Head Minimum thickness

[Hans]

What is the minimum thickness if one uses a thick gasket?

[TobyB]

Depends on what else you do.

 

Standard 'wisdom' is that half a mm is OK, with no other changes.

So if you have a +.3mm head gasket, that means 0.8 mm, for example.

But if you adjust cam timing and make sure the valves clear the pistons,

a mm is certainly ok, and you get a nice compression boost.

 

I use 1mm cut heads on race motors- simply because there are lots of them,

and since I'm adjusting cam timing anyway, it's win- win with lower piston

crowns and attending shrouding. 

And I'm not trashing 'good' heads

when the smoke gets out.

 

129mm is the accepted thickness for 'stock'.

But it varies a bit.  And sometimes I forget.

 

hth,

t

 

 

 

[Cabbage Fumes]

Since most machine shops in North 'Merica look taxed if you start talking in metric, the numbers they understand are 5.07X is a stock thickness for a production head. I've seen NOS dealer heads in the 5.08X or even .09 somethings. First cut leaves them in the mid 5.065 ish area, Blue Book says minimum is 5.060. In street cars, below 060 usually means the shop will refuse the job, unless you plead with them. Below 060 is when you have to use a thicker head gasket. The usual "thick" head gasket will bring a street use head cut into the 055s into the comfort zone for most street builders. People with blood in their eye, myself, Toby, some others on here are willing to flirt with even skinnier heads, but a lot of planning goes into using a thin head, far more than you'd think. I have a 5.012 head that takes so much math to use that I just cant.

[jireland2002]

You can get MLS (multi layer steel) gaskets up to .125" thick so you can save those thin heads down to 4.94"

 

With the normal "thick" head gasket 5.048" should be safe.

[Hans]

Supplementary Question:

I have a 121 head that is a bit over 5.060, but it has grooves from cutting ring gasket. Can I just use a regular gasket and leave the grooves?

[Preyupy]

No, you need to machine enough off the head to erase the grooves.  This is why cutter ring gaskets are not a good idea.  I know the Ti/Tii/Turbos used them and they were a good idea for sealing 50 years ago but they are HORRABLE for serviceability.  You can't even replace a cutter ring gasket with another without surfacing the head if you expect it to hold. 

 

 I had a customer bring me a Turbo that kept "blowing headgaskets".  When I pulled the head off I found witness marks from at least 6 cutter ring gaskets !!!  They just kept pulling the head off and stuffing a new gasket in place,  the head was really chewed up.  

 

I run stock non cutter ring gaskets on all of my engines below 11:1 compression ratio and MLS gaskets on all of the 11:1-13.5:1 race  engines and turbos.  

[Hans]

Seemed like a good idea 50 years ago when heads were cheap.

[tech71]

So, the cylinder head for Franzi had to be skimmed a bit, now at:       128.7mm

Block deck was also skimmed 0.004 in or 0.101mm                        

                                                                                                                              ---------------------

                                                                                                                               Now at  128.599mm,

Good to go with a 1.5mm head gasket right?

Scrouged this NOS gasket kit with 1.5mm head  gasket from Ebay for $22.50, be a shame if I cant use it. Anyone familiar with this brand? 😁

Edited October 8, 2022 by tech71

[Cabbage Fumes]

You’re still within spec. That head gasket is ancient and most likely brittle. You’ve done everything right up till now…

[tech71]

56 minutes ago, Furry Camel said:

That head gasket is ancient and most likely brittle.

Okaaay.. valid point thanks, New head gasket it is then.

[TobyB]

I've experienced hardened head gaskets, too- 

 

seconded:  may as well get new.

 

You'll also get a more- modern composition gasket.

 

t

 

[tech71]

I'm convinced, ordered a new Fel Pro head gasket.

[Dudeland]

On 7/3/2022 at 1:16 PM, TobyB said:

Depends on what else you do.

 

Standard 'wisdom' is that half a mm is OK, with no other changes.

So if you have a +.3mm head gasket, that means 0.8 mm, for example.

But if you adjust cam timing and make sure the valves clear the pistons,

a mm is certainly ok, and you get a nice compression boost.

 

I use 1mm cut heads on race motors- simply because there are lots of them,

and since I'm adjusting cam timing anyway, it's win- win with lower piston

crowns and attending shrouding. 

And I'm not trashing 'good' heads

when the smoke gets out.

 

129mm is the accepted thickness for 'stock'.

But it varies a bit.  And sometimes I forget.

 

hth,

t

 

 

 

So much of what I do daily is focused on keeping the smoke in.