650ss GPM pistons long skirts?

Don't see why not as longs as you deburr and also weigh them to check they are both the same weight. But first you need to track down the reason for the scuffing on the left side, strange how on my Commando its the left side that needed new rings and how BSA A65's seize on that side too. Being furthest away from the oil pump is a potential issue if the oil pump is past its best.

As John says , Nortons do seem to to have a propensity for drive side seizure. A lower oil supply on that side is one likely factor. There is also the possibility of a difference in the ignition timing from side to side,although the timing is more likely to be correct on the drive side!. A check on skirt clearance would be a good idea ,I would not like to run with less than 41/2 thou on that side.New oil seals in the timing cover would be good insurance. Check the piston is able to float on the pin when warm and the pin is not tight in the smallend. Running in can take longer than expected with some oils.

Previously john_mundy wrote:

Hi, I installed +20 GPM pistons during a rebuild of my 650ss 6000 miles ago (the bike ran very well). When I installed them I found that the piston skirts were long enough to touch the flywheel on the bottom stroke. I therefore removed the excess carefully using an angle grinder with a sanding disk. I've now stripped the engine down down for other maintenance and see that the left piston has considerable scuffing in several places while the right one looks fine. Since I have a new set of +20 GPMs that I can install in the lightly honed cylinders, shall I repeat the sanding trick? In short, is it OK to remove 1-2 millimeters from the bottom of the skirts?

Hello Now, why not find the original make of piston and rings made by BHB they are availableif you look on the internet, and bore sizes for the 650 are +10 +20 and + 30 oversize do not go beyondplus 40 oversize, My December 1960 built Norton 650 Manxmanis still on standard BHB piston after 57 years and I have a set of plus 10 thou in store, the piston to barrel clearances are 0.003 thou of an inch as standard you can get Omega pistons which are much lighter than the Originals BHBs has fitted in Rolls Royceengines yours Anna J

Too many zero's Anna, that clearance will ruin it.

Previously robert_tuck wrote:

Too many zero's Anna, that clearance will ruin it.

Hello Robert

Why do you say Anna has too many zero's? Three thou' looks ok to me.

Regards, Ian.

Previously ian_hay wrote:

Previously robert_tuck wrote:

Too many zero's Anna, that clearance will ruin it.

Hello Robert

Why do you say Anna has too many zero's? Three thou' looks ok to me.

Regards, Ian.

Anna has been in and edited it without saying anything. It did say 0.0003" - which is a transition fit, not a clearance fit and the engine would seize in seconds.

Even three thou is a bit too fine for me. That old wizard John Hudson advised me to allow 41/2 thou. A bit noisy when cold,but goes well.

Previously Bob Matthews wrote:

Previously ian_hay wrote:

Previously robert_tuck wrote:

Too many zero's Anna, that clearance will ruin it.

Hello Robert

Why do you say Anna has too many zero's? Three thou' looks ok to me.

Regards, Ian.

Anna has been in and edited it without saying anything. It did say 0.0003" - which is a transition fit, not a clearance fit and the engine would seize in seconds.

Thank you Bob I was unaware of the edit, hence my post.

Regards, Ian.

What clearance did you have ?.

Previously ian_hay wrote:

Previously robert_tuck wrote:

Too many zero's Anna, that clearance will ruin it.

Hello Robert

Why do you say Anna has too many zero's? Three thou' looks ok to me.

Regards, Ian.

Hello YES, you know that 3 thou is the right piston to bore clearances as standard, 4.1/2 to 5 thou was for racing engines, anything past 4.1/2 thou your ready of a rebore for road bikes.my Norton650 Manxman is on standard pistons BHB since it was new and has a 3.1/2 thou piston to bore clearance, But I have renewed the rings as the end gaps were too big, someone has overdone it with the file, and I recently reground the crank to minus ten someone had fitted the wrong shells bearings the were commandoones with holes in them ok for six start oil pump but no good of an original 3 start oil pump,

yours anna j

If the pistons are hitting the flywheel, i would ask you, does the flywheel have the machined edges ???? this is where they gave it clearance for the piston skirt. Stock GPM pistons should be fine. Picture of machined crank attahed.

Regards

Steve

Previously steven lewis wrote:

If the pistons are hitting the flywheel, i would ask you, does the flywheel have the machined edges ???? this is where they gave it clearance for the piston skirt. Stock GPM pistons should be fine. Picture of machined crank attahed.

Regards

Steve

Hello I am not a big fan of Gadini pistonsas there on the heavyside Omega Pistons are much better they much stronger and lighter too lighter than the Original BHBs yours anna j