Valve Pockes Piston Orientation

I fitted 650SS pistons in my 1960 type engine (the 650 have wider splay on the exhausts) and never thought about it,No problems.so the variance must be in the ports only.Just don't buy 650 pipes for your Dommy unless you intend to run the exhausts thro the oil tank!. 

The cut outs are diametrically opposed to each other.  They are left and right because, if the gudgeon pins are in a line, one has cutouts at 2 o'clock and 8 o'clock, and the other has them at 10 o'clock and 4 o'clock.  Next more important question: is there a front and back?  As far as I recall they are marked - if not, the lettering is the right way up when viewed from the rear of the bike.  I'm pretty certain the splay angle did not change.  Pistons from a 99 can be used in a 650, as long as the skirts don't hit the flywheel.  They need filing back to avoid the fatter flywheel.  But the pockets are in the same place, even though the 99 usually had the narrow exhaust tract spacing and the 650 had the wider.

The position of the relative valve seats and piston cutaways is the same for all the Model 7, 88, 77, 99 and 650 engines. The difference in the 650 heads lies in the route the exhaust port follows from the valve head to the outside. 

Consequently, as Robert has noted, this allows 650 pistons to be used in any of the 600cc engines. Plus for anyone really desperate standard size 650 pistons can also be used in an over-bored +80 thou set of 500cc barrels. Not a lot of people know that!!!

In the past pistons used to be marked with 'Ex' or 'In' to help orientate them in the bores. Some had the word 'front' marked instead. A few of the specialist pistons had a slightly larger inlet cutaway valve pockets to help with positioning. Whatever pistons are fitted the crankshaft may need to be re-balanced.

Robert and Dave.......can I expect some good news regarding DSN Chapter 31?

Hi Phil ,I have been working on it ,In and out of hospital and negligable IT skill may trip me up tho!.