Squish band measurement, 250 mk3 engine

[vmoto3]

Any Ducati squish band experts out there? I am assembling my narrow case 250 (mk3) engine that has a stock cyl. head and an nos Ducati 74mm 3-ring (Borgo) piston. The piston may have been made for the scrambler motor as I had to have the valve relief pockets cut-out to accept the larger mk3 valves. Numbers on top of piston are : 74 .64272/5 . When measuring the squish band, using the wire solder method, I note .033" at the very outer circumference of the piston, but this tapers down to .012" further in (towards the piston's center). The measurement of .012" represents the area of the piston that mates up to the "shelf" on the cyl. head, which is where the hemi-shaped combustion chamber ends and the squish band area of the head's casting begins. The total width of the squish band area measures about 8 mm. So the squish band tapers, like a wedge, from .033" to .012". Is that normal, or should the squish band measure out uniform (.033" in my case) for its entire width? Any input would be greatly appreciated. Thanks.
Mike

[DewCatTea-Bob]

[quote= vmoto3 ...
" Any Ducati squish band experts out there? I am assembling my narrow case 250 (mk3) engine that has a stock cyl. head and an nos Ducati 74mm 3-ring (Borgo) piston. "

____ Look-over the thread found with this link: viewtopic.php?f=3&t=419
and see if all your related questions are covered there.



" The piston may have been made for the scrambler motor as I had to have the valve relief pockets cut-out to accept the larger mk3 valves. "

____ Narrow-case Scr.pistons were all 4-ring type (with a second oil-ring located on the bottom-half of the piston-skirt) !
So rather, you must've acquired a pre-1965 / Mark-III piston (which was made to be matched with the Motocross-type cyl.head which had valve-seats intended for the standard-sized 36 & 33mm valves.
Your particular n-c.cylinder-head with 40 & 36mm valves, is actually a Mach-I type cyl.head (which became standard-issue on Mark-3 models starting with the 1965 model-year).


Duke-Cheers,
-Bob

[vmoto3]

Thanks for the info. on the piston...now have one less unknown! Unfortunately, however, the thread didn't address the issue of whether or not it's normal for the squish band to be tapered (in my case, from .033" down to around .012" If it's not, I'm thinking that the piston isn't "matched" for the head and thus shouldn't be used. Your thoughts?
Mike

[DewCatTea-Bob]

[quote= vmoto3 ...
" the thread didn't address the issue of whether or not it's normal for the squish band to be tapered "

____ I was fairly sure that that-particular detail had been brought-up once before (and that it was probably covered within that thread), but perhaps I was wrong.
__ If you care to thoroughly check for yourself,, then you could try doing a 'search' with the words squish band, to find a listing of all thread-posts which include those words, (so as to hopefully find anything related to your particular inquiry).
If you only enter just the word: 'squish' (into the search-box [as seen at the top-right of this page]), then you'll (currently) find just 66 thread-posts which include that word, (and if I'm not mistaking, I'm still thinking that one [two at most] of them had indeed mentioned something-or-another about the squish [pinch-width] not being exactly uniformly tapered, [although I don't recall it being quite as great of a discrepancy-range as you've found]).



" I'm thinking that the piston isn't "matched" for the head and thus shouldn't be used. "

____ If your piston & head are both stock,, then your concern should be unwarranted, as the cut-angle of the squish-band area is the same for both the Mark-III and Mach-I/(Mark-3) cyl.heads ! _ (So I assume that the squish-band angle of an actual Mach-I/[Mark-3]-piston would be the same as that on your Mark-III piston, anyhow..)
__ When you measured your squish-clearance,, what did you then have installed between your cyl.jug-base and the motor-case ? _ Just the original/used paper-gasket, or what ?
____ BTW, your 'Mark-III' piston originally had a 10:1 comp.ratio,, but now that you've further cut-out it's valve-pockets, it'll be lucky to still provide the 9.8:1-CR of the stock Mach-I/Mk3-piston. _ (So you may not wish to install a thicker base-gasket under the cyl.jug to increase your squish-clearance.)


Hopeful-Cheers,
-Bob

[LaceyDucati]

Mike,

The squish should be ideally parallel or slightly tapered the other way, ie. tighter at the edge and getting larger towards the centre of the chamber. A 250 should be 45 degrees or there abouts as are all 350's apart from the 350 sebring which is 40 degrees. The only possible reason I can think of the scenario you describe is you may have a Sebring head on your 250. Measure the diameter of the head where the squish band meets the head face, a Sebring is 76mm and all other Italian 250/350 heads measure 74mm. This is all assuming all the parts have not been modified in some way.

Best to try and identify the reason but failing an obvious answer, you could either machine (or get machined) the piston to match the head. 33 thou is fine but 12 thou is way too tight. For a road bike 30 to 45 thou's is fine, race bikes can go tighter as long as you are sure it's concentric and the rods good etc. Just for reference 450's are a different kettle of fish as the squish is much flatter and the actually clearance is nearer to the vertical distance.

Hope this helps

Regards Nigel

[vmoto3]

Measurement was taken with a stock Ducati gasket (appr. .034") under the jug. Cyl. head does measure 74mm across. Very puzzling. Makes sense that squish area should be parallel (or nearly so). Will look into those threads mentioned and see if others have run into this problem and, if so, found its root cause and remedy. Barring that, will either machine piston or purchase another one and see if that solves the problem. Either way, will post the final outcome of all this. Thanks for all of the info. guys.

Mike

[DewCatTea-Bob]

[quote= LaceyDucati ...
" The squish should be ideally parallel or slightly tapered the other way, ie. tighter at the edge and getting larger towards the centre of the chamber. "

____ I-myself am not an expert on squish-band angles, and I'd certainly expect the likes of Nigel to know and/or understand such tech.application more-so than myself. _ So it would be of fairly reasonable interest if he (or someone-else [such-as like Eldert]) would offer an explanation as to why it would matter any at all for the mating squish-surfaces to rather be tapered with an expanding-gap facing TOWORDS the combustion-chamber, (if not kept exactly parallel).
__ I-myself don't see how being other than perfectly parallel (with a slight-taper in either direction) could make any notable-difference whatsoever.
I can understand how it could possibly make life a slight-bit easier on the compression-rings if a tapered squish-gap had it's widest gap-opening faced away from them (and towards the comb.chamber), however it seems obvious to me that that oppositional taper-orientation would naturally tend to rather diminish the forced injection-effect which the squish-band is intended to provide.
And in fact, just as a nozzle-attachment connected onto the end of a water-hose vastly increases the intensity of the velocity-strength of it's expelling water-stream,, I'd of-course likewise naturally expect a squish-taper oriented with it's narrowest-gap/opening faced toward the c.chamber, to assist and accordingly increase & intensify the desired*effect of the squish-band !
(* It's my understanding that the intended effect of a squish-band is to provide an extra intensified RUSH -('charge') of combustion-chamber turbulence [in the air/fuel-mix], so as to cause a faster & more complete combustion-burn [thus providing improved burn-efficiency and resulting increased power-production].)
So with the properly tapered-squish providing a "nozzle-effect" to further intensify the 'squish-effect', thusly makes good natural/logical sense to me !
__ Therefore I, for one,, don't think it's any mistake that there's a taper of the squish-gap and that it's orientated in the particular direction which it has been found to be pointed towards ! _ And I'm thinking that the Mark-III's high-performance piston may quite-likely intentionally exploit the functional feature which I postulate -(referring-to the "nozzle-effect"). _ And-also,, I can see where as the squish-gap is getting squeezed-down towards it's minimum space-gap (as the piston rises near-against the head), that the ordinary squish-effect alone then merely squeezes-out it's fuel-mix.volume with gradually greater & greater linear-intensity until the nozzle-effect can next (rather 'superlinearly') finally fully-initiate it's FINAL (even further intensified) BURST-injection producing additional/bonus turbulence-charge into the THEN extra-turbulent/swirling burning-gases within the c.chamber,, (as a simple parallel squish-band/gap would-not produce such an extra-kick at that last point [right-at TDC]) ! _ (It's sort-of kinda like giving the squish-effect an intensifying magnifier-lens, isn't it !? _ And after-all, which would ya expect to most disturb the air within a bag,, blowing your breath into it with your mouth simply opened, or rather with your lips puckered [to create the "nozzle-effect"] ? )
So I'm quite inclined to believe that the angles of the squish-bands of the piston & head have been rather purposely designed so that the present squish-taper provides a rather useful function (as Ducati must've intended).
(And besides, should we really think that the likes of ourselves are possibly near same as educated engineers who are well qualified to second-guess any intended reasoning established by the designers of the piston & cyl.head ?)



" 33 thou is fine but 12 thou is way too tight. "

____ Why is it that much less than .030" is "too tight" ?
I'd expect that a tighter squish-gap would increase the effectiveness of the purpose of the squish-effect.
What negative result could there possibly be with a squish-gap that's supposedly too tight ?
__ In a related case,, I once had a 76.4mm w-c.350-piston installed within a n-c.Mark-3 engine, and it just-happened to have a squish-clearance of near-about .009" (which I thought was good). _ And it ran just fine for several thousand miles like that !
However after experiencing much hard running, it later began to develop a top-end tacking-noise which eventually evolved into a considerably louder clanking-sound that finally warranted a rather extensive investigation. _ And it was then found that the con.rod-bearing had developed near-about ten-thousandths of slop, which had resulted with piston & cyl.head squish-band surfaces that had become as barren as possible with absolutely no carbon build-up whatsoever (even-though the piston-crown was found still well covered with a fairly deep layer of carbon-deposit !).
So aside from the disturbing noise of the piston making overly-positive contact with the cyl.head, (thus no squish-gap/clearance),, the engine still performed it's usual power-production, (as far as I could readily discern).
And I certainly can't blame the original overly narrow squish-gap for making the rod-bearing go bad. _ So exactly what consequence supposedly ought-to be expected with a squish-gap that's left well-under the recommended 30-thou.minimum squish-gap ?
The only thing I can think of,, is that during engine warm-up, the involved eng.parts may expand at differing rates which may tend to at-least temporarily eat-up a fair portion of the preset squish-gap clearance. _ And-so possibly, a wider squish-gap is thus-then required to avoid any possibility of piston-to-head contact (whenever the engine is no-longer cold).


Dukaddy-DUKEs,
D.Bob

[Dave450]

Bob - as I understand it, the main reason a squish clearance of 0.030" is too tight is mechanical: the crank will tend to whip at high revs and you risk the piston kissing the head. I have experienced this on my 450 with a tighter clearance and the big end bearing was not worn. Like you, I was looking at a squish band and piston that were completely clear of carbon and slightly polished. I was lucky.

I encountered a similar problem some years back with my G50 Matchless, when an experienced engine builder did not measure the squish clearance when fitting a new piston. He was not aware that Omega had changed the design of the piston and this resulted in a clearance considerably tighter than the recommended 0.025". The result was a that the piston made a neat impression in the cylinder head. Again, I was lucky, and I also managed to save the head. (After that and some other shockers, I now build my own engines). When I consulted Andy Molnar, who makes replica G50 engines, he said that you can go tighter than 25 thou if you have a stiff crank, i.e., his crank!

Just for comparison, the G50's squish band is parallel with the surface of the piston, not tapered in either direction, as far as my measurements indicate. The squish area is also a lot narrower. I don't understand why Ducati singles have such wide squish bands. Is this necessary? Perhaps Nigel could explain?

Dave

[DewCatTea-Bob]

[quote= Dave450 ...
" the main reason a squish clearance of 0.030" is too tight is mechanical: the crank will tend to whip at high revs and you risk the piston kissing the head. "

____ I've actually heard-of that reasonable explanation before,, I simply had forgotten about it, is all. _ (And now that I think about it, I do believe that same consequence was already brought-up within a past thread-post when once before discussing the min.squish-clearance.)
Thanks for the reminder !
__ Since I had run my engine pretty-hard, the crank-flex issue may indeed have come into play,, and-thus with the con.rod & piston colliding against the cyl.head, that may've been extra-hard enough on the rod-bearing to have majorly contributed to it's developed slop.
So now I'm thinking that the overly narrow squish-gap I had, may've actually caused the rod-bearing to've gone bad after-all. _ (Whereas before, I had blamed the fact that the centripetal oil-filter had become overfilled.)



" the G50's squish band is parallel with the surface of the piston, not tapered in either direction, "

____ I believe that's also the case with most Duke-engine models, but I assume the nozzle-like squish-taper is relegated to the high-performance pistons.



" I don't understand why Ducati singles have such wide squish bands. "

____ Could be just for conveniently raising the compression-ratio,, or more likely to increase the volume of the squish-injected charge (vs the volume already resided within the main combustion-chamber volume, thus providing a more even & advantageous ratio), for more greatly intensified turbulence (for improved burning).


Dukaddy-DUKEs,
-Bob

[Dave450]

In my previous post, I should have said "the main reason a squish clearance of less than 0.030" is too tight…."