A hole in on

[roy the mechanic]

the change of plugs was not a smart move! the "p" in the number means protruded electrodes. this will not only advance the ignition timing, but also move the hot spot nearer to the poor old piston. i assume you were using omega pistons, look in the photos bit for dennis wilsons pistons. i for one have not been impressed with the results of these cast efforts.

 

[ernie]

Checked plugs before departure and they were perfect, brown and tasty. No air leak. Heads are off and the valves look perfect. I don't know if the pointy nosed plug made a difference but back pot was the same and had no problem. I rode the bike 5 miles into the nearest town on one cylinder and it ran as well as could be expected. Managed 45 mph in top gear with a dead cylinder. I rode it thus onto the ferry and off at the other end. Started easily every time and ticked over lovely. Comets must be so nice.

 

[Bazlerker]

Though I have never met Ernie - I have the highest possible opinion of him, and have no doubt that were he to set his mind to it he would find it possible - easy in fact to do just that.

 

[nobby]

Comets ARE nice Ernie, especially when they are equipped with those lovely bits from down under!
Ati and I missed the French ( we were only 200 miles from there ) but see you at the Annual? Or Dorset?

 

[Ian Scott]

Roy's comments seem perfectly valid but from my experience, I would have expected to see the hole directly under the spark plug, not in the centre of the piston crown and with the holed piston in the rear cylinder which would run hotter than the front. From Ernie's comments, the plugs looked normal on removal. So, will we get to the bottom of this?

 

[TouringGodet]

The picture does look amazingly similar to Bill's piston. Bill's incident was on the long uphill grade east of Baker, CA, so the bike was under load. Bill runs Suzuki pistons, which I believe are machined on the crown, which may have had something to do with his situation. Many at our last section meeting at Bill's place that saw the damage offered opinions, but I don't know what the consensus is.

 

[timetraveller]

Re. Roy the Mechanic's comments above. I have used nothing but extended nose plugs on Vincents since I became aware of them in the early 60s. Prior to that it was not uncommon to oil plugs on my bikes. After starting to use them it never happened again. I used them for both road and racing use (different grades) with no problems. It is an interesting point about it advancing the ignition because the explosion starts further into the combustion chamber! If the difference is, say, 3 mm and the distance across the bore is 84 mm (but with different distances to the liner in different directions because the plug is offset) then I would not think that it would make much difference. Could it be the distance to the top of the piston?

Although it is hard to be sure from a photograph the colour on the top of the piston looks to me as though the mixture was weak.

 

[Ian Scott]

The following links may be of interest, they have photos of piston damage and the cause.

www.theultralightplace.com/pistons.htm www.shopbt.com/forum/showthread.php?t=298

 

[Howard]

Re. Roy the Mechanic's comments above. I have used nothing but extended nose plugs on Vincents since I became aware of them in the early 60s. Prior to that it was not uncommon to oil plugs on my bikes. After starting to use them it never happened again. I used them for both road and racing use (different grades) with no problems. It is an interesting point about it advancing the ignition because the explosion starts further into the combustion chamber! If the difference is, say, 3 mm and the distance across the bore is 84 mm (but with different distances to the liner in different directions because the plug is offset) then I would not think that it would make much difference. Could it be the distance to the top of the piston?

Although it is hard to be sure from a photograph the colour on the top of the piston looks to me as though the mixture was weak.

Me too. I used an N9Y out of a Mini a long time ago by accident, and it was so much better than the N4 (or 5) in the other cylinder, I've used them ever since commuting, touring, playing and what I called racing.

H

 

[Bill Thomas]

Hello Howard, Roys talking about a P, Yours is missing !! Cheers Bill.