I've had Cometic head gaskets on the motor for 3 1/2 years and have covered about 10,000 miles in that time. Are they still re-usable? Cometic say they can be used up to 5 times but they don't suggest any time or mileage limit. Only reason I am thinking of re-using them is that they are not a standard thickness and may take some time to get a new pair.