Piston - Cylinder Wear

[frankfast]

Can anyone give me the wear tolerance of piston to cylinder liner (min. to max) for a 350 Scrambler?

[Duccout]

It is listed in the Haynes and Clymer manuals as 0.14mm.


Colin

[themoudie]

Morning Frankfast,

If this is a wide case 350cc engine?

From Haynes manual "Piston to bore nominal clearance" 0.07 - 0.09mm (0.0028 - 0.0035in) Wear limit 0.14mm (0.0055in)

There are 10 piston sizes; 'Standard' plus 4 'Oversizes', each with an A and B grade.

The piston ring end gap should be 0.30 - 0.45mm (0.0118 - 0.0177in) Wear limit 1.00mm (0.0394in)

The 'Sebring' 350cc narrowcase engine clearances are similar, but I cannot say specifically what they are offhand. If the engineers think any of this is "cobblers" then I am sure that they will say so in the near future.

I hope that this helps. I am now off to exercise the 450!

Good health, Bill

[ducwiz]

Extract from the french "RMT" manual:

Image50.jpg

[frankfast]

Yes, it is a WC Scrambler. For some unknown reason I replaced the original piston with a high compression item years ago and it's hell to kick over. I don't remember reboring the liner. It may have been just honed. I want to take some measurement of the original piston ( and liner) and see if it's still useful. Thanks everybody.
You've given me all I really need to know. for now.

[LaceyDucati]

Generally speaking 3 to 3.5 thou (0.75 to 0.09mm) to the skirt is fine for most Ducati 250/350 singles, maybe less on the sebring/Monzas using original pistons. If you have more than 6 thou (0.15mm) to the skirt on a road bike it's worn out.
Beware of Ducati specs as they are rather strangely not quoted at the skirt, you need the accompanying drawing in the manual to give the full picture. Clearance is stated at just under the oil ring, I think there is about 1 to 1.5 thou from this position to the skirt. Bored to 0.115 to 0.145mm at the skirt a 250/350 single will be rather slack for a road use, at 0.190 at the skirt it will be like a maraca!

Regards Nigel

Edit - originally I stated "Top ring" was meant to read top oil ring.

[themoudie]

Aye Nigel,

This is a lateral jump, but, I have a friend who has a mid 1970's 750S that he has owned since 1978. After having the engine rebored and fitting new pistons, it now smokes on the overun and sounds a bit rattly! He took the parts back the company who did the rebore for him and they assured him that the work was "within factory spec"!

Would similar clearances apply to the 750S engine pistons clearances, as the those you have quoted for the 350 engines? After all, it is just two 350 barrels on a common crankcase!

Thank you for your time.

Good health, Bill

[LaceyDucati]

Bill
The twins are to the skirt. Factory clearances apply to factory pistons, after market pistons generally have their own spec. My experience is many don't always do as they say and they rely on customers not having the ability to check! All to often over the years, I've heard "the cranks all done/ cylinders done" stories as the customer hands over a pile of bits.....Done being the word!

I had the misfortune to have to sort a crank last week for a customer, it was running miles out with a freshly sleeved pattern bend rod!! Worse still the pin was all wrong, the wrong way round with thick side shims. To overcome the now over wide crank they had skimmed the flywheel on the timing side 1.5mm.....a Professional expensive rebuild apparently...... The mind boggles!

Regards Nigel

[themoudie]

Nigel,

Thank you for the insight, I think that the pistons installed were a pair of Omega's, but I shall have a blether with the owner later in the week, all being well. The bike is well used and far from 'standard' or 'concours', but the oil consumption and the "blue haze" are noticeable.

My regards, Bill

[LaceyDucati]

They won't be Omegas as they don't make pistons for 750 bevels. I started to get them to draw some up some years ago, but never went ahead with manufacturing them. Fairly sure they never made any for anyone else, Steve wynne had pistons made at Omega before us, but they were all larger pistons as far as I know.
Nigel