I have to buy a new piston, but my cylinder is 82.55mm and looks well, which size piston do I have to take? ---82 .020--- or bigger?
What must be the tolerance between the piston and the cylinder?
Richard

email (option): bsa-m20@libero.it

Hi, the tolerance should be 0.0035" to 0.005" but with cheaper (Italian/Indian) pistons head for the higher figure but with a Hepolite or genuine BSA the figure can be to the lower end or the measurement

email (option): davmax@ntlworld.com

Hi Richard,

I think I have an original BSA +20 piston I don't need myself. Can't take a look as I am still on Crete. Quite easy to get on the Internet here. I am now at the harbour of Ierapetra and have a very good Internet connection here and it's for free

Henk

email (option): ahum@quicknet.nl

Hello Henk,
I need a m21 piston with .020". I don't think you have that one.
When are you coming back?
Maybe better stay there. Bad wheather is comming!
And Dave, thanks for your information
Richard

email (option): bsa-m20@libero.it

I do also have an original BSA M21 piston and I think it's a +20. From one size I have two and I only want to keep one spare for myself. Just a few more days on Crete. Monday we'll fly home again. The weather is still very nice here, we go for swim in the sea every day.

Henk.

email (option): ahum@quicknet.nl

Now I have a related question.. what's the difference between an M20 amd M21 piston? Are they interchangeable? Same cylinder, conrod and bore..is the position of the gudgeon pin different? I think the CR if slightly different but this shouldn't make any significant difference should it?
Cheers
Pete

email (option): petercomley@web.de

Hi Pete...The difference is the gudgeon pin center to crown hieght. Half of the difference in stroke length between the M20 and M21 (9mm) is accomodated by altering the piston dimensions to maintain the overall hieght of the engine, without having to alter either the conrod length or the cylinder height. The difference between the compression ratios of the two engines is largely down to the difference in swept volume between 500 and 600cc...and as you say, is pretty irrelevant. On modern fuels both engines will run up to a maximum 7-1 compression ratio before hitting serious pre ignition problems..Ian

email (option): ian@wright52.plus.com

And there wuz me hoping to just swop the crank and stick with the current M20 bore and piston. Foiled by the "compression height"!
Which prompts me to ask, why is it called the compression height?? What has it to do with the compression, or am I missing something once again? Presumably there are factors which determine the best combination of stroke, rod length and compression height

Cheers
Pete

email (option): petercomley@web.de

Hello Henk,
When you where on Crete,you said that you have a piston for sale?
I send the cylinder away to do a retification and the size is now 82.6mm and I think it's the wright size for a piston 82 .020.
Please send me an email with the price you want for your orginal piston.
Richard

email (option): bsa-m20@libero.it

watched a program about drug smuggling some pistons made in india had the first 30mm filled with heroin they had to drill the top of the piston to get to it i was wonder how fast my bike might go with on of these fitted zooommm!!

Hi Richard,

I have NOS +20 BSA piston and rings but no gudgeon pin. I'll send you some photos by email. The piston is an original BSA and original Hepolite.

I also have the same size Original NOS BSA M21 piston, also with rings and no gudgeon pin.

BSA actually used mm for their pistons and the piston is actually 1/2 mm oversize but that is just about +.20

Henk.

email (option): ahum@quicknet.nl