V6 Milled Head increase compression and cause bottom end to fail?

[jeepgod]

My 4runner has been loosing coolant for a while now and my mechanic has determined that it needs the head gasket replaced. I have 175K and he advised against replacing the head gasket with that high millage and to swap a used engine. He said with his experience when you mill the head it increases the compression ratio and with that many miles it will beat up the bottom end and cause it to fail prematurely. Has anyone else heard of this and is it true.

Thanks

 

[Snugg]

My neighbor has an '03 4Runner. He had his head milled and gasket changed on his top end at 180k and that was about 5 years ago. He is still driving it around and he always talks about how good it runs.

Hope that helps.

 

[RonJR]

How much do they generally mill off? Just running through some quick math, assuming a 4 inch stroke and an original 9:1 compression ratio, it comes out:
- milling off .020" raises compression to 9.33:1
- milling off .050" raises it to 9.9:1.

You could also buy new heads. Probably would cost more than a used engine, however.

 

[Brian3676]

Your mechanic is an idiot. I have done hundreds of head gaskets. On just about every car you can think of. I mill every one of them, even if it's straight. A straight head that I just want cleaned up usually get milled .020-.030". A screwed head that's warped is usually .040-.080" depending how bad it's warped. The FSM has specs, you measure the head height, or some actually give how much can be taken off. If the miniscule amount of material taken off causes your bottom end to fail, it was going to fail anyway. You have a much bigger chance of the valves hitting the piston than anything.

As said before, measure the head per the FSM and you will be fine.

 

[jeepgod]

Thanks for the input I don't think the bottom end will be effected. No I just have to decide if it makes sense to spend the $1800 in labor

 

[YetiX]

I just did mine at 178k miles. Like Brian said, your mechanic is an idiot. Or he's trying to rip you off. My heads were straight so they were just cleaned up.

Get a good mechanic to do the work and have him replace the water pump, radiator, etc. while he's in there.

 

[Phils4th]

A true mechanic would measure the head for straightness before even milling it. It could just need to be cleaned up. Might as well get a valve job while it's in there. The mechanic isn't completely wrong, raising compression too high could cause detonation, but the knock sensor would retard timing to prevent this. You shouldn't need to shave much off the head and I would honestly avoid it. It throws off the geometry for the intake/exhaust manifolds and other bolted on accessories. But that's if you shave a lot. You likely only need it to be cleaned up. If it ever overheated badly, you probably need a new head because it's warped. To throw a different engine in it is foolish.

 

[YetiX]

A true mechanic would measure the head for straightness before even milling it. It could just need to be cleaned up. Might as well get a valve job while it's in there. The mechanic isn't completely wrong, raising compression too high could cause detonation, but the knock sensor would retard timing to prevent this. You shouldn't need to shave much off the head and I would honestly avoid it. It throws off the geometry for the intake/exhaust manifolds and other bolted on accessories. But that's if you shave a lot. You likely only need it to be cleaned up. If it ever overheated badly, you probably need a new head because it's warped. To throw a different engine in it is foolish.

That's what my guy did, just lightly milled the heads, cleaned up the valves and seals, etc. It's crazy how much quieter my valvetrain is now.

Unless you actually overheated your engine, your heads should not be warped. The leak comes from the gaskets failing, not the heads warping. In the picture of my gasket below, you can see where all the black material has worn off, causing the leak.

 

[jeepgod]

Well I bit the bullet and called the mechanic and gave him the go ahead. He's a little busy and said he will start it on Monday and will take 1 week