What rings do I need, weird 250.

[Rbranst]

Hey gang- I have a 68 250 Scrambler and a 72 Mototrans 250. I am just digging into Ducati's as restoration projects.
I started with the 1968, which I bought pretty much as you see it. Cylinder and head were off, head was fresh back from what was supposed to be $1500 worth of head work. I started to dig into this bike and have all of the manuals. I was suprised to see the 8 fin cylinder on this bike had a 76mm sleeve and the piston measured 75.68 below the rings. Seems like a 350 sleeve? Once I cleaned up the piston I can see what is either 06 or 90 on the top. I am confused about what i have. it’s def a 76mm sleeve in a 250 8 fin. Since I am new I am not sure what the piston actually is and I have no idea what rings to buy. The oversize stamping on the piston is throwing me and the fact that even though I have 3-4 workshop manuals I have no idea what I am looking at.
I have found a couple of ebay postings with this particular piston style with old 250 barrels.


I have an icloud album below. Has a Japanese 21" rim on front, headlight is wrong, tailight is wrong, fenders are wrong. Are the forks correct?
I do have the original Raedelli front, I am curious if front hubs are correct. Will post the 72 Mototrans seperately.

https://www.icloud.com/sharedalbum/#B16GI9HKKG7P6Ln

IMG_1787.jpeg

[Eldert]

your piston is a M-C piston made in LA

forks and triple clamps are correct for a Scrambler and so are the hubs .

mc piston.JPG

Eldert

[Rbranst]

Thanks Eldert- I have an MC Supply ad, I don’t think they made a 350 Ducati piston doesn’t list a 350 Duc piston . BSA and Triumph yes.
What do you make of the oversize markings .06??
Sure almost seems like a 75mm .06 over in a 76mm sleeve.
Do I buy the rings to match the bore size in this case ? Still not sure what to buy since the cylinder measures 76.

[Duccout]

I have heard of pistons from a 500 Triumph being fitted to 350 Ducatis, and that piston sure looks like a Triumph piston.

Cheers,

Colin

[Rbranst]

IMG_2011.jpeg

Triumph 650- 71mm
Triumph 500 - 69mm
BSA 650 A65 - 75mm

[Duccout]

Triumph 750 - 76mm.

[Rbranst]

Ok, so I feel so dumb. I have all these factory workshop manuals and I have been thinking about this way too hard.
The piston has an 06 stamping on top. I check the books, 2nd rebore is .6 - but why is this piston measuring 75.68 and not 74.6?
I finally realized thanks to the MC supply add I have, its a .060 oversize, in inches not MM. So 74 + .060" over = 75.524. That makes a lot more sense.
That said, I only have vernier calipers and can only measure the top of the bore = which is coming up 75.96" Measuring piston bottom around skirt I am closer to 75.92mm. So I am now confident that its an MC Supply Ducati 250 piston .060" just like it says.
So I feel a whole lot better, but still unsure how to size the rings for this.
Do my rings need to match bore size or piston size? Am I measuring the piston in the right place?

[Duccout]

Well, normally you would buy a ring set to match the piston, say + 1st oversize, +2nd oversize, so they would match the piston, but you have a non-standard piston that may be hard to find rings for. I think that you will have to measure the width of the ring grooves in the piston and look through some ring catalogues. The USA is the home of the automotive industry so there must be someone who has rings that will fit.


Cheers,

Colin

[Rbranst]

Thanks Colin

[double diamond]

Look on the Hastings piston ring website. They list rings by bore size and ring width. I have found rings for many unusual pistons. If Hastings shows no stock, look on ebay for the application Hastings shows, you’ll probably find what you need.
MC Supply sold the MC brand pistons. A catalog I have shows Ducati pistons. They are forged pistons made in Japan. Actually a pretty good piston. I’ve found them in many Ducatis I’ve disassembled. Common in the US.