fouled plug on 850 Commando

Mar 1, 2007
Boy Howdy do I need help?? The left cylendar oil fouls the plug after 15-20 miles of riding my buddies 74- 850. It is oily and not powdery black when fouled. He has a Boyer and a single Mukini carburetor. It smokes lightly out the left tailpipe. He has asked me to fix it?? I pulled the head off and looked at the cylender walls. They are fine. I used some inside Mics and the cylendars look good to me. I miked the cylendars perpendicular and parallel to the crank in 12 places ranging from top to bottom. They are within tolerence at 3.032. No significant taper or egging. I am not sure how many miles are on the bike. Should I go ahead and pull the cylenders and check the rings or could it be the valve guide seals are worn or the valve guides are shot???? How do I check the inside diameter of the valve guides?? Could it be a bad oil ring. The compression on both cyls was about 140 psi If I remember right on both cylenders before disassembly. All you guys have been great and I really appreciate your advise. Thanks--Mark Cigainero
engine work

Hi Mark,
If your already that far into it you might just as well re-ring it. It would be a shame to reassemble it only to find that you are still smoking.
Regarding the valve guides, typically you measure the valve stem in three places looking for wear and taper. The inside of the guide can be measured using a "ball gauge" and micrometer also in two or three places, I am not sure what the Norton spec is for stem to guide clearance. Or an alternative method is placing a dial indicator against the head of the valve and measure any side to side movement.
Of course, if your into the head that far you will no doubt want to clean up and check the valve/seat condition, and reassemble with new valve seals.
What the heck, its still winter, you got plenty of time.

Hi Mark,
Shame you've gone that far, guide seals have been known to pop off the guide. It is possible to change them without removing the head, but while the motor is down as geo46er says "you got plenty of time".

Make sure you use Viton guide seals.
Rings last normally for 1/2 the life of the bore and on mine it was the left side smoking that told me to fit new rings.
Not sure what is available for the Commando, but most of the riders who do high mileages on Vincents use Honda Chrome rings. There is a size available that slips right into the Omega Vincent pistons. Cast iron rings seat more easily, but chrome rings are said to last about three or four times as long.

Those Japanese parts are expensive though. When I rebuilt the Vincent top-end, the little box of Honda rings cost almost as much as the complete new Vincent pistons with cast iron rings.
One local fellow has done 75,000 miles with the Honda Chrome rings and says oil consumption on a 3000 mile trip last year was about 8 ounces.

Someone on this board may know if there are any Honda or other Japanese chrome rings that will fit the Norton pistons.
Funny thing with the Japanese, when I had a 2 bit Motorcycle repair business going in the seventies, the Japanese bikes were reliable for about 10,000 miles and then they were generally not worth rebuilding, so poor and soft were most of the metals used.
Now their metallurgy is top of the field.
Cash, how do yu fit valve guide seals without removing head? Springs have to be removed. Also I fell foul of seals not lasting 2 minutes from new - RGM lists only one inlet seal, but there are 2 sizes mine were 5/16 not 9/32 inch. difference is too small diameter seal causes end to rip out of seal
You can fill the cylinder with air, just like doing a leak down test. The air pressure will hold the valve up.
And, if you haven't got a compressor smash the porcelain out of an old plug, bend a hook out of a bit of threaded bar that will pass through the pug hole. Install the plug body over the threaded bar and into the plug hole. This will protect the threads and give you something to clamp against. Position the hook over the valve head and using a nut and washer clamp the valve in place.
Now all you need is a good light, tweezers, glasses if you're over 40, a tube with a cut out, a lever, and an extra hand. You can lever off the box section head steady.

If you are interested and I can get the camera working I could get you a shot of the home made kit I use.

I thought there was only one size of seal for the 850.

One clue for oil getting past the rings is the look of the carbon deposits on the piston crown. If it's clean around the circumfrence, i.e., you can see the aluminum, that's a good indication of oil getting past the rings. If it's carboned up all the way across the top, your rings are probably fine.

Cast iron rings are easier on the bore than chrome. Hastings (US manufacturer) make rings for Nortons, both cast iron & chrome. I've been running a set of cast iron ones in my Fastback for aprox 10k miles with good results.
Hey Mark & Cash I wish I had talked to you a few months ago, I have had head on & off so many times I can do it blindfold! Re different sizes of seals, compare RGM catalogue which only lists 1 seal with Norvil which gives 2 sizes & measure your own valve stems. 5/16 part no also agreed with original Norton parts list for 850