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Cylinders possibly distort at 500Deg F?

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Andrew Campbell
Posts: 2
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Forgive me if I did any of this wrong this is the very first post I've made. I've had my 1975 R90 for about 13 years now, I should have joined the airheads a lot sooner!

I'll get right to the point, and I'm happy to answer questions.

I'm doing a total rebuild from scratch, and customizing my R90/6. I'm replacing the original cylinders with Seibenrock 1000cc kit cylinders & pistons which I already have here.

I am looking to do something different with my engine which has been vapor honed clean, cerakoting it black. Without going into all the ins and outs of cerakote finish, which I could explain, I am looking at doing V Series Cobra Black cerakote on the engine- which requires curing at 500 degrees for 1 hour in an oven. I am pretty sure that every other part of the engine and transmission would be completely fine to do this with- but I am wondering if the cylinders will warp from doing this.

I don't consider this lightly.

My background is skilled machinist, ex tool & die maker, horologist & blacksmith. I'm used to doing friction fits and heat shrink fits, liquid nitrogen shrink fits and checking fits in general. I've had items heat treated, and I know that heat treatment depending on the temperature can release residual stresses in metals. I know the normal running temperature for an airhead engine is around 280F tops or so. I'm wondering if anyone has ever mic'ed bores on cylinders and heated them up that much more for only an hour and then measured the roundness on them, ideally on a CMM (coordinate measuring machine) or just with bore gages to see if they distort.

Does anyone have any thoughts on this?

There is a high temp C Series cerakote line, which does not require oven baking to cure, that I know other people have successfully ran on airhead engines. But I have not confirmed that anyone has used V-Series- which gets a much darker black that is much more UV colorfast, and much more abrasion resistant than C-Series. Hence much more robust and longer lasting, while still having thermal cycling capability of up to 1800F (overkill I know, yes).

I am potentially willing to do this and it might perfectly well be fine but this was the only place I could think of finding people who might have intelligence in this area.



Posted : 02/19/2024 21:58
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Richard W
Posts: 2529

I'm a Mechanical Engineer and long-time motorcycle mechanic. I don't think you'll have an issue metallurgically speaking, but 500°F is much hotter than the Airhead engine runs. Seems like a "bridge too far" when it's all being done for purely aesthetic reasons, and possibly a few "bragging rights" with your buddies. This, especially since there are 1) other easier ways to achieve "black" coloration, 2) that you may not have considered what an external coating does concerning the thermals of a running engine, and 3) that it's going to look really bad and be very hard to clean after being caught in your first rain storm. 

As I have said on other boards, artsy motorcycle types too often loose site of the primary goals of motorcycle riding, which are: Ride bikes. Pick up chicks. Have fun. Believe me when I say... all chicks (past, present and future) will not give a toot about how black your black cylinders are.

Speaking as one who has dumped thousands of dollars into motorcycles only to sell them a year later for 1/4 of that price.... the loss is not in the coating, but in the fact that you are emptying your wallet chasing a dream that ultimately had its start in something you read on the internet. Here's the truth as I see it (after spending 50 years in the motorcycle business): The one thing magazine and internet authors are really good at is spending other people's money. 


Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

Posted : 02/20/2024 04:46
Andrew Campbell
Posts: 2
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Topic starter

I appreciate feedback, but...I'm really preferring actual technical thoughts here only, thanks. I'm 40 mate... I think you're imagining me to be someone much younger and flippantly swayed..

The cylinders will be taken to 500 F only for 1 hour- no longer. So even though that's outside normal running range it's not going to be raising them anywhere near melting temperature. I may very well be overthinking this, but I'm concerned whether doing that at that temperature may warp the cylinders.

I figured I was going to have to answer a lot of questions and dissaugement, so it might be helpful if I describe what I've gone through already. Everything that's being done to the bike is being done to make it more ridable, reliable, and capable of stopping better. I'll skip the mechanical stuff, and focus on the coating here- custom bike builders have been using cerakote of various types, mostly c series, for years, for a few reasons-

It's 100% UV colorfast (C & V series), meaning it will not fade like paint or powder coat from exposure to the sun eventually. So it will stay looking whatever color it's done in. It's extremely durable stuff, and c and v series handle thermal cycling like a champ- when prepped correctly, it's not going to crack off eventually like paint or powdercoat can from thermal cycling. It's an extremely thin coating compared to powder coat, but it's extremely abrasion resistant, and because it's thin, you also don't run into things like clearance issues from coating thickness. It's also extremely resistant to most chemicals out there- meaning things like the chemical pitting I got on the inside of my air box shroud from years of chemicals in the air and moisture gradually collecting and corroding the inside surface are incredibly unlikely to happen any further. If you spill gas on your painted gastank- it will affect the finish. This will happen with powder coat as well. Cerkote is impervious to gasoline. And incredibly resistant to salt spray and its effects- so your frame isn't going to rot out if you live near the sea.

To address a very valid concern of coating thickness holding in heat for an aircooled engine- this is one of the first things I thought of. I was afraid the coating would hold heat in, because it was capable of withstanding 1800F. That suggested it was insulative. I've talked with coating engineers at cerakote, and they noted that was a valid concern- but these coatings only withstand that temperature- they are so thin (sub 0.001" thick), that they assured me I wouldn't really see a noticeable temperature insulation effect against emission, due to the sheer thickness of the coating. I went back and forth with them on this for a while, because it seemed like specifically Cobra Black was designed to insulate. Cerakote does actually have heat emissive and heat insulative variants- there is a special coating specifically for piston heads I won't be using that is insulative, that's designed to keep most of the heat in the cylinder in front of the piston from leaking away, aiding combustion. So this is a thing I already thought about for a few years, and I've been assured I shouldn't see some sort of overheating condition on this air-cooled engine because of the coating thinness.

So many other ways to do this? Yeah, I've spent the last 5 years testing other solutions on parts of the bike already for this purpose, and researching everything out there that could physically do this and survive the temperature change. They all failed. Tried specialty high temp paints like VHT header paint- they eventually (rather quickly, within 6 months of normal riding) flaked off. And this was after I blasted them myself for paint prep, and made sure they were clean completely with brake cleaner and denatured alcohol so that there was no chemical left on the surface. Had high temp powdercoat flake right off krauser heat shields within a couple months after being professionally done.

I do know someone who has powder coated their engine red, on a /5, and it seems to hold up, but they had some secret sauce in their prep I think to get it to last. Powdercoaters are not all equal, and depending on the keying surface you put on with the prep as well as the lay of the spray some people put stuff on better and more tenaciously than others. But it's kind of hard to control for that. And powder coat is thicker and is likely going to hold in more heat than cerakote. Powdercoat is also like paint, it fades in UV. With red you wouldn't notice that that much, but with black you will.

I've reached annodizing as well, and found that only a rare form of anodizing that is difficult to find, called architectural anodizing, which is a specialty subset that uses metallic particulate for the coloration, is actually UV colorfast long term. And I know from machining and having parts anodized, prep work is 90% of good anodizing, and you never get a perfectly uniform finish unless you're really working with someone good who's prep is flawless. This is cast aluminum, and there can be an impregnation of oil on the surface that could affect anodizing even with blasting and cleaning. This is one reason I chose vapir honing for the purpose of prepping the engine first, it scrubs that oil completely off everything as well as doesn't distort sealing surfaces. Afterwards things can be masked on places where there are seals to do the keying black beauty 150 grit light traditional sandblast needed to bed the cerakote adhesion.

I looked into specialty chemical aluminum blackening compounds from companies like Birchwood Casey & Eastwood, things like Black Alumiclad. They were nowhere near as chemically resilient or abrasive resistant or temperature resilient as cerakote.

Even looked at Jet Hot, and talked with their engineers. Their coating is actually insulative, because it's thicker as well as different composition from what I can tell. And only a few places do it and it's a ton of money. Even the company themselves wasn't sure if it would work in my application.

So, in the end, it all came back to Cerakote. It was the most chemically resistant, most durable most tenacious and UV colorfast coating that wouldn't make the engine heat up after applying, would go on uniform and dark. That might be why so many OEM manufacturers use it on motorcycle engines for black engines. And it's been used for years in high end custom bike builds with success. Even people in the dirt bike community have had frames and parts Cerakoted with success.

I just want a specific look to my bike, for myself. I'm taking this thing to the grave with me, it's not getting sold. I intend to pass this on to my family after I go one day. I'm doing this all once to the highest degree of capability that can be thrown at it, so that further maintenance in the future is simple. A lot of rust in areas and things tattered and falling apart that were not in ideal shape when I got the bike and just I never had time to go over the whole thing fully. And I assure you it will be ridden constantly. Before I took this thing down for complete overhaul, I would do thousand mile trips on it. But I also had constant issues of leaking seals and things from just it being an old bike needing more proper care. I also am thinking of adding an ultralight camping trailer which it should be able to handle with the extra oomph of the 1000cc upgrade and brand new Del Orto PHM38 pumper carburetors. I got tired of the electrical issues I had with the points and moisture somehow getting into the wiring frame, leaky pushrod boots, leaky rear main seal from someone who installed it wrong trying to help me when I was still learning.  Basically a lot of grief with things that I wanted to just go over once completely from scratch every single part on the bike myself to make it correct so that I don't have to screw with anything again, at least within a significant time frame.

That way I can get what I want, and spend as much time riding as possible once this is done.


Posted : 02/20/2024 06:53
Richard W
Posts: 2529

Then I think the major issues will revolve around leaking oil.

Some cylinder makers cast the iron liner into the finned aluminum housing. Other makers precisely bore the aluminum housing and shrink-in the iron liner in as a separate part. When building the cylinder assy per the second option, it is common to add O-rings between the 2 parts during assembly. It is also common practice to add rubber dampeners between the fins to reduce engine noise. 

• At 500°F, any and all these rubber parts will become toast. And engine oil (which is designed to seek out very small spaces) may begin to escape.

• And there will be zero recourse with Seibenrock as far as warranty, because your personal modification will be self-evident.

That's all I can see. I wish you all the best.

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

Posted : 02/21/2024 07:02
Steven Rankin
Posts: 174
Estimable Member

I am not going to go into great detail.  

Richard gave good reasons against the plan you have.

I personally would not try heating jugs to that temperature.   The Siebenrock replacement jugs are not cast iron sleeved but are nikasail style built up bores.  

I have no idea as to how this combination of metals react to, as Richard writes, higher than normal operating temperatures.  

Frankly I have never seen anyone reach that kind of operating temperatures with their bikes.  I have never heard of anyone "baking" jugs to that temperature just to cure a coating.  So, I have no experience as to possible damage due to any distortion caused by the heat involved.  

I am cheap, I use engine black paint, matte, and it lasts a long time without the possible destruction of expensive parts or over heating issues while riding.

Richard also makes a good point that any damage will be your responsibility not Siebenrocks.    

As I say, I am cheap, I personally would not spend $1500 on an experiment just for a very minor reason.

If you do go ahead, be sure to let us know how things work out. St.

Beware! I do not suffer fools gladly! St.

Posted : 02/21/2024 13:49
James Strickland
Posts: 419
Reputable Member

Just do it. If you are super meticulous, measure the bores before and after the cerakote process. If the cylinders become de-formed, try again knowing that you ought not do that. It's only money. 

former Airmarshal, IL.

Posted : 02/21/2024 19:12


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