Whilst at the Netherlands SummerCamp my A50 started a bad smoking habit on the left hand cylinder. Having now removed the head and barrel the cause of the problem has been identified, leaking lefthand valve guides and a oil coming past the lefthand piston. The barrels are standard size and after checking with an engineer, +20 rebore size will work. So after some searching a pair of bare +20 pistons have been found - A50 engine parts have become difficult to find. The pistons I have found are genuine BSA, but the part number does not tie up with any piston books. The part number on both pistons is 68-423 and one is dated 1962 and the other 1964. You can see the numbers clearly in the picture. I have a 1963/64 service bulletin which shows a list a replacement pistons, but this number is not shown. Any suggestions?
Rob
I can find only 1 reference to piston 68 423, which shows in the September 1970 master price list.
68 423 is the part number for the bare piston, using the usual BSA convention the complete piston with rings and pin would be 68 424.
The master list shows for 68 424 use 68 438 which is a 9;1 piston but also shows 68 423 still listed.
I dont seek to explain it!
Here is the page see second column.
Thanks Julian for answering my question. I did wonder when I asked the question whether the part number was just for a bare piston. Seems odd though listing the piston without rings.
:) :) :)
I know that A50 pistons are difficult to obtain and I think my last set were hells expensive. By the time you have paid for new pistons and rebore, its almost as cheap to have the engine resleeved/lined to fit the pistons. Dependant of course on wear and condition of old pistons. I had a Sloper done that way last year. However in this case I also had the piston refurbished to std.
I truth I dont know whether or no this would work in similar situations. My thoughts are that if you can fit rings below the depth/groove level that should be OK. 8) 8) 8) 8)Depends on usage but I would think OK for 20K miles or so.
Mike 8) 8) 8)
Pistons and various quality rings can be ordered from MAP Cycle in the USA. Their pricing is very good and often cheaper than buying from the UK.
I did consider having the barrel resleeved back to standard as the pistons were useable, and then would have just needed new rings. However when I was offered a pair of unused genuine BSA +20 pistons and rings, for me it is better option. I did look at the price of A50 pistons on various online suppliers, in one case they wanted over £320 for +20 pistons. Hopefully I will get the barrels and cylinder head to the engineer next week.
:) :) :) :)
I reckon in that case you are going the correct way about it. I wanted 7.5 compression pistons---I think £320 plus postage plus Vat
Plus some funny looks from across the room.
Mike 8) 8) 8)