There has been wrong info here regarding torque values for the J-engine head bolts ...

[sam2019]

... unfortunately one of our members used them and consequently had a blow out of the head gasket.
The issue has since been resolved and he thankfully provided a link to reliable (aka OEM) service manual based information of same.

Some may remember the "cease and desist" note I once got from RE lawyers for having uploaded manuals to the forum directly (or to my own google drive account) so I won't do that, but there is a link to said manual from elsewhere:

read this thread and scroll down to "Veloman":

Classic 350 factory service manual

Considering buying a 2022 Classic 350. I have seen some discussion threads saying that the factory service manual is not available from the dealer or from Royal Enfield itself. Is this true? I do all my own work on my motorcycles and always buy the factory service manual for that purpose. No...

www.royalenfieldforum.com

 

[Dool]

I have only looked at a bit of this manual, I would be careful reading it!
It can be a bit misleading as it will show a pic of something then torque specs for something completely different, point in question.

 

[sam2019]

@Bruce sent me a PM with this info pertaining to the issue:

A new dispatch from the front lines: I picked up the new head bolts and washers from my local dealer today, and while I was there I asked them to check the torque specs for me. He went into his dealer database and pulled up the repair manual, and it says: first tighten to 15Nm, then tighten to 35Nm, and finally tighten an additional 90 degrees - NOT 150 degrees. (That makes much more sense to me, as I always thought 150 degrees seemed to be too much for a steel bolt going into aluminum alloy, but I'm not an engineer. ) You may want to check this out and update the info sheet (again), I don't know if RE made a change to the specs or what, but this new info came out of the dealers RE computer - not the internet. I would also suggest you make a note that these torque specs only apply to NEW bolts and NEW washers. If someone decides to reuse the old ones, they are on their own.
A little note of interest, the new washers are a little different from the old ones I took off. Rather than appearing to be of a two-piece design, the new ones are one piece, with a slightly smaller outside diameter, and just a hair thinner. OEM part number packaged in a sealed OEM Royal Enfield bag. Bolts appear to be the same as my old ones.
I saw Dools comment, and I was going to mention the same thing to you earlier. I noticed discrepenceis where the pictures do not match the text in a few places, I just figured the graphic people doing the paste-up were not mechanically/motorcycle inclined. The index has no page numbers, so maybe this was an early draft. However, I have also found in this manual different torque specs listed in different parts of the manual for the same part. (One specifically is the nuts holding the exhaust pipe
to the studs in the cylinder head, there are two different specs listed depending on where you are looking in the manual.) Hopefully RE will get it sorted out over time - and proofreading. Until then, buyer (and mechanic) beware