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It depends whether it's a 10-bolt or 12-bolt head. The 12-bolt has the normal ten 12mm head bolts, plus two extra 8 mm socket-head cap screws that thread into the front of the timing cover to help reduce oil leaks. You can *just* see them under the front edges of the valve cover. If you have them, you have a head with hardened exhaust seats. NOTE: the seats in the head are simply induction-hardened, not inserts added later.

Bob
 

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Better in the sense they resist wear with unleaded fuel...yes.

But worse in the sense the heads crack like crazy. There was already a flaw in Opel's design. The heads have two exhaust ports side-by-side. They happen to be the best flowing exhaust ports in the head. And the intake ports happen to be deficient. So, there is a lot of overscavenging going on, and those center ports tend to run 200-250 degrees hotter EGT's than the other exhaust ports (proven on a dyno, we see 1400-1500 degree temps from these ports on a regular basis!)

To make matters even worse, there's NO WATER between those exhaust ports, nor even an intake port between with a cool intake air/fuel charge to help even-out the temps. Every other exhaust port is surrounded by water on one side and an intake port on the other. Oh, and when they changed the casting design sometime in 1972 to the 12-bolt style, they also changed the cast iron too. It's more brittle. The exhaust seats are VERY brittle. So it's a recipe for disaster. I'd guess that 80-90% of the late 1972-1975 heads cracked.

I prefer 1971-mid 1972 heads. They have 4 cam bearings still, unhardened seats (I add the hardened seats), and thicker ports with a more ductile material.

Bob
 

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Might it be worthwhile to have the inside of the exhaust ports (at least the center two) ceramic coated by swain or jet-hot in an attempt to reduce exhaust temps? Would having the valve faces and combustion chambers coated reduce the likelyhood of cracking a head?

Darrin
 

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Even better than that: When I port a 1.9 head, I concentrate of balancing the flow between exhaust ports, and increasing intake flow. Then I design a cam with split lift/overlap. I always run more intake lift and duration on a restricted intake engine (i.e., running with a single carb). So, not only does this drop exhaust temps, it increases power, and improves BSFC. The engine uses less fuel to make more power. It's a beautiful thing.

Bob
 

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RallyBob said:
But worse in the sense the heads crack like crazy. There was already a flaw in Opel's design. The heads have two exhaust ports side-by-side. They happen to be the best flowing exhaust ports in the head. And the intake ports happen to be deficient. So, there is a lot of overscavenging going on, and those center ports tend to run 200-250 degrees hotter EGT's than the other exhaust ports (proven on a dyno, we see 1400-1500 degree temps from these ports on a regular basis!)

To make matters even worse, there's NO WATER between those exhaust ports, nor even an intake port between with a cool intake air/fuel charge to help even-out the temps. Every other exhaust port is surrounded by water on one side and an intake port on the other. Oh, and when they changed the casting design sometime in 1972 to the 12-bolt style, they also changed the cast iron too. It's more brittle. The exhaust seats are VERY brittle. So it's a recipe for disaster. I'd guess that 80-90% of the late 1972-1975 heads cracked. Bob
Did a compression check on my '72 Manta today. 60 PSI in #3 cyl. It did not change when oil was added thru the plug hole. Pulled the head and here is just what Bob was talking about with the problem caused by the exhaust valve location.
 

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