Shot peening piston crown?

[Schurkey]

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!

Are you talking about shot peening as a fatigue-resistant surface treatment, or shot blasting as a general-purpose parts-cleaning method?

HUGE difference in equipment and intended outcome.

[modok]

Piston crowns rarely crack.
When they do crack it is usually due to differential expansion from the surface being heated above 400F. It cracks as it cools. If you could peen it WHILE it is cooling then that would prevent cracking. Peening it before would do nothing or make it worse. That's crazy anyway, the damage is already done..... just for example that's where surface compression would help. It's NOT desirable for the piston to be that hot, not for what you are doing. If you piston crown cracked it's because it overheated, or a local area overheated because it was too thin. It's not the weak link in the engine.

so... solution looking for a problem, maybe it would work in some different situation.
Can you shot blast a piston top...sure. Should you? probably not, But I'd say it's better than polishing it. Still got lots of guys doing that even though it works worse sometimes. Sometimes the inside of the piston is blasted, seems to be no problem with that.
Now there is some voo-do in the surface texture and shape of piston tops.
But I'm confident it is due to either boundary layer effects, mechanical adhesion, or reflection, all influencing the burn.....not due surface compression improving any mechanical aspect of the piston.

[allencr267]

Is this your way to a thinner crown, to lighten it through strength?

[jsgarage]

Only reason I can think of is if you were having problems with top ring land cracking and were trying to strengthen 'A Particular Piston' for some reason. Personally, I would change piston design. There's some info on strengthening light alloy via bead blasting light alloy on the Web. Strength changes from bead blasting aluminum or mag reportedly happen very fast if you try this. Seems easy to go from 'slightly better' to brittle which may be the source of 'enhanced cracking'.

[DCal]

CP had some forgings delivered from the foundry shot peened inside only. These were requested for 5 Nascar engines. As each machining process was finished they were measured and then on to the next station, measured, inspected etc and so on. When the final inspection was complete it showed almost no measurable difference from unpeened forgings . The customer didn't order any more with that process, we didn't ask and they didn't tell. So I assume they worked but not to any degree that warranted the extra price.

[Momus]

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!

You are referring to shot blasting which is a gross cleaning process.

Many aluminium parts are routinely shot peened.
Peening is a refined, specialised precision process from aerospace with widely understood benefits from surface compaction.

Highly stressed aluminium components like motocross motorcycle handlebars are routinely peened.

[Taranis]

Thanks to all for the responses.

It seems that the algorithm on the site is better than Google's search, as the threads suggested at the bottom of this page and the rabbit holing that followed led me to far more useful info than using the search bar.

My thinking leading up to starting this thread was to assume that something like a CP piston for a modern MX engine is probably pretty darn close to strength-weight optimized. So if I was going to be taking material out of it, the default assumption was it would be weakened and it would be worth minimizing the effects of that in any way possible.

Or not. Found lots of discussion of tuning up piston crowns in various ways, using CNC, manual machining, and also hand work, generally with productive results. It's even effectively SOP for some builders. Some leave a grit finish from the hand work, some polish, some glass bead. Nobody mentioned pistons cracking as a downside of any of the practices.

Especially for a single-cylinder engine, testing a few variations developed through hand work and manual machining (while applying common sense, judgement, careful thought, lots of measuring and even a little science) of one piston at a time is likely to be much more cost effective than having them built one at a time. Especially since CP's web site indicates a minimum order on custom pistons of 4 pieces; Wiseco's custom form indicates a minimum of 12 pieces. That could get to be expensive development testing.

[piston guy]

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.

It's interesting Jack that when I created the spherical design everyone scoffed at it. I tried to get the company I was working for at the time to patent the design but they chose not to. BME , CP and others began doing them too. The torque increase was more than the HP by almost double and doing that with two degrees less timing told me we were more efficient. I haven't seen a Nascar piston for about ten years so I have no clue what they are doing now. The spherical design is still good and being used by some piston companies. It does favor a small combustion chamber for sure.

[Mark O'Neal]

"Patents are wonderful....as long as you want to spend the rest of your life in court defending them." - Sir James Dyson

[Taranis]

It's interesting Jack that when I created the spherical design everyone scoffed at it. I tried to get the company I was working for at the time to patent the design but they chose not to. BME , CP and others began doing them too. The torque increase was more than the HP by almost double and doing that with two degrees less timing told me we were more efficient. I haven't seen a Nascar piston for about ten years so I have no clue what they are doing now. The spherical design is still good and being used by some piston companies. It does favor a small combustion chamber for sure.

I've been thinking about that. I see Wiseco uses it on their standard bore YZ250F piston, but not the big bore versions. I don't know which came first.

One direction that makes sense is that a "chamber mirror" piston is emphasizing getting the squish charge toward the flame, but may compromise in getting the flame back out to the edge of the chamber as the piston moves away from TDC and opens the space. It makes sense to me that the spherical bowl could be a good compromise between effective squish and providing enough space for the turbulent combustion to grow back into the gap.

Also, I've read that when piston velocity goes to zero at TDC, the air and fuel left in the squish zone slow way down, and the fuel can stick to the piston or flat surface of the head, making it much more difficult to vaporize and burn. The wedge of the spherical bowl promotes movement more than a flat-round dish, but still leaves a little space so the movement can continue through TDC. Air and fuel keep moving, so the fuel doesn't stick to the metal as much.

Anyway, just some brain dribblings. I don't have simulation software to do a halfway decent job of modeling it, just thinking about it.

[modok]

The subaru EJ engine piston is an interesting example. gentle dish with a mound in the middle.
For the first few years I did not know why it had bump in the middle
then somebody told me it's to clear the rod And it's true.

[juuhanaa]

If the shot/flap peened part need a heat treatment we should not exceed (aluminum alloys 200F), or the peening affect will be neutralized.



-juhana

[Mark O'Neal]

The subaru EJ engine piston is an interesting example. gentle dish with a mound in the middle.
For the first few years I did not know why it had bump in the middle
then somebody told me it's to clear the rod And it's true.

Please keep that quiet. I have a fair number of guys that think it's a power trick. I quit arguing with them a long time ago.

[Mark O'Neal]

I'd also like to state that this seems to be a solution looking for a problem.

[Bill Chase]

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!

You guys would be shocked by the propane tank industry's aluminum fork lift cylinders. annealing furnace to soften, then weld all the spuds on, directly into a wheelabrater for a 45-60 seconds with a hard grade of stainless steel shot. every tenth part is tested with destructive measures to determine material thickness in key areas, the joggle joint is crushed to it's yield point etc. Anyway we measure blank thickness, thickness of each dome after the blank is drawn into shape, then again after blasting. Typically removes quite a bit of material. I would be lying if I said I knew exactly how much. But I can say it is substantial. When they come out of the blaster they are visibly work hardened, and so hot you cannot touch with bare hands. Sound scary? Probably around a 70% chance you have one of these tanks on your fork lift if you are in middle America. And our products are d.o.t.certified for transport when full, as well as a different cert for military use. They have to be recertified every 60 months, usually just destroyed and new ordered due to cost,but we do offer customers a recertification service if they want to pay for it.