gasket
The trouble with leakdown is unless the crack is huge, it may not show up unless the engine is at operating temperature. If you do it cold, the crack may be "shut" with carbon and or just the metal itself. Many cracks don't open and start leaking unless the metal is hot. I'd yank the heads if compression in the cooling has been confirmed and then either scrap the heads in favor of aftermarket heads or plan on having a teardown with full magnaflux (mag particle) testing to located possible cracks. We had a tank back when I was at Caterpillar that was heated up to 180ish degrees that cylinder heads were dipped into for testing. The heads would have deck plates bolted to them to block off coolant passages and any other coolant passages were capped off with a block off plate. One plate had a QD for compressed air that would be pumped into the casting during it's soaking in the hot water tank. If the headgasket has simply blown, the head surface will need to be checked (along with the block) for a true straight sealing surface. If both are flat and no cracks present, you might get away with new gaskets. Id so, you may consider swapping to head studs instead of bolts for better clamping force.
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