As the topic '
GB is not OK' ran for over 20 pages I've decided to summarise the posts for anyone who needs to know about the problem I had.
I have a 1955 A10 Plunger, with the semi-unit engine, (gearbox bolted to crankcase). The engine developed a light rattle.
I discovered that the sump strainer had caught pieces of yellow metal, (See pic 1 Left), a Little End bush had broken up, (See pic 1 Right). I also discovered that the camshaft and followers were badly worn and needed replacing, (See pic 2).
I already had a new camshaft. It was a 67-356, (designed for the Star Twin, Road Rocket & Shooting Star). (See pic 3 left) Item 67-334 is correct for this machine but I could not find one at that time. I asked the experts on this forum if the alternative camshaft was suitable for my engine. The consensus was that the 356 camshaft had a slightly higher cam lift but was commonly fitted to the standard engine and the only possible problem was that the larger cams may touch the sides of the case in the cam trough, (See pic 3 right). This might mean a few minutes working with a Dremel or similar to ease the case.
On reassembly the engine would not turn over a full revolution!
Eventually, a suggestion was made by forum member Chaterlea25 that the problem might be valve collets coming up against valve guides. I found that the collets I had were wrong for the valves. The ones I had were item 65-1624, (See pic 4 left) and were designed for the Road Rocket & Super Rocket. They have an annular raised ring inside that engages with a rounded groove in the valve stem, (See pic 4 left). They also project too far below the valve collar. They were, in fact, causing the problem, (See Pic 5). The correct collets for the valves I have are item 29-410, (See pic 4 right). They are plain inside and grip the tapered waist of the valve stem. They barely project below the collar.
My bike had been running fine with the wrong valve & collet combination for six years but putting a higher lift camshaft in caused the bottom of the incorrect collets to come into contact with the valve guides.
I rushed off to my local BSA parts seller and bought a set of the correct collets and a set of new valve springs. When the top end was reassembled the engine spun over freely! Phew, what a relief.
