Piston parts

[Bevel bob]

HI, From memory only I think the piston should have about three and a half thou clearance in the bore,measuring the thrust faces of the skirt however that's for a Borgo forged piston an Asso may be different its a pattern piston ok for pootling around but I would not use for a perforemance motor., there will be sideplay on the rod, but I don't have the figure at present.Best to have the crank stripped to check the rollers and tracks and clean out the sludge trap.

[double diamond]

If your cylinder shows fresh cross-hatch honing marks and no evidence of any burnishing of the hone marks by the piston/rings, then it may have been bored/honed and not run yet. Of course someone could have also run a ball hone through it to ”freshen it up” i.e. remove the rust from the bore, in which case the bore may be worn. You need to measure the bore with a bore gauge to determine if it’s worn. If the piston “seems loose” then it is loose. You shouldn’t be able to discern any clearance between the bottom of the skirt and the bore by just a visual check (you will be able to see clearance at the top of the piston since the piston is tapered). If you can put the piston in the cylinder and it rattles around then it’s too loose. Installing the rings will not improve this condition. If you don’t have access to a bore gauge and micrometer, a caliper that reads in 10ths of a millimeter will give a good rough estimate of the size of the bore and the piston. A “fair amount” of side to side rod play is pretty loose. In my experience, most basket case Ducati singles have worn out rods, it’s just a matter of degree. If it’s really worn you’ll notice radial play in the rod bearing. To check this, position the crank with the rod big end at 12 o’clock. Then, rotate the rod to about a 1 o’clock position and push/pull the rod up and down. Do the same check with the rod at 11 o’clock. If you notice even the slightest up/down movement of the rod at these positions (give or take) then the rod/pin/bearing is really worn. Running a loose big end will eventually lead to the aluminum bearing cage breaking up.

[single-junk]

If you can put the piston in the cylinder and it rattles around then it’s too loose.

Ok.. so if my piston is a 74.25 Asso, and it's pretty sloppy in the cylinder, then I guess perhaps someone either: really screwed up this cylinder, or, bored it to a much larger size..? If it's an actual legitimate larger size I'll be pissed that the previous owner didn't include the larger piston!

I'm going to give my cylinder to a machinist friend of mine so he can measure the bore.. hopefully get some answers.

[DewCatTea-Bob]

[quote= single-junk ...
" so if my piston is a 74.25 Asso, and it's pretty sloppy in the cylinder, then I guess perhaps someone either: really screwed up this cylinder, or, bored it to a much larger size..? If it's an actual legitimate larger size I'll be pissed that the previous owner didn't include the larger piston! "

____ Well you did get TWO 250-cyl.jugs from the same source, correct ?
So perhaps you have that piston matched together with the wrong cyl.jug. _ So you better have BOTH of their bores measured.
__ Otherwise, I'd suspect that the cyl.sleeve was possibly bored-out for a 74.4mm Borgo-1st.over, rather than the 74.25mm ASSO-1st.over piston,, which would make the bore-job .15mm -(.006") TOO-large.
(I've before found a concocted 250-engine which was still runnable with it's installed piston which was TWO-sizes -(about .016") too small for the bore-size of the cyl.jug it was matched-together with.)


Hopeful-Cheers,
DCT-Bob

[single-junk]

[quote= single-junk ...
" so if my piston is a 74.25 Asso, and it's pretty sloppy in the cylinder, then I guess perhaps someone either: really screwed up this cylinder, or, bored it to a much larger size..? If it's an actual legitimate larger size I'll be pissed that the previous owner didn't include the larger piston! "

____ Well you got two from the same source, right É
So perhaps you got-it together with the wrong cyl.jug. - So better have BOTH of their bores measured.
__ Otherwise, I'd suspect that the cyl.sleeve was possibly bored-out for a Borgo-1st.over, rather than the ASSO-1st.over piston,, which would make the bore-job .15mm -(.006") TOO-large.
(I've before found a concocted 250-engine which was still runnable with a piston that was TWO-sizes -(about .016") too small for the bore-size of the cyl.jug it was matched-together with.)


Hopeful-Cheers,
-Bob

That was also my thought, so I am going to get both measured. Unfortunately the 2nd cylinder isn't in quite as good of shape.. kinda rusty/ugly. If it does match my piston, then I guess I'll have to see about getting it cleaned up.

[single-junk]

Had the measurements done finally, supicions were correct unfortunately.

Piston is definitely 74.25, but my cylinders are 74.5
I guess the original owner misplaced them and had a 74.25 around previously and threw it in the box.

Time to find a new piston kit I guess.. wasn't really looking forward to it as they're fairly pricey.

[DewCatTea-Bob]

[quote= single-junk ...
" Piston is definitely 74.25,

____ Does that piston happen to be in like-new condition ? _ If so, then I may take it in trade towards one of my NOS.pistons,, and if it's merely usable, then I could possibly swap you a used-piston which you could use with one of your current cyl.jugs.



" but my cylinders are 74.5 "

____ Are you certainly-sure that BOTH of your cyl.sleeves are exactly that same 74.50mm size ? _ (As I'd assume that one of them is likely bored slightly larger.)
Assuming that both have the same "74.5" sleeve-bore,, then your best cyl.jug ought-to be matched together with an ASSO-2nd.oversize, or a 20-thousandths -(.020") oversize MC-2nd.over (or the like, in some other brand),
while your dirty/corroded cyl.jug probably ought-to have it's sleeve-bore honed-out to accept a 74.6mm Borgo-2nd.oversize piston.



" Time to find a new piston kit I guess.. wasn't really looking forward to it as they're fairly pricey. "

____ Whatever new-piston listed price you've already found anywhere (which I can also check-out anywhere on the web-net), I'll try to beat it by at-least 10% (total, with shipping-cost) for you !


Hopeful-Cheers,
-Bob

[single-junk]

[quote= single-junk ...
" Piston is definitely 74.25,

____ Does that piston happen to be in like-new condition ? _ If so, then I may take it in trade towards one of my NOS.pistons,, and if it's merely usable, then I could possibly swap you a used-piston which you could use with one of your current cyl.jugs.



" but my cylinders are 74.5 "

____ Are you certainly-sure that BOTH of your cyl.sleeves are exactly that same 74.50mm size ? _ (As I'd assume that one of them is likely bored slightly larger.)
Assuming that both have the same "74.5" sleeve-bore,, then your best cyl.jug ought-to be matched together with an ASSO-2nd.oversize, or a 20-thousandths -(.020") oversize MC-2nd.over (or the like, in some other brand),
while your dirty/corroded cyl.jug probably ought-to have it's sleeve-bore honed-out to accept a 74.6mm Borgo-2nd.oversize piston.



" Time to find a new piston kit I guess.. wasn't really looking forward to it as they're fairly pricey. "

____ Whatever new-piston listed price you've already found anywhere (which I can also check-out anywhere on the web-net), I'll try to beat it by at-least 10% (total, with shipping-cost) for you !


Hopeful-Cheers,
-Bob

Apologies, I believe I screwed up on the conversion.. my machinist friend gave me the measurements in inches..
I went over the math (using an online calculator this time!)

The piston is stamped 74.25, but it's worn down to 73.99 (2.913 inches) It's very clean though, I could post a pic if you like..

The good cylinder is at 74.42, the bad cylinder is at 74.58 (that's on a rough surface though, honed down I guess would be prob much bigger I'm guessing..

Would you say my choice the would be to either hone the good cylinder up, or else get a 74.4 piston? Or do something with the bigger cylinder?

I really jumped into the deep end getting into this Ducati thing didn't I? ha ha

[ecurbruce]

Bob,
You have a PM

Bruce

[DewCatTea-Bob]

[quote= single-junk ...
" The piston is stamped 74.25, but it's worn down to 73.99 (2.913 inches) "

____ Pistons are actually rather oval-shaped and-so your measured "73.99" figure was probably taken from side-to-side above the piston-pin's hole, rather than 90-degrees around where the max.diameter-measurement should really be taken from (against the front-center to back-center of the piston-skirt, below the oil-ring's groove). _ Where THERE, the piston's true diameter-measurement ought-to be found as ever so slightly less than the stated '74.25' dia.size.



" The good cylinder is at 74.42, "

____ That's pretty-much the correct dia.size required for a '1st.oversize' from the BORGO oversize-range,, and when your cylinder's bore-job was done, there was probably some confusion over the particularly required bore-size for it when it was bored-out for a '1st.over' (which actually wasn't a stock/Borgo-1st.over piston, such as your ASSO-1st.over happens to be).



" the bad cylinder is at 74.58 (that's on a rough surface though, honed down I guess would be prob much bigger I'm guessing.. "

____ THAT cylinder is probably going to be good-to-go with a 2nd.over-Borgo (74.6mm) piston, if merely honed by a hand-drill held honing-stone tool, which doesn't really increase (the non-corroded) bore-size significantly.
To make the bore-size "much bigger" by 'honing', then a heavy-duty type 'machine-hone' hone-job* would be required, (* which is an alternate operation option instead of a 'bore-job' for rather smaller increases in bore-size).



" Would you say my choice the would be to either hone the good cylinder up, or else get a 74.4 piston? "

____ Since (I think) you've mentioned that your best/"good" cyl.sleeve is already bore-finished,, then my suggestion for your first-choice, would be to have it lightly re-honed (possibly with merely a hand-drill held ball-stone type deglazing-tool), so as to properly fit with a 74.4mm-piston.
Second-choice would be to do a heavy-duty machine-type hone-job to make it properly fit with a 74.5mm-piston.



" Or do something with the bigger cylinder? "

____ Third-choice, would be to simply hone (with hand-drill driven flat-stones type honing-tool), your corroded cylinder so as to cleanly fit a 74.6mm-piston,, while fourth-choice would be to do a heavy-duty/machine-type hone-job, so as to properly fit a 74.75mm-piston.
__ Of-course fitment of piston-sizes of 74.8mm & larger could also be done as well, but possibly rather by 'boring-out' the cyl.sleeve's dia.size, (which is a more expensive operation than mere honing-out).
Your bottom-line/final-choice ought-to somewhat fairly depend on what replacement piston you can acquire most economically.



" I really jumped into the deep end getting into this Ducati thing didn't I? "

____ I-myself really don't think so, (unless you also have other parts issues which happen to be considerably more bothersome).
____ Besides NOS.piston-kits in a number of various over-sizes (up to 75.25mm), I also have quite a few used/usable bare-pistons (probably including the 74.4 & 74.6mm sizes), which I may consider swapping for your 74.25mm-piston, (with YOU paying for ALL shipping-costs both ways).
I also have new ring-sets which could be fitted to the bare pistons, but you'd need to install new circlip/p.pin-retainers.


Hopeful-Cheers,
DCT-Bob