I'm stripping a BSA 1962 B40 (SS90), I've removed the cylinder head to reveal the Piston. It appears to be a Standard diameter (79mm) I've checked the bore with an internal micrometer and it shows no real wear but it is scored, I may consider a 1st rebore (+0.020"). But I expected to see a High Compression (8.75:1) Piston and I've got a Flat Top piston. I'm a bit confused, I've seen a new piston (+20 Thou) and shows a higher piston crown, so I'm assuming that's a HC piston, it's £189 so I don't want to buy it if it's not right. I live in Salisbury (Wiltshire) does anyone know of a trustworthy engineering firm that could rebore out to 79mm (+0.020") 79.50mm
Hi Martin
welcome, I would like to reccomend one place in Bristol but he is very busy and overwhelmed with work , he has had my cases and crank for a year now.
not used these but might have to https://www.saunders-motorworks.co.uk/engine-services/wiltshire
This is a very odd one especially as this is a standard sized piston. Normally, the piston would only be changed when it is damaged or worn out but this would usually also involve the barrel being re-bored which does not appear to be the case here.
I have used two good engineering firms in the vicinity of Oxford, one of which had, pre-pandemic, a relatively quick turnaround (1-2 weeks). Obviously, they would need to have both the barrel and piston to achieve the correct clearance.
Try cox and turner 01935826816 at tintinhull off the A303.
They can do the rebore and supply quality pistons and rings.
Top quality engineering firm.
Hi, I have a spare +40 new piston for B40. Guess it is around 8:1 compression.
You might not want to go too high compression with what fuel is available these days.
I'm in a similar dilema, I need ideally a + 0.030" piston for my SS90 or +0.040"
Looking at limited availability the price of £181.60 seems rather excessive but for the 8.75 to 1 compression that's about it
But my existing piston also looks to be very similar to yours, likely a lower compression - see picture
Is a £100 standard compression B40 piston going to spoil the job - bearing in mind I bought this bike as a non runner and have no idea of performance, so nothing to compare with.