The maximum depth of material to be removed when regrinding to a level the surface joining cylinder block to hear is “0.5mm” (0.02″) according to the factory service bulletin. This limit should not be exceeded so as not to change the compression ratio beyond the standard allowance, or knocks will take place.
However, if the above said maximum limit is approached, take care to cut a new chamfer around the edge of the combustion chamber to prevent any interference with piston when at top dead center.
As further reference, the table below show the initial and final dimension measured between the surfaces which join the cylinder head to cylinder block and to camshaft cover:
1300 & 1600cc engines Initial 112 mm (4.4094″) min. allowed 111.5mm (4.3898″)
Any older Alfa head, no longer in production, is a bad candidate for milling to get compression increases. Pistons and liners are cheaper, than no-longer available heads. If you spend much time reading the BB posts, you will uncover sad stories from owners of 750 & 101 1300 heads, that have been “milled-to-death”. Some are no longer salvageable, others require serious over-bore to prevent the intake valve being shielded by the edge of the liner.
I have seen cylinder heads milled to where they were useless. A case in point, a 2600 Spider came to my shop with a blown head gasket (the two piece kind) and I sent it out to a reputable machine shop to just give it a minimal cut. When I got the head back I almost freaked out at the counter. The machinist in all good intentions milled off all of the corrosion. The cut was more than .15″ as it cut the head all the way to the edge of the valve seats. Worst part was it was on a 2600 cylinder head and cost the shop a lot of money to find one to replace it.
So where do you measure from to check your head? I prefer in the front of the head in the middle inside the chain cavity, measuring the meat of the head. I measured a 1750 head I have here and it read 111.84 mm. Effectively you are measuring from point A to point B using a very accurate set of calipers.
When a head is milled to many times, about 5 times (.002″ each time) to a depth of .010″ usually the head is toast. The hardest cut I have taken is .005″ because the head had been run with a bad gasket and it was between cylinders 2 & 3 making a groove.
When you cut to much, the valves begin to interfere with the piston. This is when you get an old used head gasket to put on the block, some modeling clay and you turn the cams with the pistons at TDC to see if they will clear. This isn’t a definitive test as it might clear in static mode, but when you run the engine and the valves are being extended, this is when you might kiss a valve and bend it. Then the valve could snap off and the “rest is history” as they say.
Let me state this, racers cut heads to improve compression ratio beyond what is normal. In addition they usually use pistons with deep pockets for the valve or they mill the piston so that the valve face will clear and not have interference. This is something you don’t undertake lightly as it takes precision and experience to make this work.
I can see your mind turning , “What if the head is beyond limits, what do I do now?” The best thing you can do is get or make a copper head gasket that is thicker than a normal head gasket and put that on. That will keep your compression down and minimize your interference. A stock head gasket material I measured was 2.05 mm if that is any help. A friend of mine Howard Jackson, who has since passed on, had a copper head gasket milled for him, he heat annealed it, put copper spray gasket sealer on it with new o-rings and it never gave a bit of problems. I have also heard of thicker stock head gaskets being offered, but I have no clue who has those.
Let me close this by saying…never throw a cylinder head away until you have exhausted all possible intake from racers, machinists etc.
You have a story about milling a cylinder head, post it to comments.
(The source for this post came from a factory service bulletin dated 12/3/1965, in which I elaborated on my experience with cylinder heads)