Measured Piston/Bore - Advice Please

[guzzijon]

I work at a high school so today I took the piston and cylinder from my 1963 Scrambler into the auto shop teacher to measure with a micrometer.

Here are the results:

Piston measured as 2.950" which converts to 74.93mm

Cylinder bore top and bottom measured as 2.956" which converts to 75.08mm

Cylinder bore middle measured as 2.597" which converts to 75.11mm

So...the piston to cylinder wall clearance is between .150 and .180 when the manual says it should be .115.

There are some visible shallow pits in the bore, the shop teacher ran a hone through the cylinder just for the heck of it but they couldn't be honed out FWIW.

Right now I'm thinking none of this is usable. Not the piston, not the liner, maybe not even the cylinder itself since it has a broken fin. Any feedback for me regarding this?

Also, do aftermarket forged pistons have a different piston to cylinder wall clearance to allow for more expansion? If so, does anyone know what that spec is compared to the stock piston to cylinder wall clearance?

Thanks,

Jon

[graeme]

Hello Jon,
Broken fins can be repaired by a good alloy welder.
Liners can be bored to the next over size, Ducati liners are very good quality.
Over size pistons are easily available.
Piston to bore clearence is usually measured 10mm up from the bottom of the piston skirt. (go by the piston manufacturers spec sheet for recommended clearence)
The bore should be the same all the way.

Graeme

[Rick]

Jon,
75MM was the largest oversize piston the factory sold. Guzzino shows a 75.5MM forged piston:
http://www.guzzino.com/dupiki25035n.html
0.5MM is less than .020", which is less than 0.010" removed from the liner wall- doesn't sound too scary to me, but others may have a different view.
The forged piston will be denser- the mfg will give you the recommended clearance.
I don't know where you live, but in the midwest USA, a good fin repair would probably cost $100, add in a new liner(you may get by with the one you have) and new piston, and waiting for a good used cylinder with a good liner on Ebay would sound good to me.
Rick

[DewCatTea-Bob]

By: guzzijon...
" There are some visible shallow pits in the bore, the shop teacher ran a hone through the cylinder just for the heck of it but they couldn't be honed out FWIW. "

____ So long as a good hone-job has been run-over the edges of the (supposed tiny) pits, they will not make a real notable difference in how the engine goes.


" Right now I'm thinking none of this is usable. Not the piston, not the liner,
Any feedback for me regarding this? "

____ It's all quite usable ! _ What's wrong with continued use of the piston (within the now honed cyl.bore) ?
__ While the Borgo-factory (who Ducati had their chosen pistons made by) only made over-bore pistons up to 1.0mm oversize, other piston-factories went beyond that point ! ...
As I've just recently stated (within another thread), the stock 250 cyl.sleeve can be safely bored-out to as much as 100-thousandths oversize (which is 76.54mm) !
__ You could go-ahead & install new parts and do all the associated refitting-work, but what you'd then have wouldn't run better or last longer SIGNIFICANTLY .
You wouldn't be able to tell the difference if you weren't aware of which set of parts were actually installed. _ So you may as well keep & use that which you have.
__ I happen to have NOS 75.25mm pistons still left in my stock of spares, which could be had for less than such listed elsewhere.
But I suggest you have fun running your Duke now (with parts you have), and put-off any actually unnecessary rebuild-work/projects for after the ridding-season.


" do aftermarket forged pistons have a different piston to cylinder wall clearance to allow for more expansion? If so, does anyone know what that spec is compared to the stock piston to cylinder wall clearance? "

____ Not much different, perhaps a couple thousandths.



Duke-Cheers,
-Bob

[guzzijon]

____ It's all quite usable ! _ What's wrong with continued use of the piston (within the now honed cyl.bore) ?

My concern is the piston to cylinder wall clearance. We measured it at between .150 and .180 (depending on where in the bore we measured) and the service manual says it should be .115.

__ While the Borgo-factory (who Ducati had their chosen pistons made by) only made over-bore pistons up to 1.0mm oversize, other piston-factories went beyond that point ! ...
As I've just recently stated (within another thread), the stock 250 cyl.sleeve can be safely bored-out to as much as 100-thousandths oversize (which is 76.54mm) !

That's good news! That means I can still use my cylinder sleeve then.

__ I happen to have NOS 75.25mm pistons still left in my stock of spares, which could be had for less than such listed elsewhere.

That might be a good way for me to go. I could just have my cylinder liner bored out to that size.

But I suggest you have fun running your Duke now (with parts you have), and put-off any actually unnecessary rebuild-work/projects for after the ridding-season.

Well, a couple things. The riding season is year-round here and this thing is like a basket-case that rolls - so many missing and ruined parts that the only thing that makes sense is to tear it all down, take inventory, and start all over again. But thanks, your info on the liners has given me a little more hope, one less thing to buy.

[DewCatTea-Bob]

" My concern is the piston to cylinder wall clearance. We measured it at between .150 and .180
and the service manual says it should be .115. "

____ Well an unused/unworn piston & cylinder-bore can indeed be at just the .115mm (.0045") clearance-amount, but acceptable limits of wear could be up near double that amount without leading to any real running repercussions, so your measured clearance which indicates merely .006 to .007" is only about a measly 2-thousandths (of an inch) greater, which is of no real concern to be worried about !
In-fact, except for some notable piston-slap, I've seen a Duke-engine still run well enough even though it was running a std.piston within a cylinder which had been bored-out to the 1st.oversize, thus it's cyl.piston-clearance was out past .016" , (which of-course wasn't good however) !


" - so many missing and ruined parts that the only thing that makes sense is to tear it all down, take inventory, and start all over again. "

____ Okay then, in that case you ought to FIRST acquire that which you REALLY need, and THEN next (later down the line), become concerned with tackling your matters of less importance.


Duke-Cheers,
-Bob

[wcorey]

A little story, not specifically Ducati but relevant nonetheless...

Some years back I bought a '72 commando with a frozen up motor (but in reasonably nice condition otherwise). The owner indicated that it hadn't seized while running but had sat for an extended period and then wouldn't kick over, turned out that somehow moisture had gotten into one cylinder and corroded around the upper rings. The chrome rings were still in decent shape as was the piston so I attempted to hone the bore until I was satisfied of it's smoothness. Unfortunately there was .009" of clearance when this was done, surely outside the wear limits, so I sent out another cylinder block I had laying around to be bored.
While waiting for the that work to be done I grew impatient and decided to re-assemble with the now oversize original jug just to see how it would run. I noticed no issues and continued to ride it in that configuration for many years and tens of thousands of miles. Eventually took it off the road in favor of other rides, 20 years later it still has that cylinder and the newly bored cylinder is still sitting on the Norton parts shelf to this day...'

Bill

[DewCatTea-Bob]

" A little story, not specifically Ducati but relevant nonetheless... "

____ And more relevant than most-any other example since the Norton's bore-size is nearly the same (only 1mm smaller), as a 250-Duke's.
__ With such excessive cyl.piston-clearance, I wouldn't have expected to get well-over 10-thousand miles of trouble-free engine-performance, but I'm not surprised that there's been little consequence (other than a little likely added noise).
So Bill's experience (at least in this regard) puts him in-the-know (along with few others), since most-all (so called) 'good' mechanics follow-the-book too closely to ever directly gain such experience themselves. _ And so the majority of such mechanic-types would tend to doubt such extremes and recommend the work required to stay within specified spec, (especially if they're charging you $$$ for the work-job!).


Dukaddy-DUKEs,
-Bob