Head work 250

[marsheng]

I had the 2 good heads vapour blasted, but after seeing the seat wear in them and being cast iron, I have had a rethink.

ValveWear.JPG

To start with the valve sizes. The originals are 40 inlet and 36 exhaust. The throat on the inlet is 32 mm. According to Vizzard, this should be 86 % of the valve, ie the valve should be closer to 37 mm. Similarly, the exhaust is 30 giving a valve of around 35.

So I'm thinking, maybe time to readdress the damaged head and make a few changes. Having smaller valves means I have a few more options in my machining.

OldHead.JPG

Looking at the new piston and the clay, it looks like I can lower the valve seats in to the combustion chamber by a mm or 2 and reshape the volume directly above the piston a bit which will improve the compression ratio. With a full reweld on the chamber, I can get new seats in, improve the CR and get the squish to suit the new piston and I was even thinking about closing up the plug hole and repositioning it with a 10 mm plug which would be closer to the center.

PistonSquish.JPG

I probably really need to decide on the valves first as there could be a tendency for them to touch each other if I raise the seats too much.

The valve to piston clearance is 4 on the inlet and 4.4 on the exhaust.

[Bevel bob]

Your comment on valve size brings back a memory that in the USA they found that the smaller inlet valve and 27mm carb on the 250's gave a better general performance for road use.

[Duccout]

I know nothing about tuning, except what I read, and I remember reading about a formula for a two valve engine (possibly written by Harry Westlake) which states that the inlet valve should measure half the size of the bore, which obviously relates to piston speed, so the 250 inlet valve should measure 37mm and this is almost the size (37.5 mm) that Taglioni used on the Pantah which had the same bore and stroke of the 250.

Anthony Ainslie (who was a very respected tuner of Ducati Singles) told me that most Ducati racers overtune their bikes in the hope of gaining massive horsepower, fitting lumpy cams, high dome pistons and massive carburettors, resulting in engines that have wafer-thin powerbands and are actually slower. A while after this I was spectating at Brands Hatch at the classic races, and watching the Ducatis coming off Clearways and onto the straight, most of them changed up and I could hear the exhaust go as flat as a pancake and the bike go no faster. I think that Nigel and Eldert will probably tell you that gas speed is the important factor, and big valves and ports slow this, narrowing the power band.

[LaceyDucati]

Wallace
Firstly as I have seen another post of yours seeking a cam, have you obtained the cam you are intending to use in this project. Also so far, I have not seen what you are intending to use this engine for when you have built it, is it a road or race engine? If you are not using the cam you intend to use to do your valve to piston clearance and ponder valve seat position then you will be making various assumptions without all the parameters.

Bringing the seats out into the chamber sounds great but the original 40mm inlet valve only just sits on the squish in it's original position. Hence the awful original pocketed bronze widecase seats as original. I suspect your heads are narrowcase, looking at the seats. Placing the seats further out will either mean a protruding seat or a remodelled combustion chamber with a flatter squish. Added to this, there is the issue of valve to valve clearance on the overlap, which will be different from cam to cam. Moving the seats out does allow a better radius at the bottom of the port, but with these engines you are always chasing your tail, robbing Peter to pay Paul and there is always a compromise of many parameters.

The piston you show is in my opinion not ideal for the Ducati combustion chamber as standard. There is no effective squish and although the milling across of the pockets will allow less interruption to flow on the overlap, it will leave potential areas of unburnt gas in the combustion chamber. For me an effective squish is an important part of the Ducati head design and losing it is likely to effect good combustion. From my experience you will end up needing to run a lot of advance maybe 50 degrees or more to get the best out of it. You could of course weld up the head and put in a flatter squish, but you will be making the chamber more distorted in shape and squishing less centrally and towards a large dome. The 250 engine is the most difficult to obtain a useful high compression, compared to the 350 and 450 which it is relatively easy to get good chamber shape and high compression.

Over 20 years ago I built a 250 for a customer and carefully tailored a lumpy American piston (to remain nameless) achieving 11.1 C.R. Before I'd only ever struggled with "lesser" pistons giving a maximum of 10.3:1. To be fair the results had always been good , but you have to experiment.... The bike seemed to perform okay, but nothing startlingly that much better. Anyway the piston grenaded it's self after a couple of meetings (the only one i''ve ever had do that in some 30 years, nothing to do with the C.R.....!) . All that I had available as a replacement was a Borgo road piston. The Borgo was duly modified and the motor rebuilt ready for the next meeting. Needless to say the performance didn't noticeably change and the bike went on to eventually win a championship. The CR was 9.3:1 and that taught me that lesson I had already annoying suspected, don't compromise chamber shape and combustion for a theoretical high compression ratio. That said there is a happy compromise and on 350's and 450's I have run compression ratios in excess of 12:1 successfully and to advantage.

Regarding port and valve sizes,

Inlet: My most successful 250 motor ran a 38mm inlet valve and I've found for a standard head a 39mm valve is a good compromise. For me it's all about porting and without major surgery you are unlikely to make best use of a 40mm valve. With a 250 unless you are going to develop power above 9500 you won't need a valve larger than 38/39. Certainly 39mm is large enough to produce power above 30bhp at the back wheel. My years of comparing inlet flow and dyno figures indicates a direct correlation if you read between the lines. Certainly poor flow figures (efficiency) have never resulted in good dyno figures.

Exhaust: Frankly there is little to be done apart from a little tidy to improve things with the standard port. The port flows very badly on a flow bench and without adding material to the floor obtaining better flow is rather fruitless. The standard 36mm valve is already to large for a 250, but there is nothing much to be achieved by using a smaller valves without remodelling the port. That said I've had a 450 on relatively standard port and 37 exhaust valve produce good all round performance and about 48bhp at the rear wheel, so the poor exhaust flow on the bench wasn't that impeding.

Throat sizes I work on 0.9 of the valve diameter. Ports around 0.82% of valve head for inlet and 100% for exhaust. This is just generalisations and I don't use round ports as it's impossible to deliver the correct area as a diameter within the space available.

There is obviously a lot to tuning and everything is linked and a successful motor is a combination of many things working in harmony. I'm no design engineer and I don't pretend to know/understand all the dynamics, all my results are from reading theory, trying to apply it, testing, observation and lots of guess work.

Regards Nigel

[marsheng]

Hi Nigel

Thanks for the reply. To start, I'm not about max HP, a top notch road is what I'm after. I'll unplug the lights etc to go racing. There are so few 250 Pre 63 in NZ that basically all I have to do is turn up at the race track to do well in the points. We have a grandfather clause that allows bikes like mine (64) to run in earlier classes as long as certain conditions are met. The reason for the road registration is there are not may race days over the year and its a shame to let this sit in the shed for 98 % of the time. PS still to decide on the cam.

Playing with bikes is what I like best, riding them is further down on the list. Just to show you what a nutter I am, here are some other photos of the bike in progress.

New frame. I didnt want to cut up the original frame so I made a new one which allows for the new lights and fairing fittings.

Right.JPG

FrameSmall.JPG

and hand made tank.

Tank.JPG

Previous project. Maico 250 1980 motor in a copied YB3 frame.

SOT_Profile.JPG

I do appreciate all the comments. What I like about old bikes is that people have been there and done that. Hopefully I can learn from them and their mistakes.

[marsheng]

What I forgot to say was that the tracks in NZ are tight. The pull out of corners is what counts here.

Cheers Wallace

[Bevel bob]

Hi Marsheng, Just out of interest how do you come to date your Mark 3 as 1964?.

[marsheng]

Hi Marsheng, Just out of interest how do you come to date your Mark 3 as 1964?.

Just a guess, the motor serial number is 96937 We have a Pre 63 group here. Other than the 5 speed box, all else remains the same as the older bikes so even if it is later, it will still comply.

From this page.
https://bevelheaven.com/data-single.htm

626 250 (n/c) DM250 96818
627 250 Mark 3 1966 DM250 96878
628 250 Diana 1968 DM250M1 96987
629 250 Mk 3 1967 97010
630 250 Diana Mk 3 DM250M3 97097
631 250 Mach 1 1964 DM250M3 97104

If you have better info, please let me know. The frame is not numbered.

[Ventodue]

Just a guess, the motor serial number is 96937 <snip>

More likely to be 1966 ...
Ref. Ian F. , "O/H singles", page 130. Range for 1966-67: 92172 - 104500.

Remember the data on BevelHeaven/held by Robert Miller are uncleaned. While potentially useful as a confirmation, they should be treated with some caution .

[marsheng]

Just a guess, the motor serial number is 96937 <snip>

More likely to be 1966 ...
Ref. Ian F. , "O/H singles", page 130. Range for 1966-67: 92172 - 104500.

Thanks for that. So 1966 it is.

Cheers Wallace