650 ss piston weights

Hi, Alex.

When I had my 1966 650SS re-sleeved a few years ago the old pistons (+ 60 thou), pins and circlips weighed 309 grams each in total, give or take the odd fraction of a gram. These obviously weren't the original pistons so I can't quote what their weight would have been. I replaced them initially with standard bore JP pistons which turned out to be much too heavy. Their combined individual weight was some 381 grams and even after I'd had them machined internally to reduce their weight they still came in at around 40 grams each lower (sorry, I didn't keep an exact note). These were still too heavy with too much vibration so I then changed yet again for some standard bore GP pistons (from RGM) and their combined individual weight was 285 grams. Vibration with these is much more bearable and their weight is much closer to the over size ones with which I'd had the crank balanced many years ago. And as a bonus the GP pistons had no problem being run in quickly whilst the JP ones were taking ages with much smoke and plug oiling.

Be aware there can also be quite a weight difference in the pins themselves so I was able to mix and match to some extent to try and get the combined weight as good as possible without going to the extreme of getting the crank balanced again.

So, your pistons are similar in weight to my over sized ones unless you haven't included the pins and circlips in your figure. Since you obviously have the engine stripped it might be prudent to have the crank balanced with whichever pistons you intend to use. I never thought of this before the first rebuild and could have saved some aggravation and expense if I'd gone that route. Then at least you will know you've done what you can to ensure vibration is minimised and won't always be wondering. It's called learning from experience or being wise after the event!

Good luck.

I recently worked on our Atlas and  had a lot of trouble trying to match the commando piston weight to the original  low comp type. It appeared that a lot of excess weight was in the thick walled piston pins and bracing to the underside of the crowns.

You would have though that 70 years of progress with modern materials and machining processes might have resulted in lighter replacement pistons in the Dominator engines..........but for standard replacement pistons forget it!!! GPM and JP pistons have much thicker walls and Gudgeon pins and consequently are much heavier than original Hepolite and BMB pistons. 

Original pistons were 280 to 300 g in weight and the crankshafts balanced to this. You will often find that replacement pistons can be from 350 to 500g giving you the choice of rebalancing the crankshaft or putting up with the vibration.  

The AN offering GPM piston weighs 285g complete with rings, pin and circlips, just like the originals it seems. Feel free to come and weigh them yourselves. 

If you like to change your pin, you will most probably get away with in a 650, but a pin is designed to work with its two mating components, the piston - this determines length, whether it will be straight or tapered, and the circlip - round wire or eyelet type. 

And yes, the modern technology will give a lighter piston, the forged standard 850 Omega's we sell are lighter than the standard GPM and yes, original pistons. 

If you change the piston weights do you not have to re-balance the crankshaft?

Yep, probably, hence the question! 

Alot of interesting info from posters here. Alex,s 650SS pistons seem very heavy, not just alongside a modern AN GPM 650SS piston but also next to the following flat top 850 pistons I,ve weighed tonight which may be of interest:

Standard used 77mm bore original AE Hepolite piston c/w pin, rings and circlips   (828cc)  392.4g

New +1.0mm o/s 78mm bore JE forged piston c/w pin, rings and wirelock clips     (850cc)  353.0g

New +1.5mm o/s 78.5mm bore JE forged piston c/w pin, rings and wirelock clips  (861cc)  364.8g

PS. Phil, are you going to sell that 961 and get back to your old Commando and 650? You know it,s the right thing to do...

There are certain rules which must be stuck to when rebuilding any Norton twin engine.

1)  Always use good quality spares if any engine parts need replacing.

2)  Either do the work yourself if you are confident enough or get the help of somebody who knows these motors inside out.

3)  If the pistons need to be changed, then make sure they are close in weight to the originals. ie. 280 to 300 g or get the crankshaft re-balanced to the weight of the replacement pistons. 

Hi Neill............in answer to your question. My first 961 headed back to the Factory for major repairs after just 10 months of use and never re-appeared. Four months later, a replacement turned up at my door in the form of a Euro 4 compliant 961. Minus the ABS. This bike broke down 3 days later and ended up back at the factory for a week. On its return the bike and battery had to be permanently plugged into a trickle-charger over-night otherwise it would not start the next morning. An irritation which caught me out many times. Meanwhile, my Honda NC750 never lets me down at just 1/3rd the cost of a new 961.

Hello, the reason I have not waded into this Thread  by now Is I have a full book of Information on Norton piston types and rings, But since you members have decided  to ignore  my inputs on these threads I feel I am just wasting my time  with you all  ,  now find out the hard way as I have,  with No help from the NOC   Yours  Anna J Dixon       

Anna,  Some of us DO value the information you post. However your style and attitude does not always go down well. Long term members have learnt to  take  the usefull stuff and ignore the rest, Be happy with that and keep posting  facts and usefull info.