More advice required

Q , I am assuming that the castings are gunmetal , any soldering is best done with silver solder ( even that the temperature in the cylinder will not be enough to melt the soft solder ) , there is here a thread on pickling with different options , I cant find it but someone here will . O rings are fine , I have used them actually on Stirling and they are OK , you can also use PTFE yarn of square section , I recently bought some here in OZ , I am sure is available there too .

If you are going to use silver solder you can ignore acid pickle cleaning. Just ensure that the parts are free of oil and grease. The temperatures involved with silver soldering will remove any grease from handling. The important point is that the joint is clean at 600 deg not room temperature - that is what the flux is for. For post-braze cleaning, if you wish to stay clear of any pickling solution, use a loved ones tooth-brush and warm running water!

If you are going to use soft solder, wash thoroughly to remove any grease, and clean with wire wool. DO NOT USE EMERY CLOTH.

For more info go to www.cupalloys.co.uk

Keith

No, you don't need it. It just makes the cleaning process easier.

There is no need to have a fear of acids, just take care when handling the stronger ones. Acids are everywhere; lemon juice - citric acid, vinegar - acetic acid, rainwater contains carbonic acid, your stomach contains fairly strong hydrochloric acid.

Personally I use 5% w.w. sulphuric acid solution in water. I always (well mostly) wear rubber gloves and am careful to avoid splashing but yes, I have occasionally splashed it onto my skin however, at this strength, if it's washed of reasonably quickly it won't burn. Goggles are also advised, especially if you don't wear spectacles. Another precaution; I have a locking cover for the tank for when the grandchildren visit.

Russell.

Hi Q
A normal bored finish when you look in detail is like a screw thread with peaks and troughs. The size and shape of the thread depends on the tool profile and feed rate. However good it looks it will still have the characteristic of a screw thread. The O rings get shaved by the peaks, if they are very small you may get away with just replacing the first set after a relatively short time. The rings seem to manage to shave off the peaks, whether it is because the material of the o ring is slightly abrasive or it gets small bits of material embedded and acts like a lap I don’t know. Honing or lapping is a good way to remove the peaks. One firm I worked for used to “ball size” bores. They were bored very slightly under size, and then a carbide ball was pressed through the bore. This removed the peaks by plastic deformation, and very smooth surface finishes can be achieved. When you use piston rings the surface finish needs to be slightly rough (a relative term) so it holds oil on the surface of the bore. Graphite and PTFE yarn seem to work with most finishes.
Hope this helps
Regards
Paul