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  • Kris

Theta ii Bottom End

When adding more power to your Theta ii engine there are few areas of concern when going to high whp levels. But even at moderate levels, there is a weak link that can result in catastrophic failure if tuning is not done properly, or the proper modifications aren't done.



A catch can setup that removes the stock PCV is the number one mod I recommend for these motors for longevity. Being that these engine suffer from excessive blow-by as it is leaving the stock PCV in place has a lot of ill effects that can even take a toll on a stock engine after time. There will be another article as to why a proper catch can setup is important and why we designed ours the way we did.


The stock pistons will be the weak point for the Theta ii engine. Cast design and there are some things to address.

Above is a stock cast piston from a K5/YF. A couple things to note. Notice the ring land for the oil control ring. Missing material. This is a common fail point to these engines.


Also notice the carbon buildup on the sides of these pistons. This would be caused by excessive oil being burnt through the stock pcv system,overly rich tune, or excessive blowby.

These carbon deposits can be potential hot spots for pre ignition issues resulting in the failure seen below.


When reaching higher boost pressures the factory tolerance is too tight for the added heat. Piston to wall clearance from the factory is

0.00079 ~ 0.00157in.

Adding boost, means adding heat which in turn causes expansion. Just to give a comparison a sheet of paper is about .004"




When we designed the KSR Forged pistons, strength was the number one priority. These pistons feature armor plating, and coating. HD Strutted forgings and improved ring lands. Below are some examples of the pistons we offer. Good for 1000whp. No corners cut.




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