size of O-ring for classic spring-loaded desoldering pump

cant you measure the diameter of the piston?

I've got a green RS one stock number 544-516 and a smaller no-name one bought when I was about 12, that's bit lethargic, and a similar sized one to the RS that I borrowed from someone who it happens I never saw again ... Sorry Trevor!)

Mine (starting to perish too) is O/D 20mm, C/S 2mm

One irritating thing about the cheap kits of replacement O rings available from various suppliers is that they are mostly 1.6mm cross section, while rings that I want to replace are typically 2mm.

Once you confirm the size you can often buy a small number for not much cash from eBay.

It's not for the piston, it's for the nozzle (I could have been clearer on that point)

I did measure the diameter of the groove, that's where I got the est. 16mm from

Heh - yeah, somewhere I might have another one and a similar Trevor to apologise to...

Thanks. that roughly fits with my guesstimate.

Like the one that comes with a replacement nozzle?

Ah, cool, thanks! That looks like the one - I wasn't able to find that for some reason.

I'll check compatibility, I think I might have enough for an RS order in the near future. A new PTFE nozzle would be worth getting after all these years...

J^n

I've found sometimes fitting two thinner rings solves the problem, especially when one bit screws into another and the O ring forms the seal.

Funny, my cheap O-ring set is the other way round!