250 street scrambler

[Sam]

Hi Bob,

I should have been more clear in my post, I had all casings bolted together with outer casings attached with odd screws I had lying around and all holes blanked off with alloy plugs, blue tack or duct tape.
I ran a tap in all the threaded holes to remove any dirt that had accumulated with the casings lying around for30 odd years.
I have a cannister with an airline and blowgun attached (like a paint spray gun with a long nozzle) that was filled with cellulose thinners, this was then blasted into the threaded holes and casings washed down.

[DewCatTea-Bob]

" I had all casings bolted together with outer casings attached with odd screws I had lying around and all holes blanked off "

____ Good-job Sam, (some others have not been so thoughtful [before going-ahead with blasting-work]) !



" I ran a tap in all the threaded holes to remove any dirt that had accumulated with the casings lying around for30 odd years. "

____ That's a good work-habit in any case, since a fairly high percentage of previous owners in the distant-past were likely not very careful to prevent dirt from getting into the threaded-holes (from the Allen-screws often being left rested-down on dirty surfaces).



" I have a cannister with an airline and blowgun attached (like a paint spray gun with a long nozzle) that was filled with cellulose thinners, this was then blasted into the threaded holes and casings washed down. "

____ It's always refreshing to learn of a fellow-mech who actually cares to do such good/clean work !
It's so very nice to feel a screw fully inserting into it's hole without a single sign of any gritty-feeling rotation-resistance !
(Anyone who doesn't really give a $#!+ about such, is not MY kind of people !)
Hopefully, more people like yourself will salvage more DUKEs !


Duke-Cheers,
-Bob

[Sam]

Thank you for the encouragement, my day of building motors in the coal shed are thankfully long gone...!

[Sam]

I did not think I would have to scoure so far to find parts,

Rings found in Australia


Piston found in the States

[amartina75]

Sam,
I hate to be the bearer of bad news, but you cannot use Borgo rings on a MC piston.
Someone on here might have a Borgo piston you can use or you can get MC rings from DomiRacer i think,
they do show up on Ebay from time to time also, DomiRacer has some MC pistons and rings and they also
have some ASSO Pistons I think, they are about the cheapest you can get besides the MC.
You will probly have to call them to find what you need.
My brother has had a few MC pistons and there are a few things you should be carefull of,
like the min. clearance is double, I think, compared to a factory piston,
and they do not go up in standard oversize, they are American standard.
If yours did not come with a spec sheet my brother can probly help you out with that.
I'm sure there are others that know more then me about this but I wanted to let you know as soon as I saw
you problem. so you can get it straightened out,

[Sam]

Hi thanks for the warning,
I was told the piston used std Ducati rings when I purchased it. I take it you are talking about the piston bore clearance? this is to be fitted to a std bore 74mm so hopefully that will be ok but a size to check would be nice.
I have been warned to check the clearance between the valve and pocket in the piston as well any further info would be appreciated. Cheers again.
Sam

[DewCatTea-Bob]

" but you cannot use Borgo rings on a MC piston. "

____ There is indeed a difference in thickness, but there have been those who've gone-ahead & fitted Borgo-rings onto MC-pistons anyhow. ...
__ Borgo made two different sets of piston-rings for 250Duke-models... The '4-ring set' has the thickness of it's two comp.rings being 2mm, while it's pair of oil-rings are each 2.5mm thick.
Their higher comp.piston-models employ '3-ring-sets', of which it's comp.rings are 1.5mm thick, while it's oil-ring is 3mm thick.
__ The MC-ring sets are also 3-piece sets, with their comp.rings being .06-inches thick, while the oil-ring is .12-inch thick, and-so that makes them merely 1.6% thicker than Borgo's 3-ring set. _ So since MC-pistons are machined to accept ever-so-slightly thicker p.rings, the slightly thinner Borgo-rings can thus possibly be fitted into a MC-piston's ring-slots.
It's been since the '70s since I've actually measured the actual thicknesses of all these various p.rings with a micrometer, but the Borgo-comp.rings should be only .001" thinner (at .059" thick), while their oil-ring should be only .002" thinner (at .118" thick). _ So while not a perfect-fit, the Borgo-rings could indeed be fitted to the MC-piston !
While so-equipped engines will run well enough, I however can't confirm that such a parts mismatch-up will last as long as normally expected.



" you can get MC rings
My brother has had a few MC pistons and there are a few things you should be carefull of, "

____ One notable-thing about MC-rings is that at-least their oil-ring is well-known to never become broken-in & seated and-thus forever SMOKES ! _ And I know-of & heard-of many MC-piston equipped Duke-engines which were destroyed due to oil-levels becoming too low because of the EXTREME rate at which the motor's oil was burned-up !



"and they do not go up in standard oversize, they are American standard. "

____ Those MC-pistons were available in 11 various dia.sizes from "0.0" to "100" (as seen stamped in their crowns), in .010" increments ! - (I've personally handled [at one time or another], all 11 different MC-piston bore-sizes !)
That makes their largest 250 bore-size 76.54mm !
__ Borgo-pistons were incremented at .2mm (.008") steps (starting at .4mm oversize), and did-not exceed 75.0mm !



" I'm sure there are others that know more then me about this but I wanted to let you know as soon as I saw
you problem. "

____ It's always GOOD for our w.site whenever others of us chime-in with anything that's questionable, like this !
__ In the reverse case, MC-rings would be too tight of a fit within Borgo-piston ring-slots.


Hopeful-Cheers,
-Bob

[Sam]

Hmm! anyone have a std 74mm borgo piston for sale then...?

Sam

[DewCatTea-Bob]

" anyone have a std 74mm borgo piston for sale then...? "

____ I have a whole-bunch of used Borgo-pistons !
Are you wanting 4-ring or 3-ring versions ? _ Four-ring pistons are either in 8:1 or 9.2:1 comp.ratios.
__ What's wrong with your original 250-piston ?


Hopeful-Cheers,
-Bob

[Sam]

Hi,
It would be a three ring and high comp if poss. the original piston was damaged it looked as if it had been dropped the ring groove was damaged I welded it up and re machined it but thought I have spent so much money re-building the motor why take a chance. It had also been lying around in a damp shed since 1980.