Scratched cylinder

[Coady]

Recently acquired some GS850 cylinders & pistons to upgrade my GS750 (and a rebuilt bottom end that I have no need for). They were already re-honed with new rings, however one cylinder has a scratch in it deep enough to catch my fingernail.

In the photo, the bottom of the calliper is at 60 mm - approximately the lowest point where I reckon the top ring would be. So a good 20mm of scratch/gouge exposed to combustion.

Since I'm planning to replace my current cylinders for excess oil consumption and they have a couple of scratches in each of them, am I going to have similar problems with this? Should I rebore my 750 cylinders and get them perfect?

[chef1366]

It'll probably smoke. If you want to use that cylinder try to find .05mm or 1mm oversized pistons and rings and get the cylinders cut or find another cylinder.
OR
Slap it together and ride it and hope it's not a fog machine.

[tkent02]

I vote for slap it together and run it.
Break it in hard, it will be good.

[Coady]

he he - I like the cavalier attitude and would usually go for it but if she keeps smokin after tearing it down a second time (did head gasket and valve seals last year - that's when I saw the current scratches) I'm gonna be ****ed off.

[tkent02]

By now you should have it down to an hour or so like Chef.

[tkent02]

I have built car engines with way worse, no smoke.
Even assembled a few with no where near the correct piston-cylinder fit, something like three times the spec. Worked fine. Ran a long time. Don't know how long it lasted but I ran the heck out of it for years.
I really wouldn't worry about that little scratch.

If it smokes after a good hard moto man style break in I will really be surprised.

[Coady]

I have built car engines with way worse, no smoke.
Even assembled a few with no where near the correct piston-cylinder fit, something like three times the spec. Worked fine. Ran a long time. Don't know how long it lasted but I ran the heck out of it for years.
I really wouldn't worry about that little scratch.

If it smokes after a good hard moto man style break in I will really be surprised.

Cool thanks. I have little experience in this area and am after opinions of people who have been there before.

I want a well operating bike (which it is other than this little oil discrepancy...) but obviously don't want to chuck away money for nothing.

[tkent02]

Who honed it?
Not much of a cross hatch pattern, at least not in the pic.
I would worry about that more than the scratch.

Looks like they just stuck it in and spun it rather than going in and out for the 45 degree pattern.

[Coady]

Apparently was honed by a reputable shop a couple of years ago, been sitting since then. Photo is a little blurry.

[tkent02]

Do the hone marks look something like a 45 degree angle?

[Coady]

more like 25 - 30 degrees.

[tkent02]

OK, I have seen it where all the lines were parallel, the hone was spun at one depth with no movement.
Kind of what it looked like in that pic. By the way, that cylinder sealed up fine and didn't smoke either, was a very fast GS 1000.

[rapidray]

I have a BRAND NEW set of RC/Venolia 70mm pistons for sale if you are interested! Ray.

[Coady]

Thanks ray, but I have the current 750 cylinders to bore for the 850 pistons if needs be.

[Coady]

Update - guess I should have known better

Well thought I'd update this thread for reference.

I've installed the 850 cylinders and pistons, and started the break-in process fairly hard. After warming up the bike I accelerated quite positively to about 7000 rpm and then engine breaked a few times in 2nd gear whilst riding around in a spirited fashion 4 - 6k rpm for about 20km.

There was no smoke on start-up and it's definitely more lively than my old 750 cylinders. In fact it goes really well.

However... having now put over 100 kms on it with two more rides and a bit more hard acceleration etc. It smokes a little once fully warmed up and a bit more after harder riding. Still no smoke at startup at all.

I'm hoping this is still the rings bedding in, but am thinking its probably that ****y little scratch. Unless I went too hard on that first ride.

I'm kicking myself, because it goes so nicely otherwise. But I did take a gamble.