out I'll try to remember to find a hardened rubber part I can cut up and experiment with alcohol and some other oils.
 

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If I remember when it is warmer out I'll try to remember to find a hardened rubber part I can cut up and experiment with alcohol and some other oils.
...and try a repeat of the silicone grease...
 

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1978 CX500 "The Grub", 1983 GL650I "Nimbus"
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You might try testing the effect of warming the part before and/or after applying the silicone grease, too
 
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