Bugger. But hey, at least you’ve found it!
I would suggest that such damage could quite easily have been started in the first 500 miles and that you’ve been living with damaged pistons for longer than you think.
My guess would be that this was put together with tolerances that were too tight. And although it’s run well for a while, all was not well under the surface. Then maybe a good run on a hot day, or a fill up with poor fuel, or similar, tipped it over the edge.
Those bores look terrible and pistons equally bad. The crosshatch pattern is even worse than terrible. Frankly, it appears to me that the engine builder was totally incompetent. ASSUMING the pistons are the correct size for the bore, then the problems was caused by: the improper cross hatch pattern (and likely an incorrect bore finish) plus poor bore cleaning prior to the assembly. Improper cleaning will produce the vertical scratches (as will poor/no air filtering and the incorrect cross hatch will create an excellent chance that the rings will never seat properly. Of course, improper ring end-gap or gap alignment can cause similar (oil burning) issues.
Fwiw, on reassembly, when you clean the pistons/bores after re-boring/honing, clean the bores with solvent like Naptha or any mineral spirits - gasoline works fine, just don't be smoking at the time - and then clean, Clean, CLEAN with rags/hot, soapy (original Dawn is good) water until a white cloth show no discoloration whatever. Then dry and wipe with a cloth dampened with WD40 to prevent rust and cover the barrels until ready to install the pistons. When you Install the equally clean rings/pistons) do not add any additional lubrication. That's right, install them "dry."
If you are totally uncomfortable with "dry," (some folks are) then wipe some 30 wt engine oil on a rag and apply that to the bores/rings. DO not dip the piston into engine oil, as the old manuals used to specify. That just creates immediate gunk/carbon in the ring grooves when the engine fires up.
Obviously, Be sure to check/verify that the rings are the appropriate size for the piston - IOW if you bore .020 oversize, you must have an .020 oversize piston and a set of .020 oversize rings. Also check/verifty the piston ring end gaps are within spec and that the gaps are appropriately staggered. CHECK all the dimensions, don't assume that the manufacturer got it right when it was labeled/packaged. I have seen (admittedly not very often) piston rings and pistons come marked as a certain size, only to find they were not.
As a side note- Strange that the Norvil 920 pistons are heavier than stock.
The RGM 920 pistons are the same weight as stock 850 standard bore pistons +-2 grams.
Roger made sure of that when sourcing the pistons. This way crank rebalancing is not required.
As a side note- Strange that the Norvil 920 pistons are heavier than stock.
The RGM 920 pistons are the same weight as stock 850 standard bore pistons +-2 grams.
Roger made sure of that when sourcing the pistons. This way crank rebalancing is not required.
Glen
I apologize if you've already posted the info and I've just missed it, but have you measured the bore to see if you can determine what the clearance actually was? Might still have a clean area at the bottom of the bore (or maybe not). Even the cast Ford pistons Fair Spares/Norvil used need at least 4.5 thou clearance if the bike is going to ever be ridden hard. I ran them at .005" in my race bike back in the '80s for quite a while with good results. It's going to be more difficult to get a good measurement on the pistons. They've certainly already collapsed enough to change the orginal clearance by at least .001", probably more. Your results really look just like what I saw back then when I was trying to sort out using forged pistons in the 920. The manufacturers (Arias, Forgedtrue, and Venolia) all said to run them at .002" or .003", and they seized every time. I didn't get any of them to work on the race bikes until I got out to .005" clearance or more.
Ken
Just thinking - as there’s not much meat on those liners, if the bore is too tight and the scores aren’t bad, might it be possible to bore it to the correct “std” size? ( And use new “std”pistons of course).
Good thinking there cliffa... probably unlikely... but definately worth checking out !
and more importantly I've had too much whiskey
Fantastic advice, I have heard of the 'dry' run-in of piston rings,
It aint that fantastic, as its a total waste of advice as you have to lube the skirts and gudgeon pins any way .
How did you not see the bores when you replaced the head ?, this problem can't have always been there ,more likely relates back to page 6 were you said you replaced needles and jets
If you are doing a dry install, the point is to NOT have oil on the rings/bores. You do, of course, lube the piston pins on assembly and you can put a smear of oil on the thrust-side piston skirt.