hi all,
My search for a suitable m21 piston has ended,
I have found a heplex +60 piston which will do fine,
My question is ,what piston clearance will I need ,taking into account the fuel that's available today,
And will the same clearance be ok when I run the bike with a side car attached,
Thanks in advance..Seamus.

email (option): Seamus,alone15@gmail.com

Is it solid skirt or split skirt?...Ian

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

Hi Ian,
It's got a split skirt.

email (option): Seamusmalone15@gmail.com

In that case the clearance is pretty tight (measured at the thrust faces fore and aft...)

.0006"-.0012" from memory, but I'll confirm that for you tomorrow as the information is in the workshop...

Personally I would recommend removing the sidecar at least for the early part of running in as it exerts considerable extra load on the engine at a time when it's best avoided...

At the very least go for the top tolerance and perhaps an extra .0005" or so with the chair fitted (not necessary if you remove it)and be absolutely certain the ignition timing is spot on and the fuel/air mixture is correctly set..In fact running slightly rich will help to keep things cool in the early stages...

If you feel you HAVE to run with the chair, go out early in the morning initially when the air temperature is lower......Ian

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

Thanks Ian,
I dont intend to fit the sidecar until I have the bike well run in,so i'll be taking her easy for a while,
(The bike I mean)
Best regards..seamus malone

email (option): Seamusmalone15@gmail.com

My BSA info. doesn't contain clearance data for a split skirt M21 piston..

However, the figures for a split skirt (85mm) B33 piston are .0006"-.00275"...

Larger pistons generally run a little more clearance and in the case of non split skirt B33 and M21 pistons that difference is .0005" for the bigger B33 piston...

As the M21 runs hotter than the B33 I would conclude that .0025"-.0035" would be about the right figure for a split skirt M21 piston (run in without the sidecar..)..

It goes without saying that the engine should be treated very carefully for the first couple of hundred miles until you are sure all is well and then run in 'by the book' for the remainder of the running in mileage...Ian

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

Thanks a lot Ian, I'm off to the rebore Man today ,and will pass on all this information to him,

email (option): Seamusmalone15@gmail.com

Just bore it to the std size plus the +0.060" you don't need to know the clearance, the piston is sized to suit, if you measure them you will see that some manufacturers pistons will be smaller or larger for a given size dependant upon what material they have made them from. You can add the extra 0.0005" as you are pulling the chair and the engine is working harder. I used to bore barrels to the clearance and then when it nipped up then you realise that they are actually differently sized to compensate for material differences

email (option): davmax@ntlworld.com

Thank you Dave for your reply,
The barrell isn't bored yet ,so I'l pass this information on to the rebore shop,
Best regards...seamus..

email (option): Seamusmalone15@gmail.com

Hi Dave ,
I am expecting parts from Hussein soon and he has asked that I post onto you piston rings which are in with my spares.
I am in north warwickshire if you are close enough , ?I am just thinking of them getting lost / damaged in the post.

Vaughan Miles
07809025565

email (option): vaughanmiles@btinternet.com