Piston question

Had a persistent oil leak from the head gasket, and rather high oil consumption, so I dismantled the top end of my Commando and noticed signs of piston blow by as you can see in the pics,there is some carbon build up below the scraper rings. I measured the bores, and noticed minimal wear in all directions (less than 0.015mm) although the cilinder walls do show glazing.
My question is : what causes this . Bent conrods seem unlikely since the bottom end has never been apart. The pistons are sill STD size. bike has run approx 90.000km from new., and oil changes and filter replacements were always done after 2-3000km.
Both pistons show this pattern, could this be caused by crankshaft flex ?
Opinions please.

Looks like nominal blow-by with a bit of excessive heat. Left piston looks very excessive.

I'll bet if you carefully measure the pistons and bores, the difference will significantly exceed the high end on the factory spec listing.

Maybe a tad too much advance in your ignition setting, as well...

Could be the result of running with too much blow by for too long. Could be one of the bores is slightly tilted.

You said it’s done 90,000km right?

I don’t know how accurate your measurement equipment is, or how accurately you measured. But surely, at 90k they are simply worn out?

Ring lands have probably worn out even though the piston to cylinder fit is still good.

Bent rods are a definite possibility. They bend very easily if you try to kick it over with a little extra fuel in the cylinder from a leaky petcock.
Check them by cleaning the top of the piston and laying a straightedge across the two piston tops. Jim

I think that it's time for a top end rebuild. You've gotten about 56,000 miles out of the stock setup. Just like humans, mechanical things wear out. Heck, I'd be happy getting that kind of life out of an old air cooled long stroke Norton. Now you have a winter project

Measurements were done with a dial type bore gauge and inside micrometer, The RH piston shows the same pattern, although hidden from view on the pics. It could be that the ring lands have worn out as Jim suggests, could it also be that the cilinder bores are out of parallel ?, although this is not easy to check. (inaccuracy in the manufacturing process ? )
Small chance that the rods are bent, although I will do the check with the straight edge.
Thanks for the suggestions so far.
btw, I have good quality measuring equipment,(dial bore gauge and inside micrometers) and do know how to use it.

Looks like the engine was smoking prior to head removal so yes to a top end job with new valves ,guides,seals,springs, head gasket. The oily mixture went past the piston rings. I'd measure again (by a good shop , the one that will rebuild the head ) and hone with new rings. 90,000 is a lot of miles so congrats on that.

Peter R, I think your assessment is pretty much on the mark. Out of alignment bores is really an obscure one but thought it worth mentioning. Get a feeler gauge and check clearance between the rings and ring lands. Take care not to nacker up the soft aluminum ring lands while doing this. From recollection acceptable clearance is 0.0015" to 0.002". I pulled some race pistons once with 0.004" after many races and seem to recall my homework telling me that 0.004" is too much and my recollection is they started at 0.002". Get with a manufacture or maybe the manual to confirm acceptable clearances.

As for bores, if you do not have access to quality measuring equipment, strip the the pistons and use a feeler gauge between the bore and piston skirt throughout the bore to check for clearances.

You may be able to install new rings, glaze bust or light hone the barrels and go. If the pistons fit are too loose, look to see if new standard pistons make the grade with proper clearance.

Yes, I would also thoroughly check and service the head.

As stated above, unless you are set up for all of this, it is best to get the barrels and pistons to a reputable and preferably Norton experienced shop to sort things out. The pistons and barrel owe you nothing at this point. You had a decent run with them.

That is great life from your engine Peter. I am totally impressed with the even wear on the cylinders as you stated:
"I measured the bores, and noticed minimal wear in all directions (less than 0.015mm) although the cilinder walls do show glazing".

I am surprised at the 1/2 thou wear, I believe that is the typical tolerance for a freshly honed cylinder;
0.015mm = 0.00059055118". We know that 1mm = .03937", or as we are more familiar; 1 mil is 40 thou.
Have you compared your cylinder micrometer measurement to the standard bore size? You didn't mention the piston to cylinder clearance; how much is that? I am also wondering if you did a compression test or a leak down test before pulling the head - if so, what was the result? Good luck with your rebuild, and share your progress.

I have a MK2a on std bore and I am on my 2nd set of rings, the NOC Commando Notes says you will be changing the rings twice to each rebore. I did not check my old rings but changed them due to blue smoke coming out the drive side exhaust on acceleration and new rings cured it the issue, pistons and bore were in good shape and I gave the bores a light hone.

Some more observations: I did the check as suggested by Jim C, cleaned the tops of the pistons, and checked with an accurate straigh edge, this check showed that the conrods are absolutely straight.
I also measured the amount of play of the piston rings in their grooves, I found 7-8 thou of play (or 0.2mm for the metric world). this seems out of spec to me.

Peter R said:

Some more observations: I did the check as suggested by Jim C, cleaned the tops of the pistons, and checked with an accurate straigh edge, this check showed that the conrods are absolutely straight.
I also measured the amount of play of the piston rings in their grooves, I found 7-8 thou of play (or 0.2mm for the metric world). this seems out of spec to me.

Click to expand...

I think you found the problem. Not unusual for the mileage. Jim

When an old engine is burning oil and coking the sides of the pistons, that doesn't mean it's bent!

I'm on my 2nd set of pistons.. still standard bore. Replaced because of similar ring land wear. The bore had little or no wear with 30K miles. It wears like cast iron!

No one ever heard of thermal distortion ?, lack of cooling in the middle distorting the bores, even worse on a sleeved set of barrels ?
Very much doubt that set of pistons has 90000km on them, but something looks a bit strange at the top edge of the top ring, all the bright shiny studs suggest its been apart before, they don't look 45 years old.

Mr splatt, I have owned this bike since 1975, and although old age and alcohol may have caused some kind of lapse in my memory, (LOL) I do know for certain that the barrel has never been off before. The cylider head has been off a couple of times due to a stripped thread of the exhaust port (around 1980). And once again due to a leaking head gasket, approx. 8years ago.
The barrels have never been re-sleeved. Thermal distortion between the bores sounds plausible, although this did not concur with my measurements.

Time to shop for new pistons. As stated in my earlier post, I got rid of my pistons with around 0.004" ring land clearance. When you think about it, the scraper and compression rings act like an integral pump/check valve. With minor factory recommended ring land clearances, just enough lubrication passes, too much clearance then too much migrates up past the rings.

Hopefully the barrels allow you to drop in new standard OEM pistons with a glaze bust or light hone; be mindful of any ridge at the ends of the ring travel in the barrel. A good shop can sort this out for you and make recommendations on overbore or not.

Well there you go , that's what an engine looks like that hasn't been molested by 25 different owners. As has been recommended, new piston and rings and carry on, and hopfully it will be another 50k before you ever see those marks again
Unfortunately new pistons will probably be on minimum size so clearance may not be perfect but it seems a shame to wear it out quicker by reboring

If the pistons measure satisfactorily you could consider getting the top land squared up and fitted with a steel spacer. This used to be quite common a repair and a machine shop that's been around for a while will know exactly what the procedure is. Basically, the ring groove is cut suitably oversize enough to square it up and a thin steel spacer is installed in the bottom of the newly cut groove. This spacer is much like one of the thin oil ring expanders but fits very precisely in the groove tightly enough to not move. It provides a perfectly flat surface for the new ring to seat/seal against and restores correct clearance. Two plusses, the machining cost is a lot less than new pistons and that steel ring-land insert will never wear out so ring clearance will stay virtually the same for life.