Milling the head too much? Below "spec"...

[Reign_Maker]

OK, so if anyone has followed this, you kow where I am with this head situation... For those that dont, here's the short version...........

Doing a build, all forged internals, ballanced, B'Pd all that great stuff... Started working on the head, oh noes! A crack... Tried to fix crack, but just chased it deeper... End result, head is a paper weight... My buddy Mark sets me up with an old head... THANKS MARK! Get it in, clean it up, pressure test it, everything is great... Get a call today from machinist: problem...

Ok, now you know the history, here is the problem...

Head has already been decked/machined before... If they do it again, it'll take it below "spec", whatever "spec" is... Frank, my machinist, is concerned it will "GREATLY increase compression"... But im using a 2mm mhg... "Whats the stock hg thickness?" I dunno?

Anyway, I told him to just do it... I gotta get it done one way or another... Just take off as little as possible, but its gotta be perfect...

So hith havein 9.5:1 Wiseco pistons, a 2mm mhg and a head that is below "spec" what kind of true compression will I have? What will it do to my power and performance? Less boost, more power? I dunno... I figure, they boost these 350Zs and make 150+ additional hp from 7psi... Am I making a mistake? Should I just hunt down another head?

Thanks...
Jake

 

[rakkasan]

uuuggghhhh

 

[Reign_Maker]

uuuggghhhh

Somehow I was hoping for a little more reasurance, but UUUUUUUGGGGHHHHHH will do... :icon_razz

 

[rakkasan]

Somehow I was hoping for a little more reasurance, but UUUUUUUGGGGHHHHHH will do... :icon_razz

My question for the machinist: If he knows it's "out of specs", then he should know how much has been taken off, so how much has been removed?

 

[rakkasan]

Addendum: I had it looked at by my guy & he said that it looked great & "very rebuildable". There's a difference of opinion, so I'm curious as to who's correct.

 

[Reign_Maker]

Its not out of spec now, but he said in order to mill as much as he wants, like 80 thousandths, it would put it just under spec... But just HOW much does it raise your compression when you mill that little bit off?

 

[rakkasan]

Its not out of spec now, but he said in order to mill as much as he wants, like 80 thousandths, it would put it just under spec... But just HOW much does it raise your compression when you mill that little bit off?

Not certain, maybe IJ can throw in an idea or two....

 

[Reign_Maker]

Well, it is rebuildable... We had to get some cam caps, because mine wouldnt work... And pressure tested, it worked great... But this head has already seen the mill... So I dunno how much has been taken off before, but some has... His concern is taking off more, will it be too much? Shaving half a mm off the head, will it really raise compression that much? I dunno... Should I worry, since Im using coated forged pistons?

 

[rakkasan]

Well, it is rebuildable... We had to get some cam caps, because mine wouldnt work... And pressure tested, it worked great... But this head has already seen the mill... So I dunno how much has been taken off before, but some has... His concern is taking off more, will it be too much? Shaving half a mm off the head, will it really raise compression that much? I dunno... Should I worry, since Im using coated forged pistons?

Hard question to answer, especially considering the amount of power you want to make. I would find out for sure how much has been removed first before I made any decisions.

 

[mk3ukr]

I suggest to cc the head's combustion chamber and calculate your CR
From JE catalogue 7MGTE head is 40cc, I think this is brand new head data.
HTH

 

[Nick M]

80 thousandths is way to much, by the book. I think it is ".039, or 1mm max. And remember the basics, the thinner the deck, the easier it will warp when hot. Regarless of what compression you are using, it is risky.

 

[Slow66]

You sure he didnt mean 8 thousanths? .080 is a TON. You happen to know if is milled down to the EGR plate or not?

 

[Allan_MA70]

you can do a few things to get the CR back down such as seat the valves in further and use dished valves, remove the squish area and make it like a group A head and generaly clean up the CC.

your going to end up with a motor thats a lot of fun but it wont be a 500hp+ monster and you will need a VERY good tuner!

 

[Nick M]

You sure he didnt mean 8 thousanths? .080 is a TON. You happen to know if is milled down to the EGR plate or not?

That is a good point. ".008 is nothing. Go ahead and do it if that is all it is. But ".008 will not really raise the compression ratio that much. Time to double check.

 

[tissimo]

yea... .080 is way to much.. thats 2mms... why would you need to shave it that much.. if its warped that bad (i dunno how) then you need another head, if its that pitted i would look for another.. I have a head i can give you if you pay shipping.. I havn't checked it, it came out of a car that threw a rod..

fwiw i shaved my old ass 83 celica head back in the day .040 and it went from 9.0:1 to like 9.7-9.8:1 from my calculations..

 

[Justin727]

slap on a 2mm mhg!

 

[Slow66]

slap on a 2mm mhg!

He is :crazy1:

It may not be enough though if the head has been surfaced a couple times, and possibly the block a couple times, and the fact hes running higher compression pistons, and the fact he wants to run big boost

What you are going to have to do is CC the head, find out how much the piston is in the hole, and use a good 'ol compression calculator to find out where youre at with a 2.0 gasket. Who knows, you may have to step up to a 3mm.....

 

[IJ.]

Stock HG is 1.37mm compressed

New head is 116mm from memory

2mm cut = dumpster

 

[JustAnotherVictim]

IJ has spoken.

 

[3p141592654]

Maybe this is obvious, but that 116.00 mm number is the head thickness from cam cover deck to combustion deck.