Why do manuals say don't remove the piston/cyl carbon ring?

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Sep 9, 2008
New Norton owner here. In the process of restoring a 26,000 mile '74 MkIIA. The shop manual states that removing the carbon ring at the top of the cylinder head and the piston will increase oil consumption. If so, I can't quite fathom how. New engines don't have this carbon ring, so do they use more oil? Not in my experience. This "advice" seem rather unfounded to me, so perhaps those of you experienced with these engines can help a brother out.

BTW, I've already got the head off and I intend to completely rebuild the engine. May not need a rebore and new pistons, but I'm sure as hell going to replace the rings, so all the carbon is going "buh-bye" one way or another.
I have never questioned the advice and never had a problem leaving the carbon there, remember these bikes have an engine orginally designed in the mid 40's and oil consumption was high as was the build up of carbon.
Re: Why do manuals say don't remove the piston/cyl carbon ri

Huffer said:
The shop manual states that removing the carbon ring at the top of the cylinder head and the piston will increase oil consumption.

My Dad said the same, I never did and have never had a problem. How old's your shop manual?
I've often heard that - mostly with respect to "old school" bikes - and woud respect it. It does seem that the combination of piston and cylinder carbon buildup performs some sealing function.

I can tell you that when I ran some Seafoam through the Snorter's cylinders not so long ago (poured through spark plug holes, let it sit, then put in the plugs, kicked it over, and killed every mosquito within fifty yards) it did seem to burn a bit more oil for a while. Although such anecdotal evidence should be taken for what, if anything, it's worth.....
carbon ring in cylinder

"Leave the carbon ring in the cylinder", how could you properly hone the cylinder and not remove it ???? Lots of good ring/piston/cylinder prep info at www.hastingsmfg.com click "aftermarket" click "tech tips".

I was told by an old motorcycle mechanic that as the cylinder,piston and rings gradually wear, carbon deposits build up around the piston and in the piston ring grooves compensating for the wear.

Removing that carbon build up would probably result in more oil consumption. Unless of course you intend to renew the pistons and rebore etc. :)
Most of my experience comes from flathead Fords here. When I was a kid you did valve jobs quite frequently to cars and boats.
If you got 30,000 miles on the early engines you did not do that badly before decarboning. By the time I grew up in the fifties I think dad was getting about 60,000 miles or so. It really depended on the way you drove the vehicle.
The accepted form was to leave the carbon on the piston sides, you greased it to keep carbon out of the rings. Everyone thought this improves sealing as by then you usually had a bore ridge. The average engine lasted about 100,000 miles so they often burned oil after valve jobs.
Modern oil, fuels, filtration, and machining have made this a part of history.
You should have seen the cabon build up I recall knocking off valves.
I think you're on the right track, Cookie. My first four-wheeler was a 1938 Austin "Ruby" (this was in 1960 or so). On a remote assignment for my employer, I was driving the 180 or so miles to the remote site when the old girl started running really badly.

I carried a fairly comprehensive tool kit (family ran a car repair business). When I got to the remote site (Salisbury, Wiltshire) I rode the bus to work and started trying to figure out the problem after work. It turned out to be two burned exhaust valves, which I replaced by working on the car in the municipal parking lot!

During the work, I cleaned off that "nasty ring" of carbon round the top of the cylinder bores and was rewarded by an increase in oil consumption of about 50 percent!

With more modern metallurgy, fuels, oils, filtration, etc., I find it hard to justify leaving the carbon deposits at the top of the bore in a rebuild. Certainly, if you're replacing piston rings and honing the cylinders, you can't avoid removing it.

IMO, it's one of those horror stories that experienced (pronounced "old") mechanics used to scare the young "whippersnappers".
Would a 1938 Ruby have been a flathead? My experience with English cars has been limited to several Jags and TR3s I've owned, a mini van I toured Oz in, and an Anglia Van (with Escort engine) I toured Kiwi with once.
I have worked on a few British cars I didn't own (worked in an SF British car shop for a bit) also but I'm quite unfamiliar with a Ruby. Oh, yes, I did do a valve job on my wife's dad's Mini in Kiwi.
Some of those engines were a joy to work on as they were so simple compared to the stuff today. On the downside they needed a lot more work.
I would also clean all carbon up while doing any internal work these days.
As you may have noted I am somewhat of an early Goldwing fan. These bikes regualy see 140,000 miles without opening the engine. It is common for riders to decarbon them with "Seafoam" or some folks dribble a bit of water into a running engine.
If they require decarboning it is usually because of extreme mileage or improper adjustment or maintenence.
Not even a motorcycle post, but maybe Jerry will let it stay!

Yes, the Austin Seven was a flathead 4-cylinder engine. It was 750cc displacement and on a really good day, managed to generate about 20 horsepower. Mine was a later model than many Sevens, built just before all auto production in England stopped for WW2.

It had the "high compression" head (6:1) and a three-bearing crank. The older ones had 4:1 compression and a crank with two main bearings - one at each end. An interesting feature was what we called "Spit & Hope" crankshaft lubrication. There was an oil gallery along the side of the block with two spray holes aimed in the general direction of the rod bearings. As the rods came by the "spit" of oil, you "hoped" it would actually get to the bearings!
The OHV engines in Nortons were certainly advanced for thier day compared to that.
It is amazing to me just how much power was developed from the same old twin and it may not have shown it's peak yet.
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