I have been told the head gasket has to be replaced. How reliable is stop leak for repairing the head gasket? How safe is it to use the stop leak?
Stop-Leak is unlikely to “repair” a breached head gasket.
In terms of safety, it is possible for that product to cause cooling system damage.
If I were you, I would not use Stop-Leak for this purpose.
When you read the claims printed on the can it sure looks promising doesn’t it?
Stop leak products have been known to temporarily stop or slow down a head gasket leak, though my sense is the rate of these “temporary successes” is low.
Be careful. If a stop leak works well enough to plug a leak in a head gasket, it often works well enough to plug the small tubes in your radiator and/or heater core. There’s a good chance you could end up with more problems than a leaky head gasket after using a stop leak.
I would not attempt to stop a head gasket leak with a stop-leak product. If a leaking head gasket has been verified beyond doubt the only cure is to replace the head gasket.
A stop leak product may clog up small passages you don’t want clogged. Don’t waste your money on these things.