B12 top end rebuild @ 55k - Replace rings? To hone or not to hone?

[Shill]

Hi all - sorry if this is an age old question, but I figure the collective experience on here is more than mine (basically zero).

So, for reasons, I've got the top end of my B12 mk2 apart. Reasons are several:

• I bought the engine used (now has about 50-55k on it), and it's always had an mild oil leak somewhere on the front, so wanted to do the valve cover and oil tube seals anyway. Probably gonna paint it at the same time, and worth doing a general inspection. When I bought it I was just enthusiastic to get the thing in and running!
• There's always been a bit of smoke out the breather filter once it's warm. The dyno guy said 'its just breathing a bit heavy', which was kind of ambiguous (like "it's not ideal but it's ok") so it's always been at the back of my mind that it smokes more than it should. I realise at this point a compression check would have put those doubts to bed... which I didn't do.
• Main thing: Last week it was only running on 3 cyls (suspect caused by a blocked jet, I ran it down to reserve for the first time in ages recently). This has happened before and has cleared itself up by the end of my road. Not this time - it stayed running lumpy, and then at some lights there was a loud pop, and when I got home there was a good amount of oil in my breather hose and filter, and on the front of the engine. So something has pressurised the crankcase? Total noob hypothesis - I suspect this is maybe just unburned blowby from the weak cylinder getting ignited (does compression alone cause blowby if it's not firing?), but I saw this and thought f*ck, something's let go. Combined with my nagging doubt about the rings anyway, and the leak path on the front, I suspected gaskets, some disaster induced by lean running on 1 cyl, all manner of horrors.

So here we are, head off, barrels off, looking for anything catastrophic, and turns out it's basically fine to my untrained eye. Bores very smooth with no scoring, pistons look fine, bit of carbon buildup on the head to clean off (suspect stem seals on 2 of the 4), oil line top orings look to be nicked so probably the source of the leak. Going to do valve stem seals and a general tidy up. Very much wishing I'd done a compression test before all this started just to have more data, but I do tend to go at these things 'like a pig at a tater' as my mate points out.

Question is, at what point would you experienced OSS dudes be expecting piston rings to need replacing? On an engine of this age, would you re-do anyway while it's apart to be safe in the knowledge of a proper rebuild, or best left alone if there are no serious symptoms? I'll be doing measurements tonight but wondering what's 'typical' at 50k. Supplementary question - there's a load of conflicting internet views about cylinder de-glazing/honing and whether it's necessary or beneficial. I'm leaning towards new rings just to be thorough, and a light de-glaze because that's the standard recommendation, but any other views?

Also, 20% off at Robinsons Foundry this Friday-Monday for Black Friday apparently.

Edited November 24, 2021 by Shill

[wraith]

I'd always give the bores a honing when putting new rings in, just so they can bed in.

Down side is, if your bores are getting worn/near their limit, a hone will just make it worse.

Best thing to do is to get your manual and start measuring parts (rings and bores)

If you have not got a lot of burnt oil on top of your pistons, then your oil leak may of been one or more exhaust valve stem seals.

Did the smoke clear when the bike wormed up or did it smoke all the time?

As to when to do a piston ring change, that all depends on how the engine has been treated over its life.

[TonyGee]

theirs a new theory going round about not honing, the goal is to have a smooth bore which helps a good seal at the rings, so why ruff the surface up again ???  does make some sence !!!!   just putting it out their   

[Shill]

5 hours ago, wraith said:

I'd always give the bores a honing when putting new rings in, just so they can bed in.

Down side is, if your bores are getting worn/near their limit, a hone will just make it worse.

Best thing to do is to get your manual and start measuring parts (rings and bores)

If you have not got a lot of burnt oil on top of your pistons, then your oil leak may of been one or more exhaust valve stem seals.

Did the smoke clear when the bike wormed up or did it smoke all the time?

As to when to do a piston ring change, that all depends on how the engine has been treated over its life.

Cheers wraith, that makes sense. I should clarify, the smoking wasn’t from the exhaust but from the breather hose from the top of the valve cover, I’m running pods so just have the breather open with a generic filter on it. It smoked more once up to temp I’d say, I’d notice it when stopped after a long run. Is some smoke normal if you’re just venting to atmosphere?

On piston and head carbon build up, couple of pics below. I didn’t expect it spotless, and kind of feel this isn’t too out of the ordinary? More experienced opinions very welcome though! The other side is cleaner, the inner pot on this side is the most carboniferous, and closest to the external leak on the front.

Will find out more when I get measuring, guess that’s the real answer eh. Cheers for the input, I know this is more standard service knowledge than OSS madness, it’s in my zuma 7/12 at least!

Edited November 24, 2021 by Shill
clarification

[wraith]

For a 50k + engine, they look very clean to me and you can still see the honing. I'd say more like 2000k engine looking at them clean pistons and bore, has the previous owner done some work?

Dose look like you definitely have some oil burning 

But I'm not a mechanic someone should be a long to confirm.

 

[Shill]

Yeah I was surprised at the condition of the bores - smooth to the touch but can still see the hone. Not sure on the history really, so a bit of a voyage of discovery!

[fab]

you said it was running ruff and thought it might be blocked carburetor, did you check oil level and smell it too make sure it wasn't a flooding carbi ?  

[TonyGee]

time to measure the ring gaps, and while its in bits do the valves and seals.