[Mac350]

Despite having 150psi compression when hot the head gasket is definitely spitting soot and looking in the exhaust manifold it could do with a decoke so I'll be taking the head off. I was thinking of taking the barrel off while I was at it but having just read this http://www.snowvalley.20m.com/bikes/dnthone.htm I'm not going to be deglazing the barrel and it occurs to me that moving the rings even a little (if they end up rotated to a new position) will mean they have to bed in again(?), so is there more harm than benefit.

From what i gather the barrel is bolted down independently of the head so its not likely to break the seal at the bottom of the barrel. I'm not throwing oil out of the breather so there doesn't seem to be much pressure leaking past the rings and good compression measured. So all I will achieve is measuring the rings for wear when I don't have performance worries that I know of other than related to still being too lean. Is it often that it has proved useful to remove the barrel just for an inspection or should I just clean the head and leave it at that?

[Paul W]

Once the head is removed, the condition of the barrel, or at least the bore, can be inspected to all intents and purposes by putting the piston at bottom dead centre. I’d not bother removing it unless you have a particular reason to do so.

[Mac350]

Once the head is removed, the condition of the barrel, or at least the bore, can be inspected to all intents and purposes by putting the piston at bottom dead centre. I’d not bother removing it unless you have a particular reason to do so.

I'm inclining towards the less effort option. I've got the service guide with lots of specs for ring gaps etc which makes it look like I should be doing that but then thinking if not knowing its broke...

[Paul W]

If the compression is good, not burning a lot of oil and the condition of the bore looks good, why remove it? There’s always the chance of breaking a piston ring if you do pull the cylinder off.

[Adrian II]

With the head off, the cylinder barrel will be held on by just one M6 nut on the cylinder base flange between the push-rod tunnels.

I agree, If the compression is good and the engine isn't burning excess oil I would leave the cylinder where it is UNLESS the top gasket joint face needs re-facing. You could check the main gasket face on the underside of the cylinder head while that is off and use a sheet of fine wet and dry on a piece of plate glass to true it if there is any slight warping or damage.

Otherwise no big deals, clean everything up and use a new gasket (not copper on these) with some Wellseal, tighten up evenly and remember to re-tighten after 500 miles.

A.

[AzCal Retred]

" it occurs to me that moving the rings even a little (if they end up rotated to a new position) will mean they have to bed in again(?), so is there more harm than benefit. "

Piston rings in 4 cycle engines normally orbit/rotate slowly around the piston. Likewise valves rotate on their seats as they operate. The most likely problem is you could break or deform one whilst removing it. Or watch the broken bits disappear into the crankcase as the cylinder slides up off the piston, as I learned the hard way when re-ringing my old CB450...

Piston rings in 2 cycle piston port designs are "pinned" so they don't rotate and shove the ring ends into open ports, ending your ride ($$$).

[Mac350]

Piston rings in 4 cycle engines normally orbit/rotate slowly around the piston. Likewise valves rotate on their seats as they operate.

so people worrying about aligning the gaps opposite to each other is a bit futile?

[AzCal Retred]

If the orbital motions are perfectly in synch they'd stay in relative alignment, but I have my doubts. Probably it matters most on initial scrub in. They'd have to jam in place with the end gaps aligned to be an issue. Likely in normal use there are intermittent "grand conjunctions", but they'd just keep rotating and that slight extra hot gas leakage event would pass. Over 100 years of piston ring experience says they do move about, but also that it's not too problematic.