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I have been trying to remove the relief valve seat [Pos. 18] from the block. Tried the M8-bolt method and all other trickery, to no attempt. The seat does not have a thread as the replacment one.

What have you done to get this culprit out?

I ran a tap into it and pulled it out with a slide hammer.

I tapped mine, screwed a stud into it, placed a thick washer over the stud so it rested on the machined face of the block, then wound a nut down the stud and pulled out the relief valve seat.

imack wrote: ↑Wed Oct 02, 2024 5:03 am I tapped mine, screwed a stud into it, placed a thick washer over the stud so it rested on the machined face of the block, then wound a nut down the stud and pulled out the relief valve seat.

My method as well

That was exactly what came to my mind when I went to bed after the post. Though I dreamt of swarf from tapping falling into the readily built engine .
I reckon a ring magnet around the tap along with some suction doohickey for the vacuum cleaner is required.

If swarf is worrying you, I would say that the vacuum cleaner method is your best bet. Tape a drinking straw to the nozzle and put this down the hole while you are tapping it. That'll do the job. It's a well used trick in engineering circles. Plus the thick washer to spread the load idea

The original manual seat has no thread. I tap them 3/8" UNC, then use the short head stud, a nut and washers to draw them out.

The only way you can be sure of getting the swarf out from cutting the thread (and what is likely lurking around the seat) is to strip the engine, pull all plugs, wash it and inspect or you can run the gauntlet.

Perhaps bonding in a stud with Loctite high strength bearing retainer may negate the need for tapping.

Thanks for all the useful hints - well, except for Spider's suggestion to strip the engine (that is exactly, what I did before this now very last task of going back to fitting a standard barrel shaped reliefe valve) - don't trigger my arachnophobia, Mr Spider
I will carefully thread it, maybe with some grease on the tap too, so the swarf will stick to it. AND some thoughts and prayers beforehand, just to make sure

You might get lucky . Sincerely, I wish you well with it. I suggest stripping it as where it fits in the oil ways, it's where the oil way to the valve itself and the oil way to Mo. 3 Main intersect, the former being un-filtered oil and the later being filtered. If it was just a single drilling, you'd stand a reasonable chance of getting all the swarf out (and if you can remove the oil pump, prior to tapping, that'll help too as there's a right angle in to that which can hold swarf), however, with this intersection and a step in the drilling here, some swarf may well find it's way in to the feed to No. 3 Main, or back the other way in to the Main Gallery.

rnp68 wrote: ↑Wed Oct 02, 2024 11:22 am Perhaps bonding in a stud with Loctite high strength bearing retainer may negate the need for tapping.

They are fitted with a tight interference fit so they don't hydraulic out. I'd say if high grade loctite was strong enough to hold a stud to get them out, then there's other issues to resolve as well. They really do take some effort to get them out.

Why do you want to pull the seat out without being in a rebuild situation? Is it sticking?

Chapeau! Thanks to Spider and everyone who supported me in my endevour (be it with tips or just mentally).
It's out - I used the 3/8 tap and said head bolt, which I had in my "scrap box" (helps to be a horder)
I used grease on the tap and closed the bores with double sided sticky tape, which closed the bores even in the pulling process.
I also just tapped the thread some 6-7mm deep. Once everything was out I sucked anything loose with a strong vacuum cleaner and cleaned the bore with greased cotton buds.

@Gary: The engine is already completely refurbished, but I did not change the seat , and the valve was one of the ball set-ups. Uncertain about the seat condition I thought it'd be good to change the whole assembly.
When I initiallly bought the engine spares the seat was not available and I only learned later that the ball is no good. So I had to bite the bullet and now change everything on a completely finished engine.