Valve Sizes

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frankfast
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Valve Sizes

Postby frankfast » Mon Aug 23, 2010 4:03 pm

Can you tell me the valve sizes for narrow and wide case 250 Scramblers?

Eldert
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Re: Valve Sizes

Postby Eldert » Mon Aug 23, 2010 4:19 pm

Hi Frank

250 scrambler intake 36 mm valve and exhaust 33 mm valve

Eldert

frankfast
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Re: Valve Sizes

Postby frankfast » Mon Aug 23, 2010 4:57 pm

Hi Eldert,

Thanks. The rockers and cams on my 350 Scrambler head need to be replaced and the prices for cams and rockers are beyond my budget but I've been offered a reconditioned head from a 250 Scrambler. With a different bore and different valve sizes and different cam profile, I'm not sure it would work. This is not a restoration but rather a resurrection. They also use different size carbs. Any advice Bob?

DewCatTea-Bob
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Re: Valve Sizes

Postby DewCatTea-Bob » Mon Aug 23, 2010 5:34 pm

____ I'm surprised that Eldert didn't also answer that the valve-sizes, (in-fact the valves themselves) are the same for both the wide-case & narrow-case 250SCR-models.
But additional DETAILs such as that, is what I myself should be exceptionally known for.


" I've been offered a reconditioned head from a 250 Scrambler.
I'm not sure it would work. "

____ A 250-cyl.head (along with it's internals, or not), can be made to fit & work quite well enough on any 350-model but, you didn't make it clear whether the intended replacement 250-head included it's tower-shaft with housing-tube. ...
The tower-shafts are different lengths, and therefore unfortunately, the cyl.head swap-job is not a simple bolt-on process (as it otherwise would be!).
__ So you'd also need to retain your 350's tower-shaft (as well as it's matched camshaft bevel-gear!), to be assembled onto the intended replacement head.
What's unfortunate about that is, that bevel-gear re-fitment-job is not a straight-forward bolt-on job, as it requires careful re-shimming of the bevel-gear alignment (due to the heads & bearings not being exactly the same).
And this kind of tech-work really should not be attempted by anyone who is not already experienced with such, or, who is not a 'master-mechanic' type.
__ So I'd suggest that instead of swapping the cyl.heads (along with their internals), that you instead consider just transferring the 250-head's cam & rockers over to your 350-head. _ As that work would be a simpler job to accomplish. _ And, the larger valves & 250-cam combo, should provide a slight mid-range advantage (that's actually not worth mentioning though).


DUKE-Cheers,
-Bob
PLEASE NOTE... If this-post is not-yet signed-off with '-Bob', then I'm still in the process of completing it,, and if not also included with 'DCT' near bottom as well, then I may edit this post's wording at a later time. - Dct.Bob

frankfast
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Location: New York (upstate)

Re: Valve Sizes

Postby frankfast » Mon Aug 23, 2010 5:49 pm

The valve sizes on both the narrow and wide case 250 Scramblers are 36mm and 33mm. But the 350 Scrambler has 40mm and 36 mm valves. The port sizes are therefore different as is the carb size. Am I correct?

DewCatTea-Bob
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Re: Valve Sizes

Postby DewCatTea-Bob » Mon Aug 23, 2010 6:00 pm

" But the 350 Scrambler has 40mm and 36 mm valves. The port sizes are therefore different as is the carb size. Am I correct? "

____ Yes, that's all correct but, it doesn't really matter, (unless the intended-swap was going the opposite way, [as larger valves may then not match with the current piston] ).
__ While the 350w-c SCR-models came stock with a 29mm-carb matched to a 29.5mm intake-port, the 250-SCRs came stock with a 27mm-carb matched to a 28.5mm port...
A 29mm-carb will still work fine with the 28.5mm port, (although ya may wish to more precisely match-up the alignment between the carb's manifold & the intake-port).
__ I've actually already done this exact same cyl.head swap-job a number of times !
And I've already mentioned (within others threads) how these (seemingly anti hot-rod) changes effect the performance of the otherwise stock 350s. -- With a stock 250-SCR cyl.head in place of the stock-head of a stock w-c 350, (with all stock-internals left/remaining as original & kept within their original-heads),
the 350's OVERALL-performance is actually improved !
As not only will such an altered 350 stay full-on (pretty-much even-steven) with a stock-350 upwards of 80-MPH, it will also trot-away from a stock-350 during low-RPM roll-ons ! - (All else being equal.)
So what's better for ya? _ A higher (illegal !) top-speed, or,, a (more-fun), much better low-end performance ??


DUKE-Cheers,
-Bob
PLEASE NOTE... If this-post is not-yet signed-off with '-Bob', then I'm still in the process of completing it,, and if not also included with 'DCT' near bottom as well, then I may edit this post's wording at a later time. - Dct.Bob

frankfast
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Location: New York (upstate)

Re: Valve Sizes

Postby frankfast » Mon Aug 23, 2010 6:47 pm

I remember from a previous post that you liked the profile of the 250 cam better than the 350 cam. I was more concerned with the port and valve sizes. I'm not sure of the effect of the 2mm difference in bore sizes.

DewCatTea-Bob
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Re: Valve Sizes

Postby DewCatTea-Bob » Mon Aug 23, 2010 6:57 pm

" I remember from a previous post that you liked the profile of the 250 cam better than the 350 cam. "

____ That's not exactly true ! _ I've only claimed that the overall-performance provided by the 250-cam is superior with the use of a stock-type muffler !


" I was more concerned with the port and valve sizes. I'm not sure of the effect of the 2mm difference in bore sizes. "

____ The in.port size difference is just 1mm, and even if it was 2mm, the difference in top-RPM performance is something you'd only be able to notice with the use of a dynamo-meter.
The valve-size difference also plays a small role at under top-RPMs.
__ If I -(who I am now) had been in-charge at Ducati back in 1967, then all 350SCR-models would have continued being equipped with the same 350-head (with 26.5mm in.port) as was produced for the 350-Sebring, (although with the 250SCR-cam instead of the Monza-cam!), and also with the 250SCR's 27mm carb.
Cuz I happen to KNOW that that combo of parts makes for a much better performing 'scrambler', than the way that Ducati equipped their 350SCRs !


DUKE-Cheers,
-Bob
PLEASE NOTE... If this-post is not-yet signed-off with '-Bob', then I'm still in the process of completing it,, and if not also included with 'DCT' near bottom as well, then I may edit this post's wording at a later time. - Dct.Bob

frankfast
Posts: 231
Joined: Wed May 05, 2010 12:35 pm
Location: New York (upstate)

Re: Valve Sizes

Postby frankfast » Mon Aug 23, 2010 9:02 pm

So, it looks like a 250 Scrambler head on top of a 350 cylinder will work fine. But you say that my 29mm carb and open megaphone may not be compatible.

DewCatTea-Bob
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Joined: Sun Nov 01, 2009 10:53 am
Location: Near SE side of Lake Michigan

Re: Valve Sizes

Postby DewCatTea-Bob » Mon Aug 23, 2010 9:21 pm

" But you say that my 29mm carb and open megaphone may not be compatible. "

____ I'd really like to understand how you could've possibly came to have THAT thought !
I'm sure that whatever "megaphone" that you happen to have, it likely ought to be compatible with most any 29mm-carb, (or other [properly jetted]- carb within 3mm difference) !

____ Hope you've noticed that my previous posts here today, were likely finished AFTER you first noticed & read them. - (You then may have missed some later added-info.)


Hopeful-Cheers,
-Bob
PLEASE NOTE... If this-post is not-yet signed-off with '-Bob', then I'm still in the process of completing it,, and if not also included with 'DCT' near bottom as well, then I may edit this post's wording at a later time. - Dct.Bob


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