Brett,
Curious as to why the case drilling "makes a problem for the next person." Will the O.S. NVA not install well on a drilled case?
Correct - you drill the hole in teh case and venturi out to fit the 4mm spraybar, and the 3.5 mm cannot be installed without rattling around in the hole, and it will not retain the venturi properly. To get around it, you have to sleeve the 3.5 mm spraybar., but only on the ends when it engages the case and venturi, because the entire point is to have it be 3.5mm in the choke section.
You can make the sleeves from brass tubing, install that on the smaller spraybar, and then use JB-Weld or something to hold it. With all that you still have to drill out the venturi holes because you still have to fit the 4mm sleeves through it. All this in order to *return it to the stock configuration* - which could also have been accomplished by leaving it alone entirely from the start.
So, this scenario which has occurred to me repeatedly:
" Hey Brett, I tried the 20FP and it's gutless and it won't fly my Twister in any sort of wind, you don't know what you are talking about!"
<<looks at spraybar - ST Clone>>"I told you not to use the ST spraybar"
"But it's better"
"It's 25-30% of the power off the top"
"Oh..."
"I have got stock parts in the van you could use - but they won't fit any more!"
"Oh..." <<walks away>>
A more practical alternative is to get larger venturis to use the ST spraybar as is but still get the same choke area. OF course, now you are headed down the modification rathole, and your system cannot compare to anyone else's -there's no baseline. So, back to random.
As always, it's just advice, no one has to follow it. I would prefer to avoid any more exchanges like the one above, having already experienced it 20 times, but it's entirely up to the pilot.
Brett