Cylinder head bolt threads - BSF or BSCY?

[BSA_54A10]

The trick to getting helicoils to work is to use the correct size drill.
Unfortunately there are a lot of really cheap kits out there that substitute really cheap standard size drill for the expensive correct size ones.

To my surprise I had no 3/8 BSF helicoil kit ( or I had loaned it to some one who had not returned it) as I did have  packet of 3.8 BSF 2.5d inserts.
I ordered a kit from roadnracebridgewater on evilbay and it is my fault because the photos clearly show it comes with a elchepo 10mm drill in place of the correct 9.8mm.
https://www.ebay.com.au/itm/3-8-20-TPI-BSF-Helecoil-Thread-Repair-Kit-14Pc-NORTON-TRIUMPH-BSA/292533902835?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
Now 0.2 mm might sound like nothing and if it was a deep thread like WW or UNC then the sloppy fit from the bigger drill would have been fine.
But on a shallow BSF thread it is enough for the helicoil to wind out or the bolts to strip out.
As luck would have it I had a W size drill in the odd size drill box & W is .386" or 9.8mm
The other big no no is not winding the helicoil 1 ( 2 is better ) threads deeper than the top of the hole.
And if you are a belt & braces type of guy, wind it 2 threads below the top. then center punch the threads above the insert to prevent it from winding out.

[bsa-bill]

Thanks guys - forum at it's best here.
I've been turning this over in my head ever since I built the RGF, it works as it is at the moment, but next year ( after I've sold the Flash  )
I intend to make the RGF more of a Flash albeit with clues from my brothers recently purchased Goldie, so will need to address the head (barrel) issue, so ideas and plans here are very helpfull