Cylinder head design.

Hallo,
my name is Bernd, I´m from Germany and ride a Bakker Rotax 256 and a TZ 350.

I´ve got a question about the design of a cylinder head for a Maico MD 250 aircooled.
Is there a thumb rule about the relation of the width of the squish band and the bore?
I´ve got several modern PVP head inserts for the Rotax. Can i just extrapolate the measures to the bigger bore?
The Maico got a bore of 76,00mm. I want to make the compression ratio to 13.1/1. So i clean the old head design by welding and get a new design on the turning machine.
The engine will be used for racing purposes.
Best regards,

Bernd

Herzlich willkommen Bernd. The squish band area is usually about 50% of the bore area. This means that the squish band width should be 14,64% of the bore.
You can extrapolate the dimensions of a good combustion chamber to a bigger bore, but you must be aware that bigger bores will usually require lower compression ratios.

Quick reply!!

We heard many things about the squish band width according to the man speaking.

Frits, I guess you're right! Do you get this 50% of the bore area (=> 14,64% bore) from your experiences or a computation based on a theory?

Rose Noire a écrit:

Quick reply!! We heard many things about the squish band width according to the man speaking. Frits, I guess you're right! Do you get this 50% of the bore area (=> 14,64% bore) from your experiences or a computation based on a theory?

Experience.

Thank you Frits for your detailed answer.

Hallo,

you think a 1,0 squish gap is enough? The engine will rev about 10.500rpm, the stroke is 54mm, the crankshaft pin is 25x64mm. Main bearings are one side roller bearing, other side ball bearing.
On my Rotax there is a 0,70mm gap. Unfortunately i don´t have experience in that direction with such a big bore.
Berst regards,

Bernd

pour comprir , pouvez mettre des photos ?
Merci

Il me semble qu'il veut parler du "jeu",clearance,entre le piston et la culasse au niveau de la squish-band.

Oui, c'est correct.

Döllinger a écrit:

you think a 1,0 squish gap is enough? The engine will rev about 10.500rpm, the stroke is 54mm, the crankshaft pin is 25x64mm. Main bearings are one side roller bearing, other side ball bearing.
On my Rotax there is a 0,70mm gap. Unfortunately i don´t have experience in that direction with such a big bore.

A 1,0 mm squish gap is too big! The purpose of a squish gap is to push the fuel-air mixture to the center of the combustion chamber, where the better concentration and the strong turbulence in the mixture will speed up the combustion process. The faster combustion will improve the thermal efficiency of the engine, reduce heat losses and prevent detonation. And if the squish gap is tight enough, there will be no mixture left in the squish area, so there won't even be anything left to detonate.
As a rule of thumb, the minimum safe squish gap for an engine with healthy crankshaft and big end bearings is 1 % of the stroke; that would be 0,54 mm for your Rotax. My second rule of thumb is: as long as you are not sure that the piston is touching the head, everything is fine.
I revved Rotax engines with a 54 mm stroke and a 0,4 mm squish gap on the dyno up to 14.000 rpm without problems.
But I have never seen a Rotax with a 54 mm stroke and a 25 mm Ø big end pin, so I have no idea what kind of crankshaft you have in your engine and how stiff it is. Therefore you'd better stick to your 0,7 mm gap; that is still tight enough to evacuate most of the mixture from the squish area to the combustion center and generate the desired turbulence.
0,7 mm is also the value used in the Aprilia racing engines.

merci

Hi Frits,

my Rotax got a 20mm big end pin. The older ones have 18mm. Stroke is 54,5mm and i use FPE crankshafts.
On the Maico the bigend pin is 25mm. My question is about that engine.
Very interesing, that you ran a Rotax with such a small gap.
Best regards,

Bernd

Frits Overmars a écrit:

Rose Noire a écrit:

Quick reply!! We heard many things about the squish band width according to the man speaking. Frits, I guess you're right! Do you get this 50% of the bore area (=> 14,64% bore) from your experiences or a computation based on a theory?

Experience.

I like your answer  
And as the coffee pub says: what else:
I reply nothing
The experience corresponds to 10 engineers minimum  

Best regards

Twenty years ago, I had found a book on the preparation of the two stock engines in the Birmingham Motorcycle Museum.
I went back in May and could see that it had been re-edited.
This book flies over the preparation of this engines.
This is a good starting point, because I was a nit in two stocks’ and it did helped me.

Hallo,

when i measure the parts in this cylinder i take notice of something curious. The support-ports have bigger a opening angle than the two main-ports.
The porting measures from top are: (stroke is 54mm)
2x Main-ports: 41-41,2mm,
4x support-ports: 38,7mm.
Exhaust port: 28,8mm
This is not a modern design i think. Can this big mismatch have any benefit or is this due to the engineering year of 1975, when they did not know better?
The opening angle of the support-ports is quite big i think. Will there be a blowback into the ports?
I have now the opportunity to weld on alluminium to vanish this mismatch or to leave it as it is.
The exhaust opening angle is not quite big aswell i think. But the engine(block, crankshaft ect.) will not bear more than 10.000 rpm i think.
What is your opinion?

Best regards,

Bernd

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Döllinger a écrit:

when i measure the parts in this cylinder i take notice of something curious. The support-ports have bigger a opening angle than the two main-ports.
The porting measures from top are: (stroke is 54mm)
2x Main-ports: 41-41,2mm,
4x support-ports: 38,7mm.  
Exhaust port: 28,8mm
This is not a modern design i think. Can this big mismatch have any benefit or is this due to the engineering year of 1975, when they did not know better?
The opening angle of the support-ports is quite big i think. Will there be a blowback into the ports?
I have now the opportunity to weld on alluminium to vanish this mismatch or to leave it as it is.
The exhaust opening angle is not quite big aswell i think. But the engine(block, crankshaft ect.) will not bear more than 10.000 rpm i think. What is your opinion?
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My opinion is that I would need more information Bernd. Are the port heights measured from the top of the cylinder or from Top Dead Center, which is not necessarily the same? What is the con rod length? How wide are the exhaust ports?
Assuming that the port heights are measured from TDC and that the conrod length is 2 x stroke, the exhaust timing will be 188° and the transfer timings are 130° and 142° respectively. 142° is high, even by 1975 standards, and 10.000 rpm will probably be too much for the exhaust ports which appear to be quite narrow. So yes, there will be blowdown into the transfer ports. By the way, what engine is it? I'd like to see more pictures of this 42 years-old veteran.

The past few days we have been dealing here with questions that have little to do with the title of this thread.
I will gladly answer all questions as well as I can, but it would be a pity if some discussions cannot be found back because people are mislead by the title of a thread.

May I suggest that we concentrate questions about two-strokes in
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This will make it easier for everyone to access the available information.

Hi Frits,

thank you for your answer. I will make some photos and measure the detailes you requested. I have to leave now, i will reply later that day.
The engine is from a Maico MD 250 rotary engine.

Best regards,

Bernd

Döllinger a écrit:

I will make some photos and measure the detailes you requested.
The engine is from a Maico MD 250 rotary engine.

Yes, I remember that engine Bernd. If ever there was a two-stroke that needed better cooling and a stronger gearbox, it was the race version of the Maico MD250. I would make those exhaust ports a lot wider above the transfers if I were you.

Don't bother about the details. It was not my intention to request them; I only wished to point out that it is not possible to give detailed anwers without detailed input. But I do not intend to start a full-blown consultancy here; I need my two hours of sleep every night.

Hi Frits,

no, i don`t expect a full consultancy. No problem with that!
Nonetheless i took some photos and have some infos.
The rod lenght is 137mm, the tdc is 0,5 down from the point i measured. My fault. But i can vary this with different foot gaskets.

I built some of this engines for racing for a friend of mine years ago. He´s happy with them. This engine is for my father so i take a little more care about the details.
The gearbox is a very sensitive point in this engine, i replaced it to a racing gearbox with different ratio in 1,2,3 and 6.
Due to the poor clutch performance i strengend the outer basket with a laser welded ring, put in other plates and built up a new pushing system.The cooling is another critical point, i plan to construct some kind of ram-air system here. The crankshaft is rebuilt new with a balance factor of 62% and a lighter piston. The inlet is changed to get a 36 Dellorto in it.

What i wanted to ask is a guess of a good opening angle for the exhaust and the transfer ports for a rpm-band between 6500-9500rpm.
What radius measurement of the exhaust port would you recommend right and left of the port bridge, without risking a broken ring?
I was not my intention to strain your free-time. Sorry, if this came over to you!
I´m happy for every little information of someone that experienced. I will put my questions in the other thread, thanks for that tip.
Best regards,
Bernd
Here some pictures:
One of the blocs, i want to use. I prepare the parts, before assembly.
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Here one of the three clutch conversion kits. The other 2 run since 2 years without any problems.
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Rebuilt crankshaft and piston.
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