Stuart Beam Piston

[Chris Taylor 3Participant @christaylor3]

Stuart Beam PistonA bit of advice please. I have just completed a very lengthy build of a Stuart Beam and I am doing the final assembly, I am a bit stumped by the piston and the supplied rubber O ring. Both the piston and the cylinder are machined to the drawing dimensions and the piston is a good close sliding fit, but I can see no way that the piston, with the ring fitted, will move, even if I can induce it into the cylinder and I assume that the entire Beam mechanism should move easily and smoothly when assembled. (I have purchased a set of piston rings from Stuart Models, but I need to make a ring compressor before I can fit them). Any suggestions?

[JasonBModerator @jasonb]

I have used O rings on quite a few engines and not had problems getting the pistion to move. I don’t have Stuart drawings for an O ring piston so what depth of groove do they show and what is the actual measured cross section of the supplied ring. Really you only want to compress an O ring by about 5%. Max

For example the 24mm bore engines that I make have 2.4mm section metric O rings and the grooves are 2.3mm deep so very little compression of the ring. You also want to lap the cylinder bore as straight off the machine is likely to shred the ring quite quickly.

Lilewise with iron ring sI have never found the need to use or make a ring compressor, they can generally be eased in by hand or if narrow a poke with something thin but not metal. A small chamfer on one end of the cylinder helps and a honed bore.

Or if you just want to run slowly for display and you have  agood fitting piston then just don’t bother with any form or rings. I quit eoften leve them out of my engines although the groove is there for an O ring if I wanted to fit one.

[noel shelleyParticipant @noelshelley55608]

A bit of flat tin plate or thin steel sheet, a little longer than 3.14 of the diameter. Go round the lower edge and in 3 places bend out with a pair of pliers. Roll into a tube and place a jubilee clip round just above your 3 bends. Place piston and ring in the tube so made and GENTLY do up the jubilee clip enough to compress the ring. Place over the cylinder bore and lightly tap the piston down into the bore – adjust the clip to suit. The 3 outward bends will stop the sheet steel from going down the bore beside the piston. This device will work for piston to 6″, I’ve done it.  Noel.

[JasonBModerator @jasonb]

Bit OTT for a 1″ ring that you can easily compress with finger and thumb

Also make sure you gap the rings before trying to fit them, only needs a 1 or 2 thou gap not the larger gap you might us eon Noel’s bigger rings.

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