Shot peening piston crown?

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Taranis
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Shot peening piston crown?

Post by Taranis »

Have lurked for awhile, just registered to post a question that I haven't been able to find any info on. I have pretty good google-fu, so if I spend a half hour or so looking and find zero discussion, it seems worth at least asking the question.

There's lots of talk about sand blasting or glass bead blasting pistons. I'm thinking about a slightly different process, shot peening. The process has probably been discussed to death in the context of connecting rods and other stuff, but I can't find a mention of it on piston crowns.

I'm working on planning out a build of a single cylinder dirt bike where none of the commercial, off-the-shelf pistons I can find are really ideal. There is a flat top that I think would work well with a little hand detailing work and maybe slight milling to get it a dish and finalize volume once all the other work is done. I'm concerned that in an 11,000 rpm application, any scratch or tool mark is a possible crack initiation site. You can polish them out, but there's always a chance of some being a little deeper than you want to polish. And then a lot of people I respect by reputation say to go the other way and blast them.

In theory, shot peening should work all the microscratches (and possibly some less-micro scratches) back down, uniforming the surface, giving it a minimum radius for any surface texture, and putting it in compression. So it should have benefits usually identified with glass beading, but without the risk of adding microscopic or larger pits, and no possibility of embedded fractured media, etc.

Curious what more experienced people think about this. Am I overthinking it? Overconservative? Nothing to worry about with hand tooling marks (gentle filing, careful grinding, sanding, etc.) on the surface, or am I right to be concerned?
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Re: Shot peening piston crown?

Post by digger »

surface texture is probably for helping formation of nice free coating of insulating carbon.

Not sure there is much to be gain from shot peening crowns as crown failure is usually overheating or gross overload not your typical cyclical fatigue in a highly tensile stress regime
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Re: Shot peening piston crown?

Post by englertracing »

If the piston you want to uses is an existing motorcycle piston don't expect too much material avaliable for removal

Can probably get cp to make you a piston for 260 ish dollars
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Re: Shot peening piston crown?

Post by BCjohnny »

By no means a complete reply but true shot peening is mostly used to deter failure modes related to tensile forces, specifically near surface

It places the 'skin' of the component under pre-tension such that tensile cracks become harder to form and propagate

Is the piston crown subject to predominantly tensile forces ?

As said, heat is the major source of failure initiation, and all the peening in the world won't change that

And you basically have to ask yourself if the process actually added value, why isn't it used more widely ?
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Re: Shot peening piston crown?

Post by PSA »

Aluminum tend to behave very differently than steel, one part of this is that the oxidize layer is a lot harder than the material itself.
I believe I read somewhere that some people that used/tried micro peening on piston skirts had more failures, but I can't recall right now where I read that.
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Re: Shot peening piston crown?

Post by mag2555 »

If you have ever seen for example a late crappy cast iron SBC head like the 882 casting come out of a shot peen machine ( that has been loaded with fresh shot ) with all its machines edges now rounded over by 1/4” you would then know that shot peening is not a process to be used on anything other then steel’s or cast iron!
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Re: Shot peening piston crown?

Post by Taranis »

Piston is a CP shelf piston. They only list the one version for the engine and bore size. I am aware they do custom, but the minimum order is four. That's likely a lifetime supply (or more) for a single cylinder engine the way it will be used. Would want to have proven the design before spending $1000+ to be locked into them. My thought is that once I validate the performance of the design, I'd work with CP to have a batch made replicating it in such a way that their engineers were able to bless it structurally, etc.

The final details of course depends on measurements that can't be obtained without parts in hand. The bike is a Yamaha WR450F with a +3mm big bore. Target compression ratio would be mid-12s after unshrouding valves, etc. in the head. CP advertises the flat top at 13.5:1, which I assume includes factory gaskets. Most single cylinder motorcycles have separate cylinders, so you have both the base and head gaskets to play with when setting up squish clearance. Cometic will make custom thickness gaskets. OE squish clearance tends to be in the neighborhood of 0.060. I'd target 0.030-0.040, probably right in the middle of that. I don't know where CP put the clearance with this part - need to measure. But if I'm reducing clearance, that increases compression even more.

There are shelf pistons with dish and lower compression, but all the currently available pistons I've been able to find photos of (including CP's shelf pistons for the quad version of the motor that has a different wrist pin diameter) just have a round dish machined over the whole thing, which throws away a really big part of the squish area provided by the head. Understandable for a generic shelf piston when the manufacturer doesn't know what any given builder will have done with the head. The idea I'm pursuing is having a piston that is flat topped wherever the head has squish area and dish that matches the chamber area in the head as much as possible. Essentially, to thoughtfully use the piston crown as the bottom half of the combustion chamber, rather than treating them as only somewhat related.

The process I imagine is first detailing around the valve reliefs CP cuts, then CCing it and figuring out how deep to mill a dish in the center, leaving all the squish area around the edges. The other reading I've done on this site about final tuning of piston crowns encourages me that the concept isn't on the face of it going to cause a thermonuclear explosion under my seat. There was at one time a shop building pistons that I would just buy and roll on, but they closed up and the pistons haven't been made in a dozen years. I'm at this point investing in due diligence to verify there's a pathway to the place I want to get to, and that the price should be at least sort of reasonable, before I start spending money to fill my garage with machines and parts.

Connecting rods have cyclic stress; tension around TDC and compression around BDC with cylinder pressure overlaid on the inertial forces. The piston crown surface, similarly, likely sees more compression around TDC and tension around BDC, if you think of it as a disc being moved up and down by force applied at its center. That is not a totally unreasonable starting point for thinking about a modern performance motorcycle piston. So if tool marks on the surface create enough of a stress riser/crack initiation site, one can at least expect it to see tension during the operating cycle.

The oxide layer on aluminum that's not anodized is so thin I am not aware of it being significant in the structural performance of the material.

Shot peening can be applied with a variety of equipment and methods, some of which are inherently adjustable in their process variables. Obviously, it would need to be done in a process tailored to the parts and the material to obtain desired results.
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Re: Shot peening piston crown?

Post by lefty o »

you seemed determined to do it, so do it. personally pick your piston manufacturer , call them, and have a piston made. its only 1 piston, so cost should not bankrupt you, unlike a set of 8.
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Re: Shot peening piston crown?

Post by dannobee »

Taranis wrote: Tue Feb 15, 2022 12:34 am
I have pretty good google-fu, so if I spend a half hour or so looking and find zero discussion, it seems worth at least asking the question.
If you spent half an hour looking and found nothing, there's probably a very good reason why. Collectively, we've built thousands (tens of thousands?) of race engines and have never heard of anyone shot peening a forged race piston. Especially on the crown. There can be an argument made for peening the skirt to reduce friction, but modern coatings would reduce the friction more than simply shot peening.

The fact that no race piston manufacturer advertises shot peening anything should speak volumes.
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Re: Shot peening piston crown?

Post by piston guy »

Taranis,
Pistons do not crack because of material surface imperfections / scratches. Aluminum cracks because of FLEX which causes "work hardening" and cracking . Shot peening would compact the surface to some degree and provide a nice surface for carbon build up as has been mentioned. I don't see any benefit from the process on the surface , though it is often requested on the UNDERSIDE of a piston for surface compaction / stress relief but it MUST be done before ring lands are cut and skirts turned as the forging "moves" because of the process.
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Re: Shot peening piston crown?

Post by PackardV8 »

Taranis wrote: Tue Feb 15, 2022 9:17 am
There are shelf pistons with dish and lower compression, but all the currently available pistons I've been able to find photos of just have a round dish machined over the whole thing, which throws away a really big part of the squish area provided by the head. The idea I'm pursuing is having a piston that is flat topped wherever the head has squish area and dish that matches the chamber area in the head as much as possible. Essentially, to thoughtfully use the piston crown as the bottom half of the combustion chamber, rather than treating them as only somewhat related.
Like you, I've been accused of overthinking. Back in the day, I spent money having custom piston dome machining to mirror the combustion chamber. Then, one day here, a NASCAR builder with dyno proof, told me they made more power with a simple symmetrical dish than with the custom shape.
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Re: Shot peening piston crown?

Post by rcull »

When preparing pistons for thermal barrier coatings and dispersants, etc., techline and probably others blast the piston with Garnet or Aluminum Oxide. This prepares the surface for the coating. It would definitely clear and minor starches which sounds like your prime objective.

Perhaps you could just coat or have someone coat the piston tops with a thermal barrier?
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Re: Shot peening piston crown?

Post by David Redszus »

As has been correctly stated earlier, piston crowns are not subject to stress failures (valve impacts excepted), but they often
fail due to excessive heat, mostly due to pre-ignition.
The oxide layer on aluminum that's not anodized is so thin I am not aware of it being significant in the structural performance of the material.
Performance pistons are very often anodized to provide a hard surface that penetrates aluminum that will resist the
erosion effects of detonation. While there is some heat barrier effect due to anodizing, it is minor.
Shot peening can be applied with a variety of equipment and methods, some of which are inherently adjustable in their process variables. Obviously, it would need to be done in a process tailored to the parts and the material to obtain desired results.
Shot peening produces local surface stresses that help strengthen the material. Aluminum is often "shot peened" using glass bead instead of steel shot. Bead size and air pressure are critical to ensure a stressed surface.
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Re: Shot peening piston crown?

Post by Malvn »

Here is a link plus their is others that do the same with pistons that are for high out put racing engines :)
viewtopic.php?f=1&t=25658
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Re: Shot peening piston crown?

Post by Mark O'Neal »

No.
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