250 street scrambler

[joe46ho]

Sam, If you recall I was the one that sold that MC piston to you a while back, it did not come with rings because i didnt have any, and when you asked about where to get a set, I told you to find a set of standard bore high comp borgo rings (which are also hard to find) just as bob has stated they are only .001" thinner on the compression rings, and .002" thinner on the oil rings. The reason you couldnt find any mc rings is because from what i understand they where JUNK. (even back when they where being made and sold people would substitute other brands of rings for the MC rings because the rings sealed poorly ) The pistons where nice, they where forged and machined here in the USA but the rings where either made in Japan or Taiwan and had a lot of sealing problems, furthermore other engine builders have noted that the side clearance of the MC rings in relation to the piston was too tight, and with any buildup at all the ring can freeze/stick in the ring land which further amplifies the oil burning problem that bob mentioned... So as far as i am concerned using a set of very slightly thinner borgo rings would be ideal, even if you had a set of mc rings and a set of borgo/AE rings i would still say to use the borgos...

One reason this is possible on your piston is because it is a standard bore piston (this would NOT work on a overbore MC piston if you where trying to use standard factory oversized borgo rings, NOR would MC rings work on a borgo piston)...In other words if you ever come across a non-standard bore 250 ducati MC piston the size is 74mm + XXX (in english measurement) so a 74mm-020 piston would require a 74.508 mm Bore size and you wouldnt find a set of borgo rings that bore size... this is kind of confusing... I have no idea why they did it this way.
But this really doesnt apply to you because you have a std. bore mc piston... (still just because it is "standard" do not assume that the new mc piston is 73.89mm (or whatever) in diameter, measure it to be EXACTLY sure what it is)

The most important thing is DO NOT ASSUME ANYTHING when building an engine (even if you had it bored, and/or honed professionally), my advice is measure your piston (at the proper location, with a micrometer, and NOT a caliper) and measure your bore (top to bottom, in line with the wrist pin, and also perpendicular to it) Determine your piston to cylinder wall clearance and if it is within the MC specs Then install the mc piston, with the borgo rings, and you will have no problems, as long as you measure, inspect, and assemble everything else correctly... (be sure to also check the wristpin to piston fit and correct if needed, and also the wristpin to connecting rod bushing fit)...The two people i have spoken with that have ran a lot of these have used MC's "racing" spec of .006"- .008" (or .152mm-.203mm) Piston to cylinder wall clearance...

I am also posting a pic of the original MC instruction sheet that came with their forged pistons like you have, if you cant see it well enough i can email you a better quality pic.

If you really dont want to use the mc and want to use an old worn 74mm borgo i have about 8 you can take your pick of for free... (i have no clue why you would want too) and i would send you a refund for the MC if you wanted to send it back to me.

Cheers, Joe

Here is the instruction sheet from MC

[Sam]

Hi Joe,

Cheers for clearing that up, from a low to high in one afternoon as I stated about the clearance earlier I thought it would be ok on a std bore but will check the piston / cylinder clearance before I build anyway. I was not wanting to use an old piston seeing as I have replaced everything else it would have been the wrong compromise.
Sam

[DewCatTea-Bob]

" engine builders have noted that the side clearance of the MC rings in relation to the piston was too tight, and with any buildup at all the ring can freeze/stick in the ring land "

____ I can confirm that I've come-across a number of MC-pistons with their rings stuck-tight within their ring-slots.
The pistons are good but, their rings are indeed quite poor !



" One reason this is possible on your piston is because it is a standard bore piston (this would NOT work on a overbore MC piston if you where trying to use standard factory oversized borgo rings,
...In other words if you ever come across a non-standard bore 250 ducati MC piston the size is 74mm + XXX (in english measurement) so a 74mm-020 piston would require a 74.508 mm Bore size and you wouldnt find a set of borgo rings that bore size... this is kind of confusing... "

____ What's being stated here is that once ya get into overbore-sizes, the over-sized pistons & ring-sets are not STANDARDIZED in identical increment-jumps (between all manufactures), and so therefore over-sized ring-sets are then of-course not naturally interchangeable (as-is).
However, it's been a fairly common practice to actually possibly prefer p.rings which are the NEXT increment-size larger, so that the ring end-gap can then be ground-down to achieve the desired gap for the cyl.bore that's being used as it happens to be bore-set at. _ Or likewise,, if it just happens that the next-sized ring-set is ALL that happens to be available for YOUR piston-size, then ya 'adjust' THOSE rings to fit your cyl.bore (by grinding-down the resulting end-gap to suit).
I-myself have done this ring-size adjustment many times, and have heard of others who did-so even with rings that were TWO incremental-steps larger !
So with the previously given-example for instance,, a 20-thousandths overbore-piston with a size of 74.5mm, could quite EASILY accept Borgo's 2nd.over/74.6mm rings, by simply grinding the end-gap (about .3mm) to achieve the desired p.ring end-gap ! _ And THAT example is a rather SIMPLER-one to achieve, since the adjustment happens to be even less than one-whole incremental-step, (which is what I've always preferred when faced with such an issue) !
__ For any of us who are willing to accept such ring-resizing work, I still have a fair selection of Italian-made 3 & 4 ring-sets in incremental-jump sizes of .25mm up-through 75.25mm.



" I am also posting a pic of the original MC instruction sheet ,
if you cant see it well enough i can email you a better quality pic.
Here is the instruction sheet from MC "

____ I've adjusted that pic (as now seen below), so that it can be seen better up-close.


Duke-Cheers,
-Bob

[Sam]

Cheers that will save me on borrowing Christinas glasses..

[StewartD]

Hi Sam,

Great to see such good photos of your project under way. Its always interesting for others to see the methods that are used and it really boosts the motivation of many others to get into their own projects.

Hope you don't mind a comment on the engine bolts. You say you are using 8mm stainless steel bar to fabricate new ones. I think the stainless will be a lot weaker than the original bolts. I had a 1974 disc brake 450 Ducati which I got rid of in 1990. I remember it had strength grade markings on its engine bolts of either 8.8 or 10.9. These markings indicate either 640 MPa or 900 MPa yield strength bolts. Sorry I don't remember the strength marking, maybe someone could check a late model. I don't think the earlier singles would have had these markings as they are an ISO standard.

A brief explanation of strength grade markings: The strength grade 8.8 means two things; the first 8 means that the Ultimate Tensile Strength (U.T.S.) is 800 MPa, and the .8 means that the Yield strength is 0.8 x U.T.S. or 0.8 x 800 = 640 MPa. Similarly 10.9 indicates 1000 MPa U.T.S. and 900 MPa Yield strength. Low strength or commercial grade bolts are grade 4.6 which is 400 MPa U.T.S. and (400 x 0.6 =) 240 MPa Yield strength.

For comparison ordinary hot rolled structural steel is generally 250 MPa Yield strength. These are the angle irons, channels, I beams or (R.S.J.s) and I columns used in large scale construction. Square hollow sections are available in 250 and 350 MPa yield strength steel.

The yield strength is the figure that we should be considering. If a bolt is torqued up to a level under the yield stress it will stretch slightly. Once the bolt is undone the bolt will return to its original length. The steel has a certain elasticity if it is not taken beyond the yield stress or the elastic limit. If it is taken beyond this limit it yields and will not return. It is the elasticity of a correctly torqued up bolt that keeps the engine plates pressed up against the crankcase bosses with considerable force. This force would be lost if the bolts elastic limit was exceeded and the bolts were permanently stretched; that is they yielded.

Anyway, back to your project: Stainless steel of 316 grade is listed at 515 MPa U.T.S. and 205 MPa yield strength. This means your bolts will be weaker than even commercial grade bolts (240 MPa Yield strength). Stainless steel socket head cap screws listed in the Unbrako catalogue have only 550 MPa U.T.S. The yield strength is not listed.

If you go ahead with the stainless steel bolts it might pay to make up an extra bolt and torque it up till it fails. Note the torque achieved before failure and then use, say about 3/4 of this torque on your engine bolts. Then keep your eye on them for a while. As yours is a 250 motor with wide crankcases you may get by with the stainless steel bolts.

The wide case Ducatis are a lot stronger design for the rear engine mounting than the narrow crankcase models. The narrow c/c models got by with 2 M8 rear engine bolts and the drive chain load was overhung in relation to the back engine bolted surfaces. The wide c/c models have M10 rear bolts and the drive chain loading is within the bolted surfaces. This means that the drive shear loads at the engine c/c bosses is a lot lower, and the larger bolts is an added bonus.

For anyone with a 350 or 450 wide c/c I would avoid using any bolts of lower strength grade than standard due to the higher torque that these engines will produce. Higher torque of the motor gives directly proportional higher chain tension loading. Also I wouldn't go to a lower strength bolt on any narrow c/c model due to the marginal design of its rear engine mount.

When I was 18 (in 1973) I lost the rear engine bolts of my Mach 1 and replaced them with hardware shop bolts. They were probably grade 4.6, but back then I had no clue. Any way, they loosened up, the engine moved about and the crankcase bosses were badly worn and the frame lug holes were elongated. This of course created a few problems in the recent restoration.

Good luck with the rest of your project, keep the photos coming.

Cheers,

Stewart

[Sam]

Hi Stewart,

Long time in replying I will keep an eye on the engine bolts cheers for the heads up but as you say not much power in the wee 250, Just pulled this project back out into the open, I have had a good run with work and think I see a wee opening in the work schedule to give this some attention by the end of November. Does or has anyone fitted a high level exhaust to the wee Ducati and have any pics they could post up, I think this would be a better looking setup for this style of bike thank you in advance.
Sam

[Sam]

Surprised no one has made or put on a high level exhaust to the scrambler, I was thinking something along the lines of the BSA and Triumph B25/50 ss range, pipe along and over the timing side casing and a wee reverse cone megaphone!

[graeme]

Hello Sam
Maybe an RT header might fit?
Graeme

[Sam]

HI Graeme,

I have done a search looks like a lot of money for not a lot of toy, one thing what dia is the 450 header 1 5/8 or 1 3/4? that would be larger than the smaller motor which I think is 1 1/2"

Sam

[graeme]

Hello Sam,
The header on my RT is 38mm OD. 1 1/2
Same as my 450

Regards
Graeme