Material for small motorcycle header

[quaipau]

Hello everyone, and greetings from Germany! Please excuse my somewhat rusty English.

I’ve been reading this forum for a while and now I have a question for you. I’m sure I’m not getting any better advise anywhere else.

I have a small project starting up. Goal is to make long lasting parts for motorcycles which have been mostly forgotten by their manufacturers.

Pilot project is a small simple piece for a Honda CN250 scooter. 250cc, 19HP, nothing hot. The part is the actually quite short header joining the head and the muffler.

Originally it is a carbon steel part made of a flanged tube, 77mm (3”)in length, held in place by a bent thick plate welded to the tube.

Searching for a manufacturer some questions have come up, for which I could use some help.

I’d love to offer exhaust parts in stainless steel. The muffler, project nr. 2, is easy enough to get done this way. But the header’s thermal profile is quite extreme. I’ve been offered three methods for building the part, none of which is identical to the original, as well as different materials:

1. SS plate, rolled, formed and welded. Quite labour intensive. Material: pretty much anything I’d like. Closest in form and function to the original.

2. SS CNC. Not as expensive as I expected, but by far the most expensive. Beautiful. Pretty much any material I’d like.

3. Inverted casting. I never expected it to be so cost effective. Materials: 410, 304, 316 and 316L, probably others if I ask nice enough. Part is completely different from the original, thicker walls (at least 1/3), I can do a gorgeous design around this method.

I’m sufficiently familiar with the first 2 options, but casting baffles me a bit. Are there any obvious issues with cast SS Headers? Brittleness is something that worries me, but I have no experience with stainless steel castings. Is there any obvious choice in material? My clients are willing to pay for a part that will last, so that is the priority.

Thank you all for your help!

Paul

 

[EdStainless]

The only issue that I see what casting is the thicker walls. This will change the thermal profiles, change the hot spots maybe, and cause more thermal related stresses. Thinner is better for parts like this, you would rather have them deform if the stress gets high since the other option is cracking.
I would think that option #1 is likely the best.

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P.E. Metallurgy, Plymouth Tube

 

[BUGGAR]

The header gasket may have some affect on, or from the header material as well. Especially concerning service lifetime.

 

[Lou Scannon]

You probably don't want to design against TMF failure by seat of the pants, but rather by actual testing or at least FEA TMF analysis. You could start the FEA by validating your TMF model with the original design, material properties, and actual crack sites. Goal would be for the new design to show vastly superior TMF resistance.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[RodRico]

What is "inverted casting?" I couldn't find it via Google.

Rolled, formed, and welded is common when making tubes of any kind, possibly because it's largely insensitive to wall thickness. There's a lot of labor involved when prototyping but the price would come way down in a modern factory through use of fixtures and robotic welding machines. The up front capital costs for tooling are significant, but the per unit cost in production is low if the production volumes are high. From your picture, it looks to me like the original may be a piece of stock tubing with two rings and a curved plate welded on. Have you checked if the tube stock can be bought off-the-shelf? If so, the cost of this option would come down. If you're really lucky, the two rings welded on the pipe might come from pipe stock, so all you'd need to fab would be the curved plate (which might be stamped from a blank).

CNC is costly, especially in prototyping, due to set up time and holding fixtures (tooling). Once fully automated, it get's cheaper, but it's still quite time consuming and ties up expensive assets, so it's never cheap. If the basic part can be made using another process such that machining only takes off a mm or less of material, the cost of machining comes way down.

I imagine some of the prototype shops would offer casting using a 3D printed mold (or a 3D printed model burned/melted out from the mold). This approach would require thicker walls than the other methods because of the need for metal to flow in the mold. I image the up front cost would be lowest at the firms that can 3D print a sand mold directly. The parts would need to be cleaned up after casting, however; the flues must be removed, the surface bead blasted, and critical interfaces machined.

Finally, you've forgotten an option: 3D printed stainless steel. The on-line shop 3Diligent (

http://www.3diligent.com/3d-printing-service/powder-bed-fusion/)

is one of many on-line shops offering the capability. Their site says they can do parts as large as 350mm x 350mm x 380mm with wall thickness down to 1 mm and feature size down to 0.125mm. The surface finish is a bit rough (comparable to casting), so the part may need post-processing. Every shop I've seen offers that service as well. There's a cool example of what's being done with 3D printed metal at

http://www.motohive.in/light-rider/

(note the article says the surface finish was accomplished by glass bead blasting). The parts were printed by a German firm APWorks (

https://apworks.de/en/)

and they offer stainless steel as well.

I don't think I'd do anything until I had spoken with the 3D printing shops. There's no limit on designs (though not a big deal for your current part) and very little if any set-up and tooling cost. Their cost per part may be a bit higher than the other options, but it doesn't sound like you're entering a high volume market, and you'd have the advantage of not having to keep a lot of inventory (the other processes usually require pretty large orders to get the price down).

 

[btrueblood]

Make sure you get the 3d print shops to quote a fully sealed design, no porosity.

 

[michaelwoodcoc]

quaipau,

this is cool, and this is something I'd make myself. Stainless. I'd probably get the exhaust bolt/stud flange water jet cut, then I'd brake it to the angles myself. I'd try and modify the design for 90degree angles. For those steps on the pipe, I'd hope you could find a similar sized pipe/tube, lathe the inner diameter and outer diameter to the right size, cut them on the band saw, if your band saw is not straight enough you'd have to face them after this operation, then weld them all up myself. TIG is fun. It looks like the OEM part hardly has any welding.

I'm not sure how bending would affect the stainless steel, I'm sure some grades of stainless are fine. In the past I've just tig welded slighly dissimilar stainless steels together, as long as the filler rod was acceptable to both metals. some metals we had around the shop were unlabeled so I took the chance but no failures.

looks like you could also get most of it cnc machined on a lathe, then weld on the bolt flange, and the other little step/flange on the pipe, but i think it'd be most economical the other way.