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What causes a head-gasket leak?

inquiring minds want to know: 1) why do head-gasket leaks occur; 2) is there some way to prevent them; 3) is there some way to stop or slow the leakage down without having to spend the $2k to fix?

Different reasons for each type of car,what kind of car are we talking about?

The reasons range from abusive operation to poor design, poor materials, poor quality control upon assembley.

Ask the question about a SAAB and then ask the question about a SUBARU and then ask the question about a GM v-8 that recently had it’s heads off.

The gasket has to deal with high temps and high pressures, probably the most stressed gasket.

running engines to hot causes a lot of this, along with others that wre mentioned above.

Some causes.

  1. Overheating
  2. Age and coolant rot.
  3. Faulty materials/design. Rare.
  4. One that I may veer off of from the rest of the world and that is the lack of cylinder head bolt retorquing in regards to all aluminum or aluminum head engines.
    I understand the principle of Torque to Yield bolts but I’m just not completely sold on it.
    Back in the 80s Nissan had a policy of head bolt retorquing (on their dime) and did away with it in a cost-cutting move. This was soon followed by numerous head gasket leakage problems from an oil pressure port. Just one example.

If the head gasket is leaking externally some goop 'em may stop it. If the gasket is leaking into the combustion chamber then it needs to be replaced.

Two things. An episode of overheating expands the engine beyond its normal limits, stretches the head bolts and releases the torque on them, allowing the gasket to fail. Many of these failures can be prevented by having the head bolts re-torqued anytime overheating occurs.

The second reason is not as rare as OK thinks. That is simply poor engine design. Great care must be taken to maintain head gasket integrity in todays aluminum engines. Different manufacturers use different techniques, some successful and some not.

Re-torquing the head(s) at 20,000 miles and again at 60,000 miles can’t hurt anything and is cheap insurance. This job can be difficult and time consuming so it’s seldom done in todays “if we can’t do it in an hour, we don’t do it at all” world of auto repair. Most mechanics cringe at the thought of removing a camshaft in order to re-torque a head…If they are going in that deep, they would rather take your $2K and replace the failed gasket…

In engines where head gasket failure simply can not be tolerated, the engine is designed so no head gasket is used. The head and cylinder(s) are cast as one piece. Aircraft and racing engines are often built this way…

I totally agree with Caddyman about overheating loosening head bolts. On every engine that I’ve rebuilt or performed a head gasket or valve job on it always gets run for a while, shut off, and the head bolts are retorqued.

In almost every case those bolts are able to be nudged a bit more after a heating and cooling cycle.
I’ve also never had to redo one either.

Back in the 70s Subaru used to recommend a head bolt torque every 15k miles. (all aluminum engine)

If you have a cast iron block and an aluminum head, the block and head expand and contract different amounts and at different times as they heat and cool. The head gasket has to tolerate this relative motion. Under ideal conditions, the head gasket will tolerate this for somewhere around 15 to 20 years of daily heating cycles. Then it has to be replaced. It is not a bad design, just was not designed to last forever.

If the head gasket is that old and it is just leaking water or oil to the outside, and the car has not overheated, there is no reason to suspect a warped head so the job is a simple R&R.

Why $2k? What kind of car? I replaced the head gasket on my '91 Volvo last year (260k miles). Total bill was about $100, including the 1/2" breaker bar and the socket that I had to replace because I broke them. Those bolts were really tough to break loose. I spent all afternoon and evening doing it, but most of that time was cleaning out vacuum ports and other stuff that is hard to get at with the head in place.

1995 Ford Explore Sport 4x4 w/160k …

perhaps, just like me, the miles traveled have taken their toll.

so … looks like i fall into category #2: age and coolant rot.

isn’t there an antiinflammatory that i could pop in the engine?