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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

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

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

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.

Aye George,

Mmmmm! Borgo piston crown depth, could be variable and would certainly account for the dinking!

Either retain the double solid copper washers under the plug or lift the head and barrel, then replace the cylinder base gasket, with an alloy spacer and two composite gaskets, to allow sufficient clearance from the spark plug electrode.

Something similar was posted in a thread on the forum, back in 2021. However, it was the piston touching the exhaust valve! Jordan was involved in that thread as well, I see. Here is the link: Piston touching the exhaust valve

Hope the BIG move all goes smoothly for you in 3 weeks time and once you have your feet on the ground, maybe have a blether. PM if you prefer.

Good health, BillR

Hi George,

Most Ducati singles engines that I have seen came from the factory with the aluminium spacer under the barrel; I don't know why this is so, but could it be that yours is missing and this is causing the problem. I believe that these spacers come in different thicknesses, so they must have been to allow the factory to adjust head clearance to the piston. Seems a pretty inefficient way of building engines......


Cheers,

Colin

One of the advantages of bevel drive engines is the ease with which the compression ratio can be changed, with the ability of a coupling in the system to allow variation in length. This was useful for race tuning.

Compression plates?

I may be wrong, but none of the late MKIII widecase engines that I own are fitted with compression plates at the base of the barrel and I do not recall seeing them fitted to other road going widecase engines. I suggest that this is an earlier addition made to narrow case and not the wide case engines. I suspect that George's engine is a wide case type? Hence, his enquiry, as to "Why?" and "Is it an isolated occurence, or something more widely occuring?". I have seen the plate fitted to narrow case race bikes, but I am not sufficiently a "rivet counter" to be able to give chapter and verse, by model!

However, there is a solution to the problem, if 2 solid copper washers beneath the spark plug aren't the desired remedy. Mdina Italia, supply from Nigel's stocks, a range of aluminium alloy cylinder base gaskets that will solve the problem. N.B. there is also the "standard" thickness composite base gasket and another composite base gasket of 1.5mm thickness for the 250/350 engines! Maybe therein lies the problem? The OEM engine build used a 1.5mm composite gasket, whereas George used the thinner type?

Here is the link to the various gaskets available from Mdina's website:Mdina_Italia_Ducati_single_gaskets_website

Good health, Bill

Hi Bill,

The compression plates are a bit of a mystery; maybe Nigel knows more? My 350 Sebring had the thick plate under the barrel and both my wide-case Mototrans engines have thick plates also (I can't be more specific because I have never measured them). I don't know if the plates were to account for variations in the barrel length or to vary the compression ratio.


Cheers,

Colin

Hi All, many thanks for your input, I also have never seen a std. late 1970s 'Italian' widecase with an alloy compression plate, I have on many narrow case motors and in particular on monza's and sebring's. Bill you may have hit it with the cylinder base gasket thickness I can't remember if I used the 'thick' or the 'thin' one.....I think it was the thin one? however surely even a full mm of gasket thickness wouldn't account for around 2.5mm of spacing the plug out? Cheers George