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Sand Casting an air cooled head

Sand Casting an air cooled head

(OP)
Background: I'm modifying a vintage 2T race engine (porting, intake, combustion chamber etc..) In stock trim, this motor needs better cooling.
I've found a cylinder head (sunburst style fins) that has more mass and more fin surface area, which I want to modify and duplicate in a casting.

I've also found a foundry that will cast this head for me for a minimal fee

My question is around the alloy that they use: 535 ALMAG. I copied this description from http://www.lbfoundry.com/535-almag-aluminum-sand-c...

Aluminum alloy 535.0 is an aluminum magnesium alloy that does not require heat treatment to reach full properties. It has excellent corrosion resistance and machining characteristics. Milling and turning speeds are 4X faster than alloy 319.0, and 16 to 18 times faster than gray iron or malleable iron.

The anodizing rating is excellent and the color is satin white after anodizing.

The polishing rating is also excellent and the castings can be buffed to a silvery white color.

The weldability rating is poor and it also is not recommended for leak tight or pressure type applications.

Typically it is used for parts where dimensional stability and shock resistance is important, such as instruments. It is also used for marine hardware, ornamental fittings and castings where lighter weight and high strength is a prime consideration. Almag 35 is 10% lighter than average cast aluminum alloys, 64% lighter than gray iron and 69% lighter than bronze.


My major concern is that it's not recommended for leak tight or pressure type applications. Yikes! The wall of the combustion chamber will be at least 14mm thick. Should I go for it? Or is it a waste of time?

Thanks for any help or opinions! Todd

RE: Sand Casting an air cooled head

I wouldn't for the negative reasons that you stated not to mention how things would change in your high heat and vibration application. There are plenty of good materials that meet your application requirement so using one of those from the start would give you a higher chance of success and allow you to concentrate on the other details of the project.

RE: Sand Casting an air cooled head

Almag has poor castability. 380 series aluminum might be a better choice.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

RE: Sand Casting an air cooled head

(OP)
Welp... Because I don't fully understand why this material has a bad rating for "leak tight and pressure type applications" maybe I should ask: Why is it so bad? I don't want voids in the casting which would make hot spots. And I'm a little worried about the heat cycle + Pressure from combustion. But for a $free casting, and only one choice in materials at this price, would I be able to overcome this material's shortcomings by making the walls of the combustion chamber thicker? Or anything else I can do to make this work? Thicker walls would help in stabilizing the temp so I think that's going to happen anyway.

Does anyone here know this material's behavior compared to other's?

Thanks again, Todd

RE: Sand Casting an air cooled head

Sorry, had my diecasting train of thought engaged. 356 or 355.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

RE: Sand Casting an air cooled head

When I see not to use it on leak tight or pressure applications that makes me think that it's a brittle material that is susceptible to hairline cracks and other casting defects. This would result in catastrophic failure which is the worse case scenario and I don't see increasing the material thickness eliminating a catastrophic failure from a hairline crack. Material properties are developed from testing and in service performance. To use this material in an application that it isn't good in may require extensive research/development in manufacturing process that wouldn't seem worth it for a one off piece. Even if you could get a perfect and defect free casting the material may still be wrong for the application. You could always do your own pour and have the casting shop that is giving you the good price make the mold. I've melted aluminum at my house many times and can be done fairly cheap.

RE: Sand Casting an air cooled head

tdub13-

The most common sand cast alloy for engine blocks/heads is probably A356-T6, and A357-T6 has a bit better properties, but both alloys have slightly lower RT properties than A535. However, what matters more are the elevated temperature properties of the materials. The flow/fluidity characteristics of the molten alloy can be a concern when trying to sand cast thin section features like cooling fins. But since your casting has a 14mm wall thickness in the area requiring pressure tightness, I don't see this presenting an issue. You should also take care when designing the mold tool to make sure the cylinder wall surfaces will be free from subsurface defects/porosity that will be exposed after machining.

Prototype qty sand castings are always expensive, even now that there is equipment that can 3D print sand molds directly from a CAD model. So for one or two custom A/C motorcycle cylinders, you might be better off making them using a combination of CNC machining and welding. You mentioned that you require increased cooling from your cylinder design. With A/C two-stroke cylinders/heads, the heat transfer efficiency around critical areas like the combustion chamber is helped by optimizing the conductive heat path from the chamber wall to the cooling fins. So what you want is the minimum practical chamber wall thickness and the highest density of cooling fins on the adjacent outer surface. Increasing the heat transfer area of the cooling fins by simply making them longer is not an efficient way to improve cooling. It is better to keep the fin length shorter, and increase their density by making the fins thinner and reducing the air gap between them. You can get at least two times the fin density using CNC machining that you can with sand casting. Here's an example of what can be achieved with a machined A/C cylinder head (well before CNC machines were available).

Hope that helps.
Terry

RE: Sand Casting an air cooled head

I've read somewhere in what is now antiquity that superfine head fins, as in a/c cylinder heads similar to the one Terry showed us, were in some cases cut in multiple setups with simple fixtures, using power hacksaws. They didn't even bother to cast the fins; they just cast the heads like elephant feet with ports, and cut away what they didn't need.

The closest I have come to actually doing anything like that is cutting fine slots in an aluminum sheet heatsink blank using a table saw. Woodworking tools work pretty well on aluminum; you just have to beware of the chips, which come off hot and stick to your skin.


Mike Halloran
Pembroke Pines, FL, USA

RE: Sand Casting an air cooled head

MikeHalloran-

That A/C aircraft cylinder head I linked was machined from a die forging. The cooling fins, intake/exhaust ports, combustion chamber, etc were all 100% machined using 1940s technology.

RE: Sand Casting an air cooled head

Those fine pitch fins are designed for an application where the is a lot of fast moving air directed over them. A dirtbike engine like this one is going to be slow moving and possibly coated in mud. I don't think the fine pitch approach is appropriate.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Sand Casting an air cooled head

(OP)
Wow, Excellent thoughts from all! I don't post much, but really cherish all the great minds that frequent this forum!

This application is a "free air" design in which the air passing by the engine is not forced by a fan or directed by any shrouding. Average Speed is somewhere around 20mph.

I have a great PDF file that I'd like to share, but don't know how. Here's the Synopsis in JPG format:



My thoughts, so far:

In this case, The wide fins make sense and having a good draft will aid in transferring the heat to the outer edges of the fins, as well as ease of casting. More mass in the cylinder head will buffer the peaks of the heat cycle in high throttle / low velocity applications such as a long hill climb.

There are some fins which have been truncated at the factory for clearance of other components. These are what I wish to build back to original height. I think epoxy may do the trick for the pattern model. But there are also large voids that need to be filled in (combustion chamber, and on the outside of the chamber). Can I use modeling clay or will that stick to the sand?

Thanks again for all the help!

RE: Sand Casting an air cooled head

Aluminum cylinder heads for air cooled four stroke engines typically have alloys containing a couple percent nickel. This seems to help high temperature stability. In particular, it helps four strokes retain their valve seats. An old piston alloy, 242, is a favorite.

Modeling clay may be too sticky for patterns and would tend to deform when the pattern is rammed. The softest material I can remember being used in patterns is a hard wax. Pattern makers would take a tool with a ball on the end and use it to form filet radii. This is called 'waxing filets'. You might stop by a pattern shops and ask.

Remember that a casting made from your existing head will have a few percent shrink.

RE: Sand Casting an air cooled head

I have an old tech paper regarding electronics heat sink design from a Massachusetts tech company.
I seem to recall free air fin spacing needed to be > 1/4" started to prevent viscosity slowing down convection, which is the primary mechanism heat is removed from fins. All typewritten and xeroxed It was an important discovery to me at the time because thermal glass manufacturers were bragging on how their windows with a 5/8" airgap were so superior to those with 3/16" gaps, and even the Shelter institute was advocating a BIG air gap when home building thermal windows.

My fine collection of free air cooled motorcycles seems to support 1/4 inch as a minimum spacing

RE: Sand Casting an air cooled head

Alloy 242 has a long history of usage for air cooled engine cylinder heads. 355 alloy works as well. 319 alloy with higher silicon content (to where the composition becomes similar to 380 die cast is popular for high volume production of engine blocks/heada at some of the OEMs. I would not consider the 535 Almag.

RE: Sand Casting an air cooled head

Quote (Tmoose)

I seem to recall free air fin spacing needed to be > 1/4" started to prevent viscosity slowing down convection, which is the primary mechanism heat is removed from fins.
I'm sure that fits in electronics where the heatsinks may be sitting in stagnant ambient air... but did that paper give any info on forced air currents over those fins?

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Sand Casting an air cooled head

Melting down old two-stroke heads might also be a possibility. You could probably assume that the original manufacturers used the correct material.

RE: Sand Casting an air cooled head

Hi MacGyverS2000,

You asked " did that paper give any info on forced air currents over those fins? "
I don't recall, but I should have been more specific that I believe the scope of the document was vertical fin spacing for free air cooling of stationary objects relying solely on self generating convection.

I suspect the OP is working with bike velocity induced cooling, perhaps with airflow mucked up with fairings and certainly obstructed by frames and wheels and stuff.

Here is the fin spacing that won a few world MX championships back in the 60s. Ground speed on the order of 45 mph or so max and often quite a bit less.
http://cdn.supercross.com/images/stories/features/...
The sunburst style was adopted by the conservative factory and then incorporated on production bikes.
http://tenwheel.com/imgs/a/b/m/t/f/1973_cz_400__cz...

This one came close.
http://www.wischt.com/machine/MJC-VR-right.jpg

Here is the fin spacing used on the legendary yamaha TZ race bikes in the few days before water cooled heads.
Ground speed a bit higher than the CZ.
http://www.motosvit.com/Yamaha%20TZ750/1972%20Yama...

Even fan cooled snowmobile fin spacing was fairly coarse. Mostly The fins just got shorter.
http://www.snowmobileforum.com/attachments/vintage...

At what point die and sand cast manufacturing, optimum fins size and spacing for cooling, or some other thing set the dimensions is comPLETEly unknown to me.
My first and perhaps final thought would be, it worked fairly well for them.

RE: Sand Casting an air cooled head

Fundamentally, the 1/4 spacing seems sound. A "Tesla" turbine, as I recall, is nothing more than closely spaced plates (sans blades or fins) on which the medium acts, via viscosity/friction of the flowing medium.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

RE: Sand Casting an air cooled head

(OP)
Thanks for all the replies! I've contacted the foundry foreman and asked about hard wax and also the possibility of melting down some heads and cylinders that I have. I'm wondering if melting down pistons would be a good choice of material, assuming that pistons would be a different alloy than cylinders and heads. They are more abundant for sure!

RE: Sand Casting an air cooled head

The pistons are generally a hypereutectic alloy which require special molds, casting, and cooling techniques.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Sand Casting an air cooled head

(OP)
Aha, Yes now I remember. Hypereutectic meaning the alloy has a high melting point... It's beyond the eutectoid football shape on the graph. It's been a while but I remember the graph!

Time to start gathering up all my doorstops. :)

RE: Sand Casting an air cooled head

With aluminum casting alloys hypereutectic typically implies a silicon content above 12%. There are some sand casting aluminum alloys that have a silicon content above 12% (like A390). Using scrap pistons as your raw material would not be a good idea, since you would have no idea what the piston alloy was composed of. Most auto engine pistons are either die cast or forged, and the alloys used are not suitable for sand casting. It is important to know exactly what the composition is of the alloy you are using for your casting. In order to achieve the desired properties you need to use a specific alloy designed for the particular casting process you will use. Also, the pattern maker will design the mold for using a specific alloy based on its characteristics like shrinkage, hot tearing, fluidity, etc.

In reality, the raw material cost will not be a huge factor in the price of your casting. If the trimmed casting weighs say 10lbs, then the amount of metal that will be poured would be something like 20-25lbs. The cost of aluminum casting ingot is probably not much more than $5/lb, so the material cost for a casting should be less than $150. And the raw material cost will be modest in comparison to the cost of constructing and preparing the mold tool.

Good luck to you.
Terry

RE: Sand Casting an air cooled head

The preceding posts give good advice. I agree that an alloy commonly used for air cooled heads should be used. 356T6 is common. The essential property is the strength at high temperatures.

14mm chamber wall thickness seems on the thin side. Thick, solid aluminum is strong and a good heat spreader. The larger the area of fin base the more fins can be accommodated. The base is larger with a greater wall thickness. The only way to increase cooling over the existing design is with more fin area, so this suggests more fins on a larger fin base and longer fins.

RE: Sand Casting an air cooled head

Some of the photos linked by Tmoose provide examples of how to improve heat transfer efficiency with an AC motorcycle cylinder. The MJC example shows fins that have "joggles", and the purpose of these joggles is to trip up the boundary airflows, which increases the heat transfer rate between the fin surface and the cooling air. The Yamaha TZ example shows slots in the fin edges which creates a similar effect by mixing the core flows.

RE: Sand Casting an air cooled head

Here is a pretty typical example of cooling and basic intercooling of an air compressor. Both are in essentially the same airflow coming from the bladed pulley/sheave.

http://asnowsquall.smugmug.com/photos/449037160_7Y...

The tight fin spacing on the HOT intercooler line is in contrast to the cast cylinder fins. My hunch that may be due as much to the differences in manufacturing methods for each component as the cooling requirements.

RE: Sand Casting an air cooled head

Hi Tbuelna, I can not say with certainty the reasons for joggly fins and cut fins, but reduced radiated noise and reducing bore distortion when hot were considerations for US street bike markets and race bikes of the era.

regards,

Dan T

RE: Sand Casting an air cooled head

(OP)
The Montesa fins look pretty "trick". I do recall that during testing, the magazines mentioned that the fin design was superior in collecting and holding dirt and mud. (not good) :) I'm intrigued by the cuts in the fins on the TZ cylinders. It appears to be more of a mechanical resonance breaker, but I can imagine the cuts may entice airflow deeper into the fins than without the cuts. I may consider this design feature by machining after casting. But it does bring noise into question:

Mechanical "noise" is at least a small design factor on these bikes: Nobody wants to hear the "jingling" of the wrist pin bearing amplified. My bikes need to pass a sound test at tech inspection. There are a few different ways to combat engine noise and "fin rattle" that I've seen, but they all seem to be Band-Aid approaches. Yamaha frequently uses rubber dampers between the fins of their cylinders and on the heads, they bridge the fins with the casting, as seen at the top of this thread. I'm not sure what to do here: I'm also wondering if my design (more taper in the fin, larger root width) will aid in keeping the fins from becoming sounding boards for the bearings and combustion.

Thanks everyone for your thoughts and comments. I really appreciate all the help with this project! I'll post pics of my pattern as soon as I get something nice to look at.

RE: Sand Casting an air cooled head

(OP)
Before:


First steps. Clean and cut the webs out. Parting line will be centered behind the middle remaining webs and over the spark plug area. These webs may be continued across the entire head.

RE: Sand Casting an air cooled head

It's interesting that the area of the head with the highest thermal input (ie. around the combustion chamber and spark plug port) also has the least efficient heat transfer path to the cooling airflow.

RE: Sand Casting an air cooled head

(OP)
Agreed 100%. This is what I aim to "fix" before casting. In the terrible paint pic below, if you use your imagination and patience with my bad "art". I've tried to show where I'm adding material to the top side of the chamber, especially as it loses altitude going away from the sparkplug. I'm going to attempt to fill in the concave curves and make the entire thing a nice convex dome.

I also want to widen the root width of the fins. I plan in using a popsicle stick to create the radius at the bottom of the fins. Hopefully I can extend the fins a bit to regain the surface area that will be lost.

I'll also rebuild the fins that got truncated at the factory to clear the pipe. And connect the fins that were cleared in the middle for the studs. So, all the fins will be continuous and I will be able to machine the sparkplug hole and stud locations wherever I need them to be.

RE: Sand Casting an air cooled head

Always wanted one of those Maico 490's, I was riding 125's in those days. Later I did have a CR500 with proper water cooling, that thing was a monster. 5th gear wheelies were easy. Everyone I let ride it broke a bone.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Sand Casting an air cooled head

(OP)
My YZ490 is the same way. I don't let anyone ride it. The YZ125 is a great bike and it has a lot of untapped potential in the porting and combustion chamber design. In vintage MX racing, we're not classed by displacement, only by year and technology: The EVO class around here is for drum brakes, air cooling and non-linkage suspension. I've won just as many races on my 125 as I have on the 490! I like building the bikes and motors because there's gobs and gobs of performance improvement that can be enjoyed by the builder.

RE: Sand Casting an air cooled head

(OP)
I've finally gotten back to this project. Old School pattern making in process. Wood, bondo, spot putty and a lot of massaging to go. Here's what it looks like now. I'm building up the truncated fins, then extending all the fins and adding mass over the combustion chamber and flowing it out towards the edges.

RE: Sand Casting an air cooled head

RR350 (LM119) developed for use in Piston Aero Engines by Rolls Royce and has good high temperature properties.

Solution treat 540 degC for 6.5 hours quench in boiling water and the age for 16 hrs at 215 degC

RE: Sand Casting an air cooled head

(OP)
Thanks for the tip! I will look it up.

RE: Sand Casting an air cooled head

tdub13-

Why go thru all of this trouble when you can design and optimize your head using 3D CAD and FEA? And then have it made using DLMS in almost any aluminum alloy you want? If you limit yourself to using sand casting for this head then you are restricted to using aluminum alloys that have lower mechanical properties.

When it's all said and done, the cost of using DLMS won't likely be much different than the total cost of having the head sand cast.

RE: Sand Casting an air cooled head

"DLMS" = ? ? ?

Google was of no help.

RE: Sand Casting an air cooled head

Try "DLMS metal" as your google search term.

RE: Sand Casting an air cooled head

Aye, that will do it!

RE: Sand Casting an air cooled head

(OP)
Those are great suggestions. I've got a bunch of heads designed in solidworks, but I'm trying to stay in the spirit of old school engine building because I race Vintage MX. I do have a friend right down the street who could easily SLA my designs and I could have them cast. Or go directly to DLMS, but it would feel like cheating. I have ultimate respect for the old school pattern makers and their skills. I think that this head will have a little more "soul" if it's hand formed. The imperfections are what will make it perfect. ;)

RE: Sand Casting an air cooled head

Porsche 911s use an alloy with a relatively high Yttrium content and these heads have always performed well even on high boost Turbo charged engines.

I did have the Alloy designation but am struggling to find it now.

RE: Sand Casting an air cooled head

@FennLane, I would LOVE to see that spec. Everyone suggesting 356 or similar... are you sure about that? Doesn't 356 loose it's temper right at 350-375°F? I know it's used in liquid cooled engines all over the place, right next to Viton exhaust valve stem seals. but as far as air cooled with cylinder head Temps upwards of 500-550°F, 242 is the only thing I've seen used due to its high copper content picking up the temper temp. I've recently been looking at 4032 as a billet air cooled head... but can't seem to find high temp data for it.

RE: Sand Casting an air cooled head

(OP)
I'm still picking away at this project in my spare time. So please post up any suggestions you have for aluminum alloys that would be good for this application. Thanks!

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