Is R stamped on the engine referring to rigid?

[squiggle]

Looking at the at the engine number, picture attached:
ZA7 is the year & model
S is Star twin as opposed to sprung
Does the R refer to rigid?
Is HC is high compression.

Ted

[JulianS]

No. The case is from a Star Twin, a premium product which was supplied with a plunger frame only.

I dont think there is a definitive answer to the "R".

[Swarfcut]

 Ted. That engine number puts it as a 1949 Star Twin. HC indicates high compression.

 Looking again at your intro picture, the bike has the later type of Longstroke timing cover, identified by 3 (rather than 2) bolts along the bottom edge.

 Without a crank, rods and pistons, and maybe a poor cam and followers, this engine will be an expensive project. Without paperwork a later type engine to match the frame model year will be easier and cheaper. Post '53 frames have a brazed on sidestand lug, early ones don't, so you can get a rough date visually. Toolbox is from a rigid frame, but often fitted to your plunger frame as this later frame was introduced, running alongside the rigid version. All frames are marked A7,  followed by S if the frame was a sprung (plunger) type.

Swarfy.

[squiggle]

From what I can gather from the frame number ZA7S 24935 this was manufactured in 1951 and would have had an engine number starting AA7S. Like you say I was planning to find the later engine for the frame so I don't hit obstacles trying to piece the 2 variants together, been there before. I will still source parts for the 1949 engine and see where it ends up. Hopefully Stamford Hall, the Banbury run and Kempton will provide some bits to move it all forward.

Talking of crank shafts are the stamped with the part number.

[Swarfcut]

      Cranks come with strange stamped numbers, more like factory batch identity rather than crank type or part number.

   A7 and A10 small journal cranks are identical in style, the only difference being the stroke.  The earlier Longstroke crank has a narrower flywheel and  distinct almost added on bobweights in the flywheel periphery.  On my crank the factory balance was by a deep groove machined into the centre of the flywheel rim, rather than the numerous drillings more usual on later designs.  This is the only crank that will be suitable for the Longstroke cases, as bore and stroke are different from the later A7.  Conrods for Longstroke are steel, so crank and rods at a jumble are fairly distinctive but somewhat rare. Plenty of images on t'internet.

 Swarfy.