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Hey guys, need help(again) hunting down a spark plug. Engine I have is a 350, early 70’s(72) but not clear on model it came from. Below are pictures of the spark plug that was in it when I got the bike.

Have a look at the Lacey Ducati site, it lists spark plugs for different models. Off the top of my head I think that the 350 uses a B6HS NGK.

Colin

It appears you have a thread repair insert that came out of the head with the spark plug. The spark plug should be a 1/2”x14mm thread. It also appears the repair insert is too long and is protruding into the combustion chamber. You will have to address the spark plug threads in order to be able to replace the spark plug. The plug shown is an ND, perhaps one heat range too cold but might be ok depending on state of tune.

Thanks for the insights. It didn’t even occur to me that it was a thread repair insert. The length is correct as it makes no contact with the piston but seems to certainly leave the electrode quite covered. Would be better if it was more exposed for a better burn - so a few more threads off would be beneficial eh?

And there was me thinking 'what a weird spark plug'. That happened to me once when I had the plug thread repaired by a local car firm; when I picked up the head they had left the top of the insert proud of the plug seating area, and when I queried it they told me that was the correct way it should be. I called Alf Hagon to ask him what he thought, and in his brusque manner he told me that it was not right and that if I removed the plug the next day, then the insert would come out with it. He was right and it did, so he repaired it with a normal helicoil.

Colin

The length is correct as it makes no contact with the piston but seems to certainly leave the electrode quite covered

Objection, the length is incorrect !
It must not be greater than the length of the plug's thread. The electrodes must not be covered, and any fractional part of the insert must not protrude into the combustion chamber.
I was always an opponent of any sheet metal inserts. I always made good experiences with Helicoil, Baercoil and the like.

Hans

ducwiz wrote:

The length is correct as it makes no contact with the piston but seems to certainly leave the electrode quite covered

Objection, the length is incorrect !
It must not be greater than the length of the plug's thread. The electrodes must not be covered, and any fractional part of the insert must not protrude into the combustion chamber.
I was always an opponent of any sheet metal inserts. I always made good experiences with Helicoil, Baercoil and the like.

Hans

Got it. So the only thing ‘correct’ about it is it hasn’t made contact with the piston. Clearly it was run this way prior, which is kinda crazy! Ok I will trim it appropriately. I have a NGK B7HS 5110. Any experience with them? Good choice?