250 MK III Exhaust Valve

[matzb]

I recently bought a complete Ducati 250 engine (narrow case) on EBAY. The exhaust valve measures approx 41-42 mm. Would this be a Mark III head?

[DewCatTea-Bob]

" The exhaust valve measures approx 41-42 mm. Would this be a Mark III head? "

____ WOW! _ Not necessarily, as the stock Mk3-ex.valve is 36mm !
__ What size is the intake-valve?
To tell if ya have a stock Mk3-cyl.head, it's intake-port should measure about 29.5mm.
But if you don't have stock-valves, it's doubtful that who-ever installed oversize valves, would've left the ports at their stock sizes...
__ It does not pay to install oversize-valves unless ya also increase the valve-seat opening, which only a larger valve can then properly cover.
That's why the larger valve-set that Mk3's (& M1s) have, are also matched with larger valve-seats !
So I don't think someone (who's smart) would install such a large valve into a (non-Mk3) cyl.head that doesn't have the larger valve-seats ! _ And even so, such a larger valve should not help unless the Mk3-type valve-seat has also been ported-out along with it's related port.
__ Back in my day, many cycle types -(bikers) assumed that larger-valves (alone) would improve a motorcycle's power... And thus I've come-across many small-valve type DUKE cyl.heads that simply had the larger-valves installed. _ Not really smart.
____ Please let us know if that cyl.head looks to be otherwise stock, or well modified.
Cuz if not ported-out from stock, it could be that somebody had experimented with proving something (unconventional) which I had told them may improve performance.


Hopeful-Cheers,
-Bob

[Eldert]

are you sure it the exhaust valve ?
i dont think it possible to fit a valve that big without the valves tangling in the overlap period
even if you sink the valves real deep into the head .
the biggest valves i ever fitted was 42 intake and 37 exhaust and had to sink the valves pretty deep
later on when i got better flow thru the ports i went back to smaller valves

Eldert

[DewCatTea-Bob]

" without the valves tangling in the overlap period
the biggest valves i ever fitted was 42 intake and 37 exhaust and had to sink the valves pretty deep "

____ But that would also depend on max.overlap-lift !
So which cam were you using then Eldert? - (A G&W no doubt!?)


DUKE-Cheers,
-Bob

[Eldert]

my bad , i forgot to mention that was with a race cam ( in this case a VeeTwo item )
but even with a very mild cam there would be no HP gain . the exhaust valve has to be opened against
the gas pressure in the cilinder . with a bigger valve it would absorb more power .
the surface on the exhaust rockerarm has therefore a harder life as the one on the intake side

a guy i know here made rockerarms out of titanium and could not figure out why the exhaust rockerarm kept breaking
and the intake not

Eldert

[matzb]

I should take the time to proof read my question before i post it. The intake valve is approximately 40 -41 mm. Exhaust valve is approx 36 mm. Also the cases on this motor do not have the traditional stamps on the top front of the cases.
My scrambler is stamped DM250 on one side of the case and the serial number on the other. The only numbers on the cases of this motor are M353 on the primary side and 346 on the chain side. These numbers are stamped on the bottom of the cases.

[DewCatTea-Bob]

" The intake valve is approximately 40 -41 mm. Exhaust valve is approx 36 mm. "

____ The stock valve-sizes for the large-valve heads are 40mm-intake & 36mm-exhaust !
__ Did you measure that head's intake-port size yet?


" Also the cases on this motor do not have the traditional stamps on the top front of the cases. "

____ If those case-halves appear as if they were made originally blank, then they were most likely not original production-line cases, but rather replacement-cases that someone had ordered.
Do you have any pix of that area of the motor where the DM/motor-number should be?


Dukaddy-DUKEs,
-Bob