head gasket bore vs block bore question (already searched)

[supraguru05]

sorry for another dumb metal head gasket question i searched and could not find a anwser here is the problem

im finishing the rebuild on my 7mge. the head and block are both machined good enough for a MHG now im picking which gasket. i dont want to wait 10 days for a cometic so im looking into a HKS bead. but heres the problem the bore of the head gasket is 86mm which is bigger than the STD size which i belive is 83ish. so im wondering if thats going to cause a problem or not since i did not have to bore the block out.

the gasket i want to use is a thickness of 1.2mm since im a NA motor i figure its ok. barely any material was removed from the block

my bad im in the 7mgte section but this question really has the same answer for 7mge and 7mgte

 

[pimptrizkit]

most bore's are 83mm, and over bore ti 84/85

not 100% been awhile since i saw bore specs on the these engines.

i dont think that the hg bore being bigger will effect the seal, so long as it's sitting on the deck of the block all the way around each cylinder.

 

[bigaaron]

86mm hg will work fine on a 83mm or larger bore, I have one on my own engine.

 

[supraguru05]

thanks alot guys ordering today

 

[JustAnotherVictim]

IIRC having a hg with bigger bore than what you need also lowers compression slightly.

 

[saxman47]

IIRC having a hg with bigger bore than what you need also lowers compression slightly.

makes logical sense, you're increasing the cylinder volume slightly

 

[Nick M]

makes logical sense, you're increasing the cylinder volume slightly

I don't think that is the only problem. Funny thing, this was just brought up in another forum....

That gap is where the air and fuel will swirl quickly and mix thoroughly, and it pushes the mixture to the combustion chamber. And guess what, that lowers the threshold for detonation.

If you are running good gas, and low boost, or none in your case, you can probably "get away with it'. But I would suggest the proper head gasket.

 

[jdub]

Dead on Nick

It's called the "quench area"...Google it guys.

 

[saxman47]

I don't think that is the only problem. Funny thing, this was just brought up in another forum....

That gap is where the air and fuel will swirl quickly and mix thoroughly, and it pushes the mixture to the combustion chamber. And guess what, that lowers the threshold for detonation.

If you are running good gas, and low boost, or none in your case, you can probably "get away with it'. But I would suggest the proper head gasket.

learn something new every day :icon_razz

 

[JustAnotherVictim]

I'm confused now because I did google quench area and so far all I have seen are positive things about it.
Example:

Excessive cylinder pressure will encourage engine-destroying detonation and no piston is immune to its' effects. An important first step is to set the assembled quench (a.k.a. "squish") distance to .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance (the distance your piston is down in the bore). If your piston height (not dome height) is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .040" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. Take note, on the exhaust cycle, some cooling of the piston occurs due to the closeness of the water- cooled head.

Right or no?

 

[Nick M]

If you are moving the quench area further from the combustion chamber how is that good?

The tech note above is correct, but I am talking about moving it away from the combustion chamber. He has a 7MGE, not 7MGTE with significantly increased boost pressures.

 

[JustAnotherVictim]

Oh I see.
That makes sense now.

 

[supraguru05]

yea sorry about the 7mge post in the 7mgte section but i went ahead and ordered the gasket anyway. it should hopefully not make that big of a difference the thing i was concerned about would be the head gasket failing because of the extra size. the issue was it looked like cometeic was the only company the said it had a 83mm head gasket bore. all the hks were 86 or 88.

 

[Doward]

jdub and nick hit it on the head - as a matter of fact, I brought up this point over on mkiiitech.com - I don't understand why so many in the Supra community ignore something as important as quench area!

When I rebuild my 7M, I'll be tracking down the proper sized head gasket for my build.

 

[supraguru05]

jdub and nick hit it on the head - as a matter of fact, I brought up this point over on mkiiitech.com - I don't understand why so many in the Supra community ignore something as important as quench area!

When I rebuild my 7M, I'll be tracking down the proper sized head gasket for my build.

the problem is the only company that seems to make a 83mm MHG is cometic and i cant wait 10-14 business days for one. so im forced to get a HKS if someone else makes a 83mm bore MHG i would buy it

 

[tig321]

the problem is the only company that seems to make a 83mm MHG is cometic and i cant wait 10-14 business days for one. so im forced to get a HKS if someone else makes a 83mm bore MHG i would buy it

I believe greddy makes an 83 and 85mil, cometic is 84 or 86mil. At least thats what I could find.

 

[supraguru05]

man i am really off the ball this week your right. cometeic seems to be 84 and 86 greddy has a 83mm. ive heard some bad things about greddy's though. that being said ive changed my mind yet again and am going NA-t with gte electronics (i already have everything). so im looking hard at the 3mm greddy gasket to lower my compression im just kinda in limbo on wether i really plan on running over 15lbs of boost. if i keep good control of my timing and fuel and use a 2mm hks stopper i should be able to run the higher compression with decent amounts of boost. and get the benefit of faster spool with the higher compression i just need to make up my mind. thanks for your help everyone any more input is still appreciated

 

[jdub]

You do realize that a MHG requires both the block and head to be decked to a very smooth surface (HKS, Greddy - 30 RA or less, Cometic - 50 RA or less). When you say "good enough", the both the block/head need to meet these specs.

Since it looks like you are going to use NA pistons, the compression does need to be reduced. But what you don't want here is that big of a gap at the top of your cylinders, especially if you are going to run 15 psi of boost. You are asking for detonation and that will destroy the motor pretty darn fast. At minimum you should size the MHG to match the cylinder diameter....Cometic will make a MHG sized to the spec you need.

To do this right, you need turbo pistons, a MHG thickness to retain stock head height, and ARP hardware torqued to ARP spec.

You're kinda putting the chicken before the egg here.

 

[supraguru05]

i already have arp studs, for the head and main caps. i had the block and head resurfaced by a local speed shop. i already have the shortblock assembled thats why i dont want to change pistons. i have a full turbo motor that i could rebuild but it would just be easier to use the na one im already arms deep in. from what ive read on here and sf the raised compression from the NA pistons isnt completly bad if you manage the fuel right. i think id be fine running the higher compression pistons with a 2mm HG and 10lbs of boost on the stock turbo with wideband it woudl all just depend on what my a/fs look like. i would also be adding a maft pro unit probably at the end of the autocross season

btw thanks for discussing this with me and not flameing me lol

 

[jdub]

If you can feel ridges on the block or deck surface with your finger nail, it's not smooth enough for a MHG

I understand what you're doing and at least you understand the higher compression pistons will cause problems on a turbo motor. If you go this route, at least make sure the MHG matches the cylinder diameter. Using a larger diameter MHG will leave a gap at the top of the cylinders...carbon will form in this gap, creating a hot spot. This can cause pre-ignition of the air/fuel mixture...not quite as bad as detonation, but almost. You really, really do not want this to happen. Bite the bullet and call Cometic to get a correct diameter MHG.

What else did you have done to the block machine work wise? Did you have the rods re-sized?
I assume you are using new bearings and the clearances are to spec.