Head Gasket thickness selection

IJ.

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20 thou isn't going to make much of a difference to the CR or gasket used, the "stock" bore MHG's are already oversize in the hole.

Stoppers are only available in the 86mm bore but are overkill for 99% of 7M builds.
 

Needboost

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20 thou isn't going to make much of a difference to the CR or gasket used, the "stock" bore MHG's are already oversize in the hole.

Stoppers are only available in the 86mm bore but are overkill for 99% of 7M builds.
are the cometic 1.9mm headgasket any good or the titan 2.0mm
 

IJ.

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Cometic make the gaskets for Titan, don't start stressing over a .1 difference in the real would unless you're building a class race engine it's going to make 0 difference.
 

Needboost

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Cometic make the gaskets for Titan, don't start stressing over a .1 difference in the real would unless you're building a class race engine it's going to make 0 difference.
well i was really wondering about how good they are i saw where the hks stopper 2.0 is almost 400 dollars where the titan and cometic are around 180 wondering what the real big difference was
 

IJ.

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1 extra layer that's rolled/folded on the Stopper, as I said up there the Stopper is overkill and will still blow if something goes "wrong".. none of the HG's for a 7M is bullet proof some just take more abuse before letting go, the danger here is if there are other weak links in the chain they will fail before the HG.
 

Needboost

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1 extra layer that's rolled/folded on the Stopper, as I said up there the Stopper is overkill and will still blow if something goes "wrong".. none of the HG's for a 7M is bullet proof some just take more abuse before letting go, the danger here is if there are other weak links in the chain they will fail before the HG.
ok well i guess i may go with the titain 2.0mm gasket since you said cometic makes the gaskets for titan and that 2.0mm is the size that i would need thank you for the help
 

Skeezix

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OK so I decided to bury the needle on the car, twice, bad fuckin idea. Now I'm back to another build (took out #6 rod and main) and am thinking of going MHG this time. The Top Line OEM replacement never blew (with ARP studs). But when I had the motor done six months ago we had the head planed a bit and the block resurfaced, now don't know exactly how much was taken off. Read this entire thread and could not find where you measure the block. The main saddle, now are we talking measure from where the cap meets the block,or where the bearing would sit in the journal? Seems like measuring from the journal may be difficult but I don't have/never used verniers. Help would be appreciated as I don't want my car in the garage all summer again.
 

Skeezix

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Also my block has already been bored 40 over would I need to jump up to the 86mm bore gasket or is the 84mm just enough seeing as how stock is 83mm and 40 over is 1mm?
 

JJBToyotaSupra

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Trying to figure out my compression ratio using this calculator: http://www.csgnetwork.com/compcalc.html but when i enter all the stock values i get 8.7:1. Isnt stock 8.4:1? Anyone else have this issue?

Also, im changing the HG on my GTE. Just got a stock size fel-pro composite HG because i didnt wanna take out the engine. The machine shop said they took .040" off my head, so now im not sure what to do. I understand the concept of adding what was taken off to the gasket to maintain compression and squish/quench but im wondering would i be able to get away with a stock gasket with that much taken off the head and just running stock boost stock engine? Would my compression be too high and damage the engine or would my valves hit the pistons? Toss the head in the garbage an get a new one? Appreciate any input, im afraid to put it together now lol.
 

JJBToyotaSupra

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Yes sir. These are my number:

1 = Inches 2 = Millimeters: 2
Enter Cylinder Bore Size: 83
Enter Piston Stroke Length: 90.98
Enter Head Gasket Bore Diameter: 84
Enter Compressed Head Gasket Thickness: 1.37
Enter Combustion Chamber Volume In CCs: 40
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-'): -19
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : -.5

Gives me a C/R of 8.7:1 am i doing something wrong or is that calculator a little off?

Also IJ, in your opinion would it be ok to run my head with .040" machined off on my stock HG on stock boost? I just need my car driving again, wont be driven very hard. Planning to rebuild it all next year.
 

IJ.

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Pistons in a 7M are 20 thou out of the holes at TDC, from memory CC volume is 49cc but it's been awhile since I was hands on with one.

You'll have to listen carefully for any sign of detonation and maybe pull some base timing out of it if you hear some.
 

mcammarn

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so i took my block in to machine shop to get rebuild, and they didnt have to take nothing off from the block. they said it was good still.
for the head, they took off 6 thousands. (.006)

so from what ive been reading in the 14 pages,lol, is that i wont need a mhg since the block didnt get resurfaced?
and my end hp goal is roughly 350.
what thickness of hg do i need then if i go with compsite or mgh?
thanks
 

mcammarn

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Stock will do it 6 thou is nothing to be concerned about.
will stock be good to run around 350hp? like fel pro, oem toyota. theres a hg in the gasket kit i got its from ITM.
im just dont want to end up pulling a head off from bhg.
 
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mcammarn

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CORRECTION: machince shop called me and they told me they did shave some off the block. they just didnt charge me for it.
they said 3thousands (.003) was taken off.
so now ive got to send my block in with the timing cover to also get that shaved to now.

so with a total of .009 taken off, will this effect the size of hg needed now?
 

TheMaxx

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Use the gasket you have now if you plan on doing a full rebuild, since you will be opening the engine back up anyway. If you don't plan to rebuild it just get a MLS and be done with it.
 

IJ.

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If the Surface RA is within Spec go Metal, a 1.5mm would get you close to stock CR/Quench.

If the surfaces are too rough a stock Composite isn't going to have a negative effect.
 

mcammarn

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im sorry but whats RA? and lets say i do get 1.5mm mhg and its to thick, will that cause a problem?
If the Surface RA is within Spec go Metal, a 1.5mm would get you close to stock CR/Quench.

If the surfaces are too rough a stock Composite isn't going to have a negative effect.
this was just rebuilt, unless your talking about top end.

Use the gasket you have now if you plan on doing a full rebuild, since you will be opening the engine back up anyway. If you don't plan to rebuild it just get a MLS and be done with it.
sorry for all the questions, just want to do it right the first time.
 

Numba1Stuna

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What's up guys? Finally at the stage in my complete rebuild to measure for my head gasket! Got my digital calipers in the mail yesterday and put them to use today. So I've read this thread a couple times when I first joined and just ran though it again. Just wanted to post up my numbers and get the seal of approval from the great IJ.

So stock numbers are:
1) 1.37mm for the head gasket.
2) 116.00mm for the head.
3) 198.50mm for the block.

My readings were:
115.60mm for the head, difference from stock being .40mm
197.88mm for the block, difference from stock being .62mm
Add the stock head gasket size 1.37mm to .40mm and .62mm and I come up with 2.39mm head gasket size needed.

Does that sound correct to you guys, 2.39?

Would you recommend a 2.4mm head gasket?

Thanks in advance!
 

IJ.

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Not sure if a 2.4 is available, 2 and 3 are for sure but there will be a slight compromise using either, 2 is going to give you more off boost throttle response but depending on fuel you maybe be limited in boost and timing, opposite with a 3 but it'll be a bit doughy off boost.
 

Numba1Stuna

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Awesome, thanks for the response. Did a little looking on ebay and found this head gasket: http://www.ebay.com/itm/330800056593?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

I have a question about it before I buy it though... I still have the stock bore on this block, 3.26" or 83mm. I noticed that this head gasket has an 84mm bore. Is that something to be concerned about or will it work without issues? Not sure if it needs to exactly match bore size or not, so that's the big question.

As far as thickness goes, my measurements, if they are correct, tell me that I need a head gasket at least 2.39mm in thickness, this head gasket is .095" or 2.413mm. So I think that part is good, right? Also, with it being closer to what I need exactly rather then choosing a 2mm or 3mm, how will that affect what you had mentioned in the previous post? Just kind of balance it out? Is there anything else that I need to be concerned about? Thanks again.
 

IJ.

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The larger bore in the gasket won't affect anything (I ran the 86mm Stoppers on a stock bore) I wouldn't get too caught up in the fractions you're talking about, in reality unless you're in a race class and trying to Win you'll never notice a difference as a layer of carbon is going to have more CR change.
 

Numba1Stuna

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Thank you again IJ. I plan on keeping the 90 stock(my 91 is the powerhouse). So it is finally decided I guess lol. 2.4mm thickness with 84mm bore.

I'm going to start a for sale thread either today or tomorrow to help fund the rest of the needed parts/gaskets!

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Numba1Stuna

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Ok, once again have a question. I have slight electrolysis on the bottom of the head still after the mill. I'll attach pics to show you. Is this anything to be concerned with when using a MHG?



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Numba1Stuna

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I did a search for electrolysis, but could not find anything relevant to this issue...

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Grandavi

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electrolysis? Are you talking about the corrosion on your head? Im not a machinist, but your head doesn't look machined well enough. That looks like corrosion from a leaking head gasket that sat far too long...

my comment and thought.. but I am not an expert.
 

supraguy@aol

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I would lay a head gasket over that head surface, to see if any of that corrosion will impinge upon the sealing surfaces.
If it does, you either gotta shave it more, or toss it.
 

Numba1Stuna

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Well, I guess I can do up to .6mm more off the head and just get a 3.0mm MHG. They don't make them bigger then 3mm right? I'll stick a composite on there and see if it is in the sealing area.

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AbsoluteSpeed

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They do make a thicker HG but are very difficult to come by.
When I built the motor in my first supra the machine shop ended up using a copper gasket that was 6mm (I think)



Honestly you would be much better off finding a new head it you need anything a over a 3mm gasket.
 

fienloco

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Rebuilt my 7m .40 over on bore 4thou off the head, 8thou off tge block 2.0 MHG. Performance now is null. Any suggestions on the math or size of gasket I should have gone with or should go with?
 

B shaw

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If compression is your only issue and you have the funds, why not buy pistons that help to lower compression? I did that with my 89t because I was going with forged internals anyways so it seemed like a good idea to me. Compression ratio comes out to a bit under stock, which isn't a bad thing. Am I correct in that line on thinking?
 

VegaSupra

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Just read every post in this thread. Excellent info here.

One question I didn't see an answer for though...

Is it ever a good idea to open up a perfectly working 7M, resurface the head, and replace a stock HG with a 1.37mm or 2.0mm MHG?

My Supra is basically all stock and original from 1991. I do see in the original owner's maintenance notes that he retorqued the head bolts at 46K miles. (now has 72K miles)

But here's why I ask... over the next few weeks, I will be installing the following all at once.

57trim CT-26 upgraded/rebuilt by Albert
Intake (basic open air cone type filter)
Full 3-inch widemouth downpipe from Driftmotion
Full 3-inch exhaust
3-inch test pipe from Titan
Type RS BOV

I will not be raising boost on purpose (not buying a boost controller), but with the 57trim, and all the extra breathing room, I am guessing this will push me well over 300hp and will most certainly raise the "stock" boost level to at least 10 or 12 psi. I actually wish this weren't true, but I guess there's nothing much I can do about that.

If you were me, would you be concerned about blowing your stock original retorqued HG? What would you do?