The BSA A7-A10 Forum

Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: PatM20 on 16.02. 2023 17:28

Title: Pistons, which way round?
Post by: PatM20 on 16.02. 2023 17:28

Hi, dug out some +20 pistons for my 1961 swinging arm A10. I have new rings. Question is, as I didn't make any notes, I can't remember which on is for the left or right pot and which way round they go. They are flat crown low comp pistons. Telling left from right I will struggle with, but orientation? Any help great fully received. Thanks.

Title: Re: Pistons, which way round?
Post by: muskrat on 16.02. 2023 19:24

G'day Pat.
If the pistons have any stamping s on the crown they will be read as facing forward. Valve pockets are usually bigger on the inlet (but ours are the same).
As for left or right, use feeler gauges between skirt and bore and get them as close to equal as possible.
Cheers

Title: Re: Pistons, which way round?
Post by: Colsbeeza on 17.02. 2023 00:30

I remember in the old days pistons had the gudgeon pin approx. 1mm forward of piston centre (or was that rear of centre??) *dunno2*. I believe this was to ensure that the piston tops were leaning forward after ignition, thus no clunk as piston rolled over due to the change of direction of force from conrod (if that all makes sense). Made sense if you had a lot of piston slap. I forgot to measure whether my A10 pistons had offset gudgeons or not?? If so that might be useful to determine front or back if there is no markings.
Anyone know?
Col

Title: Re: Pistons, which way round?
Post by: muskrat on 17.02. 2023 08:41

G'day Col.
Pistons for "cooking" road models the pin was center. I had MC racing pistons where the pin was off set for the reason you said. I can't remember now which way they went either. I'll dig a set out on Monday. Going down to The Creek tomorrow to watch some old fellas on old bikes go round and round.
Cheers

Title: Re: Pistons, which way round?
Post by: Colsbeeza on 17.02. 2023 11:07

Hi Musky,
I dug out an old set of A7 pistons this arvo, and the gudgeon is central. Looks like no help then for PatM20.
Have a nice day tomorrow, watching "some old fellas on old bikes go round and round."
Col

Title: Re: Pistons, which way round?
Post by: PatM20 on 17.02. 2023 19:05

Yes, my gudgeon pins are central. I also found some scratched arrows that I must have made when dismantling. Now only down to which is left and which is right. 50:50 chance!

Title: Re: Pistons, which way round?
Post by: trevinoz on 17.02. 2023 21:39

Correct me if I am wrong but I think that solid skirt pistons are non-directional but split skirt are.
Obviously this doesn't apply to all makes, Norton for one.

Title: Re: Pistons, which way round?
Post by: Greybeard on 18.02. 2023 12:01

May not help but here are a couple of pics from when I last had my pistons off.

I did scratch identification marks inside the skirts when I had the engine apart in 1972.

Title: Re: Pistons, which way round?
Post by: PatM20 on 20.02. 2023 13:03

Hi Trevor, I think they are non-directional when new. Mine were in for some time, so wanted to put them back where they were. I'll chance it with new rings and a hone.