So in a fit of paranoia, I got a spare B20f head from the junkyard. It was cheap, and right there. But dang, heavy! I've never pulled a B20 head before...
And I noticed something right away- where the valves pocket into the edges of the head, the pocket extends right up to the edge of the cylinder.
From working on other engines, I've come to expect a ring around the head, right at the edge of the cylinder. The books call it the 'squish' zone, a ring that almost touches the piston at TDC to push the flame front away from the sharp edge of the cylinder- to- head interface. In fact, they make a pretty big deal about it...

So what's the deal? Where's the squish?

What does this mean for Seb and me, looneys who're turboing our old pushrod lumps? Are we gonna detonate for days?

Inquiring loonies are wondering... maybe the B21FT makes more sense after all...

t

Obviously every B18/20 head is like this and it works well, however machining in a squish area, couple of ml deep is a useful modification to lower the compression a little for big bore or turbo motors and it seems to let the engines run better on low octane petrol, if it was important for standard motors Volvo would have done it in the early sixties

I found this item below somewhere ages ago: I don't know how true it is, but is sounds interesting and relevant to the discussion. If anyuone has had any experience of something like this, I would be very interested.

Regards

JohnH

"The very best thing I have found to avoid all the problems of low octane fuel is a trick I picked up from Nissan. When they increased the capacity of one their engines, rather than re-design the cylinder head to cope with the increased compression pressures, they just performed some simple surgery, and it goes like this!
Remove cylinder head. Have your friendly engine machine shop machine circles into the head, their position and diameter to match the location of the "fire ring" of the head gasket. I think this is called "fly cutting" in the US. Make each circle about 0.040" deep but no deeper due to the possibility of weakening the structure. Have them "cc" the combustion chambers to make sure they're all the same after they give the mating surface a very light skim. Re-fit head.
As near as I figure the theory is that while this lowers the compression ratio marginally (about half a point or so) more importantly it alters the burning characteristics of the flame front on the "power" stroke. The decreased degree of "squish" and lower level of turbulence results in a slower, more controlled burn. This is particularly effective in getting rid of "ping" or "knock" (detonation). The most expensive part of the exercise is the top-end gasket set.
The machining is remarkably easy (cheap) and it’s a permanent cure. I've done this a couple of times with nothing but positive results. My wife's '84 240 runs smoother, quieter, has marginally more power and uses less fuel on standard ignition timing.(even though after numerous head shaves in the hands of a previous owner it still has a about a 10.5:1 compression ratio). I guess if its good enough for Nissan, it'll do!"