piston and rings

[ajleone]

I can't believed this happened twice.

I have 2 125 Broncos that I am working on. One engine was running and the other one had been disassemble, cleaned and reassembled with not much "known" about it from the guy I bought it from. A month ago, I removed the heads on both, inspected the piston, rings and con rod play - everything seemed ok, so I put them back together. Yesterday, I thought "let me take one more look before I put both engines back together and back on to the frames". So I pulled one head and cylinder off - still looks ok and then went to the other. WHen I pulled the cylinder off the piston, the lower ring fell apart and a few bits dropped down into the engine case ! I was able to retreive most of the pieces and then removed the oil sump and fished out a few more pieces - I think I have all of them. So then I turned to the other engine, and when I picked up the piston to check the play, the lower ring fell apart on that one too !!! I love making work for myself. with flashlight and tweezers I think I got all of the pieces out on that one too, I hope.

This brings me to the question of where to get a lower ring and what size. Both pistons seem to have the same number stamped on the top:

55 2-63523/36 I know 55 stands for 55mm, but have no idea with the other numbers mean. It is a little ironic that both engines have the same stamping ??

I used my el-cheapo mic and they both measure 55.15 -ish

I was told that Domi racer has rings, but on their website I only see 98 Bronco rings listed.

ANy ideas ? Should I just forget the lower ring and put these things back together before something else breaks !

Tony

[captpaul]

I don't know with out looking all over my piston ring boxes, for an oil ring for you, as I am restoring a bronco also.
No I don't have any,Part #s for your 4 ring piston, rings 0250.47.493-standard-- o,4 oversize 0250.47.593-- 0,6 over size 0250.47.693-,
0,8 oversize0250.47.793-- 1mm oversize, 0250.47.893!
So if you can correctly measure your piston/cylinder bore. these numbers will get you the correct rings. If its hard to find your correct size ring set, you can buy next size larger and file fit to your cyl size, It's not kosher, but over the years some times it was the only I could finish a resto.Does your piston say "Borgo" or"Asso" as the # i gave you was factory "Borgo" not an Asso.
Capt Paul

[ajleone]

Capt Paul,
Thanks for the info. The one piston is stamped borgo and I cannot see what is stamped on the other.

Do you know the original size piston diameter for the 125 Bronco ? Mine both seem to be about 55.15 ish.
Tony

[captpaul]

I can't tell you from here but it has to be measured with a metric caliper(digital is much easier) A 6 inch metric/inch is about 12-15$ at Harbor Fright.You need one to measure every thing from valve size to port size ect.makes part measuring a snap.

[DewCatTea-Bob]

" Do you know the original size piston diameter for the 125 Bronco ? Mine both seem to be about 55.15 ish. "

____ Both pistons must be original/stock parts, since the 125's factory-bore was 55.2mm, (not 55.0mm as ya would assume).
Pistons are actually (ever-so-slightly) 'oval' shaped, and so their 'size' should be read at 90-degrees around from the ristpin-hole & about 1/8th" below the upper oil-ring .
__ You just need 1 std.ring-set, then you could put your 2 used oil-rings on the bottom-slot of each piston, since that lower location is not as important to have a oil-ring on.
And if you can't find a new ring-set, then don't worry about having the lower oil-ring, as they'll run okay without.

____ Everyone who pulls the top-end from a DUKE should know to stuff a rag around the con.rod as soon as possible, before completely pulling the cyl.jug all the way off & away from the piston. - (Not just to keep things from falling-inside, but also to keep the rod & piston from clanking against the motor-case.)


DUCATIly,
DCT-Bob

[DesmoDog]

It looks like Domiracer has oversized rings for the 125 but not the std size.

Here's a hint for dealing with them - they upload a pdf price list every week or so. Download that and you can search by part number. Type in the part number (no spaces or dots) and it will turn up with a price and availability if they carry it. I'm amazed at some of the stuff they have BTW.... but don't buy any of the stuff I want because a lot of this seems to be very limited supply!

I've forgotten offhand where the price list is on their site. It's a small link in the middle of some text in the contact us section? Ordering info section? I'll have to look for it again and update the post...

[Pete]

Here is a 2nd oversize piston with rings on Ebay

http://cgi.ebay.com/ebaymotors/NOS-DUCATI-MOTORCYCLE125cc-SINGLE-CYLINDER-PISTON-10-1_W0QQcmdZViewItemQQhashZitem58859dfddeQQitemZ380198845918QQptZMotorcyclesQ5fPartsQ5fAccessories

[ajleone]

Craig and Pete - Thanks for the tips.

Sorry Craig, but Domi racer will have one less .4 over ring sets less today. I think I will go this route and file down the end .4 mm - Does this sound Kosher ?

BTW - The correct original piston size for the 125 Bronco is 55.2 so it appears that both pistons I have are original.

[DewCatTea-Bob]

" Here is a 2nd oversize piston with rings on Ebay "

____ That aftermarket-piston's box-label has been 'hand-marked' = " 2 ND oveR ", and thus the auction-listing states it to be the "2nd oversize"... I would not trust that to be actually true, cuz the size that's 'factory-printed' on it's box indicates it to be a full 1mm over-size! - (Which should equate the '4th' over-size.)


Cheers,
DCT-Bob

[DewCatTea-Bob]

" I think I will go this route and file down the end .4 mm - Does this sound Kosher ? "

____ No, not really! ... If you're getting a ring that's for a 55.6mm piston, then you'd have to file-off about 1.2MMs -( 47-thousandths / .047" , total) off the tip-edge ends of the ring. _ (That's a lot!)
If you do go-ahead & try that idea, you should file-off a little at a time, until you've attained the correct 'end-gap' (when placed into the cyl.bore). _ (However that's not really as important as with a compression-ring.)
I myself wouldn't bother to do all that messing-around, since the lower oil-ring really isn't needed.


GoodLuck,
-Bob