whats different about the 7m

[ilikebigbutts]

well i've replaced stock mhg's on 3.3 chrysler motors a couple times and just watched one of our techs put a stock mhg on a 2.7.the way he (and I) prepped the mating surfaces is he just used a wire wheel on his pneumatic drill and removed most of the gunk, then used a roloc pad to ensure smoothness...he has done a couple dozen 2.7's like this and never had a problem including his brothers and son's intrepids that have more than 40k on the motors (2.7's are notorious for rodknocking)...

now this got me thinking because from what i understand in order to install a mhg on a 7m special machine work is required...now i swear i searched but gave up and i'm just asking why is this so?i dont care getting the machine work done i just want to know why?iron block makes a differance?just because the head design sucks?just wondering...

aslo, what the hell does RA mean?i swear i searched for like 10 mins....

 

[MassSupra89]

RA is a roughness average. Basically how smooth the gasket surface is.

The machine work and smooth RA is required when switching to a Metal Head Gasket, they are much less tolerant to imperfections in surfaces.

 

[donnys90T]

It's a gasket. A metal gasket. Think about it. That said I know lots of people who don't do the block work with MGH and never had problems, including yours truly. The head was done very well though.

RA is a measurement of surface roughness. Often done using a set of visible standards. Best done using a profilometer.

 

[ilikebigbutts]

so all i have to do is ask the machine shop to machine to mgh ra specs? like 20-50 i think i read somewhere...

 

[MassSupra89]

Depends what gasket you get. Check with the gasket manufacturer

 

[jdub]

The danger in using a wire wheel to clean the surface is inducing low spots. The tight cylinder spacing on a 7M makes it fairly unforgiving when using a MHG. Like Donny said, there are success stories using a MHG on blocks that have not been machined...they better be squeaky clean though and I would not use a rotary abrasive to do it.

 

[Dan_Gyoba]

Well, a 4cyl, or a V6 has a much shorter head, and often has more space in the cylinder walls. This makes the head gasket surface more forgiving, though those blow head gaskets when done like that, too.

The extra length of the 7M head also introduces problems, particularly with the aluminium head and cast iron block. This is a combination that blows head gaskets. The 7M just has a few more cards stacked against it is all.

 

[ilikebigbutts]

The danger in using a wire wheel to clean the surface is inducing low spots. The tight cylinder spacing on a 7M makes it fairly unforgiving when using a MHG. Like Donny said, there are success stories using a MHG on blocks that have not been machined...they better be squeaky clean though and I would not use a rotary abrasive to do it.

yeah i know what you mean about making low spots but usualy a roloc pad gets rid of any marks made by the wire wheel.

 

[themadhatter]

All i did when i did my headgasket repair was clean the block very well put a percision straight edge on it to to check for any warpage and had the head resurfaced(by a race shop that knew i was using a mhg)

 

[Poodles]

long engine/head vs short engine/head

not to mention the higher pressures we're dealing with on a turbocharged engine...