Head gasket: standard or MLS?

[User4574]

I've been watching the rebuild threads over the past few months while getting ready to dig into mine. Had a hand surgery that took priority first and am almost ready to start in.

Seems like most of the gaskets are standard. I have a standard from the complete Felpro kit and also have a Felpro MLS because at one time it was recommended as "it's just better".

Is it better? Pros/cons? Snake oil? Any reason to use or not use the MLS over the regular gasket (since I already have it)?

 

[Rickyd]

I've been watching the rebuild threads over the past few months while getting ready to dig into mine. Had a hand surgery that took priority first and am almost ready to start in.

Seems like most of the gaskets are standard. I have a standard from the complete Felpro kit and also have a Felpro MLS because at one time it was recommended as "it's just better".

Is it better? Pros/cons? Snake oil? Any reason to use or not use the MLS over the regular gasket (since I already have it)?

Use one with the same compressed thickness as stock so your lifter preload doesn't change.

 

[topfuel83]

I used the Fel-Pro perma-torque MLS coated head gasket that came in the upper gasket not I bought. So far so good.

And I'm not sure you need to worry about lifter preload to much. The factory service manual lists an acceptable range for pushrod length. My head got shaved 0.010 inches and the machine shop said I didn't need to worry about pushrod length, though he did ask what gasket I planned to use.

 

[User4574]

Use one with the same compressed thickness as stock so your lifter preload doesn't change.

So are you saying the MLS isn't a viable substitute but should only be used in certain situations?

Is it about the thickness of the MLS or the strength? Because it was mentioned to me that it is more a strength issue to avoid breaking and wear better than a traditional gasket.

 

[User4574]

I used the Fel-Pro perma-torque MLS coated head gasket that came in the upper gasket not I bought. So far so good.

And I'm not sure you need to worry about lifter preload to much. The factory service manual lists an acceptable range for pushrod length. My head got shaved 0.010 inches and the machine shop said I didn't need to worry about pushrod length, though he did ask what gasket I planned to use.

Thanks. I wondered if you did or not since you said one was on previously. Do you think the head was warped prior to your removal and the gasket helped bandaid it?

 

[sab]

Thanks. I wondered if you did or not since you said one was on previously. Do you think the head was warped prior to your removal and the gasket helped bandaid it?

An MLS head gasket won't make up for a warped head. In fact, MLS gaskets typically have a tighter flatness tolerance requirement for the block and head. The advantage of the MLS gasket is that the embossed rings around the cylinders and multiple layers better prevent temporary leaks if the head lifts off the block (every so slightly, for milliseconds) due to extreme temperatures or combustion pressures. Those short leaks drive combustion byproducts, which contain acids, into the head/block gap, and those acids etch the gasket and head/block surfaces on a microscopic level, leading to head gasket leaks, eventually (tens of thousands of miles, I suspect). I wouldn't think a stock 4.0L would really benefit from an MLS gasket since, as old as the 4.0L design is, it's not likely to see crazy temps or pressures in stock form. That's just a hunch, though, since I have no personal experience building 4.0L engines.

 

[lumberjacklloyd]

Use one with the same compressed thickness as stock so your lifter preload doesn't change.

Hydraulic lifters have ~0.1" of travel with 0.04-0.05" being the happy medium. Definitely something to be aware of, but i think you will have a hard time finding a standard head gasket that effects lifter preload that much...

 

[Rickyd]

So are you saying the MLS isn't a viable substitute but should only be used in certain situations?

Is it about the thickness of the MLS or the strength? Because it was mentioned to me that it is more a strength issue to avoid breaking and wear better than a traditional gasket.

I'm saying old lifters won't want to stray from their worn in preload. I don’t think it matters what material is used on a 4.0l so long as it is suitable for street engines,just that there are many thicknesses and chamber opening sizes to choose from.

If you are going to fixate on something it should be compressed gasket thickness and the size of the round hole for the chamber.

 

[topfuel83]

Thanks. I wondered if you did or not since you said one was on previously. Do you think the head was warped prior to your removal and the gasket helped bandaid it?

I'm thinking the MLS gasket that was on there was actually factory. I found some evidence Jeep might have switched to an MLS toward the end of the 4.0 production.

If the head hadn't been previously off it could only have warped when I removed it, even though I tried reverse torquing the head as I removed it to prevent warping. Don't know for sure though. A Carfax report doesn't show engine work, so if the head had been off before it was done independently.

And the Fel-Pro Permatorque MLS is supposed to be designed to seal imperfect surfaces:
https://www.felpro.com/technical/tecblogs/inside-permatorque-mls-head-gasket.html

 

[topfuel83]

I'm saying old lifters won't want to stray from their worn in preload. I don’t think it matters what material is used on a 4.0l so long as it is suitable for street engines,just that there are many thicknesses and chamber opening sizes to choose from.

If you are going to fixate on something it should be compressed gasket thickness and the size of the round hole for the chamber.

So far my original lifters aren't giving me any issues with changing to a new preload. I didn't want to replace lifters as I had read that sometimes they can wipe out the cam since the cam lobes were worn to the old lifters.

 

[Landon427]

In my case (2.5L, 1999), the machine shop decked the head 0.009" and the block 0.005". Since the Felpro MLS head gasket was about 0.015" thicker than the original one, the overall lifter preload was nearly the same after the rebuild vs before. Other things such as new pushrods and lifters made only very small differences in my case. My lifter preload after the rebuild was a little over 0.061", as near as I could tell. I didn't check it before the rebuild.