250 desmo piston hitting plug

[blethermaskite]

So I am mulling over selling my 250 desmo (getting old and all that) I havn't had it running for nearly five years so it got a bit of a 'going over' ready to start again. When I took the spark plug out I found I had two solid copper washers on it instead of the normal crush washer, and this is where older age starts to get scary I could not remember why the copper washers where there.......until I put a new plug in and tried to start it .....not a cough, pulled the plug out and the gap was closed tight.....AHH....that's why I had the two solid copper washers in there .
Here's the question why on a completely standard engine (with no mods that I could see during the extensive rebuild) would there be a clearance issue between the plug and the piston? (it has a new std 250 desmo Borgo forged piston), std. thickness cylinder base gasket, correct plug, plug threads in the head are as new no messing gone on there........I mean I know 'why' its doing this, but I don't know how this could happen with all std parts? Cheers George

[themoudie]

Aye George,

Battering your spark plug eh!?

What do use as your "Standard" plug? Champion L81/NGK B7HS? They have a reach, of 15mm, base of the thread to the outer edge of the earth electrode, or as given in the catalogues 12.7mm (½"), from the bottom of the washer to the outer edge of the earth electrode.

Has someone skimmed the head too much, to increase the squish, or is the flange on the top of the liner thinner than "standard", or been skimmed to improve the mating surface, between it and the head? Thereby, reducing the distance between the piston crown and the plug electrode. Did you do the "extensive rebuild", or had it already been rebuilt when it came into your care? Just asking, not accusing. Maybe a squiffy, last of the line, assembly job? The same as my 350 MKIII muff and liner!

Thank goodness the wee dodger didn't start, or else you could be hammering the big end, knackering it, the little end, piston and all!

Being a sucker for punishment, I've been for a 130 mile canter around the unclassified roads of Angus and Perthshire today on the 450. Would you mind PMing me about your mullings? And, I must start the 350 MKIII rebuild!

I would like to know what you conclude is the likely reason for the battering.

Good health to you, Bill

[blethermaskite]

Thanks Bill, I am guilty of the extensive rebuild.....all with genuine Ducati parts.....I am only the second owner, the first owner was an idiot who thrashed the life out of it until the big end expired at about 6000 miles. I was the first inside this engine so unless as you suggest as a late disc brake model it might have been built with less than perfect 'end of the line' parts I can't quite fathom out why it should be like this.
The only reason I can come up with is that the new Borgo piston is a little higher compression ratio than the factory std spec. It looked the same as what came out...but.....I didn't measure it critically ......maybe the squish profile was different?? I'm unlikely to pull it apart at the moment (moving to Scotland in three weeks!) I'm too busy with box's, Il'l stick the two washers on the plug again for now Cheers George

[Jordan]

Ideas for dealing with the current problem of piston hitting the spark plug earth electrode:

- Find a plug with a recessed electrode.

- Fit extra or thicker cylinder base gaskets, standard fitments in some models.