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K0-K3 series 750's head gaskets v K4 onward - and head gasket leaks

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the-chauffeur:
To add another facet to this thread . . .

 . . . I may have mentioned I've long suffered with a seep/leak from around the front left outer joint between the cylinders and the head on my K2.  Having tried a number of gasket combinations, together with heavy duty studs and various types of sealant, I still seemed to be getting nowhere.  And then I found a post on the US forum buried deep within a much longer conversation - clearly I wasn't alone in having suffered this problem after rebuilds . . .

"Early 750 head leaks from around the front edges?  I have a theory about this.  When I've torn my engines apart the knock pins from the cylinder to the head were really hard to remove and I had to destroy them to get them out.  Not sure what size they were (I think they were 20mm) but if you check the parts book the pin is called out as 22mm long but is no longer available (I think it was a mistake). 

Later models show the same parts as 18mm long.  When I checked the depth of the hole in the cylinder and the hole in the head and the thickness of the head gasket it totaled 20mm.  In otherwords if you have 20mm pins, you bottom out in the bottom of the holes without adequately compressing the gasket.  Therefore the pins are jammed in making them hard to remove.  I believe that's where the later models used the 18mm pins.  I've used 18mm pins on my rebuilds and so far no head gasket leaks. 

I did add the APE studs and the Bulletin #42 o-rings so maybe that did it but I see no down side to using the 18mm pins.  Like I said, just my theory.  Its just strange that most of these engines leak."

I've followed the microfiche fairly closely every time I've reassembled, and it turns out that was kinda my big mistake.  At some point in the past I replaced my locators with the 'correct' ones for a K2 - the 22mm ones - and they're too tall. When sat in the top hole, there was 11mm showing and when in the bottom, there was pretty much the same, which suggests that best case scenario, they'd allow for a 1mm thick gasket to crush flat.  But then I put the cylinder head on top of the barrels with the dowels in place and there was a gap that was more than 1mm all the way along the front.  Given that the head gasket averages 1.30mm thick, that's not a lot of crush. 

The most recent disassembly has left me with a 'used' head gasket that looked pretty much the same as it did before it went in - there was very little noticeable pressure on the fire rings and it was still the same sort of thickness around the front holes when measured against a new one (whereas the rear edge seems to have compressed by about 0.5mm).  As always, there was a thin layer of oil on the head gasket and yes, it was the cause of the leak at the outer left (it was only a matter of time before the outer right went the same way).  There was also a carbon lip on the cylinders from where I assume air/oil has gotten in via the wrong entry points - that came off with wire wool.

This time, I'm using the later dowels, which will allow me to get a proper crush on the gasket.

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