Possible to shrink expanded O-rings


I have four tiny but very expensive O-rings used for dust scraping around pistons in motorcycle brake calipers. Thay have expanded due to an oil leakage and I hope that it's possible to shrink them again somehow.

I know very little of polymers, but the O-rings are brake fluid resistant and apparently not transmission oil resistant. Looking at this list:

The rings could be made of EPDM (Ethylene Propylene) but that is just an unqualified guess.

The rings feel like they haven't changed properties beside the expansion which makes me hope that it is possible to shrink them again. What do you, dear readers, think of this?

Thanks in advance.

Reply to
Jes Vestervang
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BTW, I tried soaking them in brake fluid for some days which made no difference. I wrote that they are tiny which in this case is approx

35 mm.
Reply to
Jes Vestervang

Since this is the brakes you are talking about, do you really want to take a chance that the rings are o.k.?

It looks like the oil has swollen the rings and has not exuded back out. You could try soaking the rings in a solvent in order to increase the mobility of the oil, but at the risk of overswelling the rings and having them rupture. Heptane, toluene, gasoline are all options. They are all flammable options as well, so keep it away from ignition sources, have good ventiliation,...

John Aspen Research, -

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"Turning Questions into Answers"

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Reply to

I have considered that too, but there are two rings per brake piston. The inner one that keeps the brake fluid inside haven't been damaged at all while the outer swollen ones are for keeping dust outside. Because of that I think that the risk of a sudden failure is absolutely minimal. I wouldn't recommend other to experiment with this, though, and I only do it because it's my own bike and I feel confident that it won't failure suddenly.

I left the rings in gasoline over the night, about 6½ hour, and after

10 hours of drying they have shrunken to their correct size. When I removed them from the gasoline they had swollen to around the double circumference but they seem perfectly free of ruptures now (even when stretched a little). It seems that your tip has worked perfectly, so thank you very much! :-)
Reply to
Jes Vestervang

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