How a "Block Style" cylinder head is measured for thickness.
 

How a "Block Style" cylinder head is measured for thickness.


This is a fairly simple thing to do.
This will also help in setting the timing belt and calculate combustion chamber size.

New head thickness is 5.200
The Mitsubishi factory manual says a combined total of .008 may be removed from the block and head.

AERA (Automotive Engine Rebuilders Association) says .015 can be milled from the head.

So by AERA specs the Min thickness will be 5.185


The tools needed will be few.

Something to use to clean the head gasket surface
And a set of venier calipers,
Paper and pen.

The first thing to do is remove the head from the engine.
Remove both the exhaust and intake manifolds.

Now on the work bench, remove the cam caps
https://www.miataturbo.net/attachmen...ine=1337375941
The cams and followers.
https://www.miataturbo.net/attachmen...ine=1337375941
Next roll the head over so the head gasket surface is up.
https://www.miataturbo.net/attachmen...ine=1337375941
Now the head gasket surface needs to be cleaned of any old gasket material.
https://www.miataturbo.net/attachmen...ine=1337375941
I use a stiff backed long body board.


Now roll the head so one port side is up,
I will start on the exhaust side.
https://www.miataturbo.net/attachmen...ine=1337375941
Measure from the valve cover rail to the head gasket surface.
I will also use the cam cap mating surface on the top, it will help keep the calipers square.
Make a note of the measurement.
This is Exhaust Front
On this head it was 5.193
Next I will move to the rear of the head.
https://www.miataturbo.net/attachmen...ine=1337375941
Make note of this measurement.
This is Exhaust Rear
Here I measured 5.190

Next roll the head over to the Intake side.
https://www.miataturbo.net/attachmen...ine=1337375941
This is the Intake Front
The measurement is 5.191

Now to take the forth and final measurement.
This is the Intake Rear
https://www.miataturbo.net/attachmen...ine=1337375941
This measured 5.188

So now with the 4 measurements.

EF=5.193 IF=5.191

ER=5.190 IR=5.188


We see the head is not parallel to the valve cover rail.

We can see that from side to side there is .003 difference
And the same .003 from front to back.

Note also this head was belt surfaced before.
This is not a badly surfaced head for being belt surfaced.
The combustion camber volumes will still not be equal.


This procedure will hold true for any "box style" cylinder head.

Other related links

Mazda info

1.8 NB 97-04

New thickness 5.276

Min thickness 5.267

Mazda 1.8 BP 90-97

New thickness 5.276

Min thickness 5.267

Mazda 1.6 90-95

New Thickness 5.276

Min thickness 5.267

Specs are from AERA Head and block 2009-2010

Good stuff. When cleaning the gasket surface with that board, is there sandpaper on it? (what grit?) or is it more like a paintscraper thingy?

Oscar... Do you mean this method? :D

https://www.miataturbo.net/showthrea...665#post862665

You may like this one too..

https://www.miataturbo.net/engine-performance-56/head-gasket-repair-how-clean-block-surface-w-pistons-installed-65074/

Ohhh so this is how you tell if a head has been milled?

Fae, yes it is and how much.

Any reason at all why you don't use a micrometer as opposed to a vernier? I'm not making any criticism I just get a bollocking at work whenever I'm spotted using a vernier haha

HAH! almost responded... the month on this thread had me fooled.... necromancy is EVIL!!!!

Vernier, you mean a caliper?

When measuring straight surfaces a caliper is more versatile. To use a micrometer you have to have straight surfaces on each side that you're measuring. In a case where you have a head that is dug up for the piston chamber you just cant use it.

Now of course there are very very precise calipers just like there are micrometers. The shop i used to work had one that was almost half an inch thick to ensure it doesnt bend.

They used to be called vernier calipers, vernier in latin means very near. I never call a digital or dial caliper a vernier, just the really old style that only uses the slide for its increments.