During the first 2 1/2 years that I owned my Stafford I constantly had problems with clack valves that would not reseat correctly and promptly started emptying the boiler.  This problem was not quite unique to my Stafford as I did witness it happen on another couple of Stafford's on visiting engine days, but my Stafford seemed particularly prone to clack failure.  As a result I designed and built my own O Ring clack valves, and from that day on I never had any further problems.   So as I was already modifying the injector system (see other pages of this website) I decided to make a new pair of O Ring clacks and fit them to the Feldbahn while the injector pipework was being rebuilt. The photo on the left shows the parts of the O Ring clack valve and shows the shuttle "as machined" and when fitted with its O Ring.  All the parts other than the shuttle, which is Stainless Steel, are made from Phosphor Bronze and the O Ring is VITON to withstand the temperature of the boiling water (172 degrees Centigrade at 120 psi).   Both the top and bottom caps of the valve can easily be removed for cleaning, and as the O Ring seat is on the face of the bottom cap there is no difficult machining of blind holes / valve seats to do.  It takes me about 1 1/2 days in the workshop to make a pair of valves but their performance is superb and having used them for 18 months on my Stafford I am convinced of their superiority over normal ball valve clacks.  The only downside is that the O Rings need to be replaced after about 50 hours use as they tend to flatten and leak when there is no boiler pressure (e.g. before steam is raised). O Ring clack valves require the shuttle that carries the O Ring to be guided as it moves up and down (open / closed) so that the O Ring will seat around the hole in the inlet, unlike a ball bearing valve where the loose ball simply drops into the hole.  This can be accomplished by having a fluted stem on the shuttle which slides up and down in the inlet port bore but that requires the inlet bore to be increased beyond the feed pipe bore so that the area available for the water to flow is maintained.  This will obviously increase the dimensions of the clack valve.  To overcome this size increase I decided to put the shuttle stem on the top of the shuttle and guide it by using a tubular extension of the valves top cap.  The length and tapered end of the shuttle stem were made so that the stem could be engaged in the caps guide hole before the cap is screwed down to ease maintenance of the valve, and obviously the guide hole in the cap has an opening into the valve chamber at its upper end to avoid any possibility of the shuttle stem becoming hydraulically locked as it moves up into the guide hole.  Full details of the O Ring clack valve can be obtained by downloading a JPEG copy of the CAD drawings from the Stafford section of this website by clicking here (use a "right click" and "save as" to download the image to your computer).