Thunderstorm Pistons - Caveat Emptor

This is something I came across recently and I thought I'd share it.



This is a competing brand of piston, a very well-known brand, and in fact, at least one of our competitors ships this brand of piston in their engine kits. This piston is designated by the manufacturer as being a match for Thunderstorm heads.


As you can see, the dome angle is 10 degrees. However, the squish band shelf in the Thunderstorm head is 15 degrees, and in fact the factory Thunderstorm piston has a matching 15 degree  dome angle.


Why is this a problem? Well, ideally you'd like the squish clearance between the piston and the head to be in the .030" range; this promotes good chamber turbulence and efficiency because the fuel gets squeezed out of the squish band while the piston passes through TDC. We talk about squish clearance and how to optimize it in the General Instruction Sheet that we provide with all engine kits.


However, the shallower 10 degree dome angle of the piston shown above will not not allow a proper squish clearance when used with a standard 15 degree Thunderstorm head. The squish band and the benefits thereof will not be realized if you use this piston under a stock Thunderstorm chamber.


When I asked the manufacturer of this piston about the 10 degree dome angle in a 15 degree chamber, they told me that by making it 10 degrees, they could use the same piston under the 10 degree Lightning head (Lightning heads came stock on XL1200S models as well as some 96-98 Buell models). And it was better to have too shallow of an angle under the Thunderstorm head than too steep of an angle under the Lightning head (which of course is true, but kind of misses the point).

So basically, they're offering something that really doesn't work well so that they can minimize the number of different part numbers in their book and pistons on their shelf.


As someone who works really hard to find every last horsepower for his customers, I just can't imagine that. It's outside my scope of understanding.

There's another problem with trying to double up that application, too. The Thunderstorm chamber is 67cc, the Lightning chamber is 62cc. Using the same piston for both applications is either going to result in a high CR under a Lightning head or a low CR under the Thunderstorm head, depending on which chamber the dome is sized for. I'm not sure if they advertise two different compression ratios depending on the heads or not, I didn't ask.

Here is a Hurricane piston designed for a Thunderstorm head:



As you can see, it has the proper 15 degree dome angle, which matches the standard Thunderstorm chamber squish shelf. The dome volume of this piston will give a 10.5:1 compression ratio under a 67cc Thunderstorm chamber.


Likewise, if you buy a set of Hurricane pistons for a Lightning head, you get a 10 degree dome angle to match the 10 degree squish shelf angle of the Lightning head, and a dome volume that gives 10.5:1 compression under the Lightning head's 62cc chamber volume.


We also have 30 degree pistons for higher performance applications. The 30 degree dome angle has advantages, but it needs more dome height (and thus tend to give higher CR) for a given amount of squish area. And of course, fitment requires machining the combustion chamber out to 30 degrees to match, which we can handle, we do it all the time.


The moral of the story here is to be careful what you buy. Don't let the cost saving measures of your vendors compromise your result.


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