Re: Head Dimensions

[Glenn Merrell <gmerrel@domain.elided>]

Hello Wendy,
	Well, you need to hear more about this from the usual suspects.

I'm looking at a technical dispatch dated 3rd March, 1975, titled "Cylinder Head
Skimming", sighed by one F.K.Lumkin, Technical Liaison.  The notice states that
the minimum recommended thickness of the cylinder head from the bottom of the
camshaft bore to the cylinder head face is 4.427 inches for a Stag and Dolomite,
4.745 inches for a Sprint.  Common speculation is that the cylinder head can
only be skimmed once at the standard -0.010 inch to reach the minimum
recommended thickness, netting 4.437, and as Mike Wattam states +/- 0.003
tolerance from original machining nets original thickness at approximately 4.444
inches.  This seems to match the measurement of a set of heads I removed off an
engine that had never been apart, as the thickness was 4.445 inches.

	Two points were made by previous posts that I believe are good advice:  1.
don't mix an old style Mark I head and Mark II head on the same engine unless
you verify that they are indeed the same, that all water and oil ports match the
gasket holes; and 2. Tony Hart's recommendations on when to apply regular vs
extra thick gaskets should be followed as sound advice.  Indeed there is plenty
of room on the head for skimming before the valve face protrudes past the head
surface, about - 0.035 inches seems to be the thickness of the head face
shoulder to standard valve face.  Tony Harts advice also seems to match a set of
heads I have that measure 4.425 inches on one skimming with the valve to
cylinder head surface at least 0.025 inch below the head face.

I'd like to add a third and fourth note:  3.  Make sure the gaskets you are
using are at least 0.055 inches thick for standard gaskets and not 0.040 inches,
the latter being superceeded with the thicker gasket, and - 0.015 inche
difference could cause you trouble when you crank that engine through the first
time after a rebuild.  4.  an engine rebuild may call for the block to head face
to be "decked"  -0.005 to -0.010 inches.  If this is the case, the pistons may
protrude above the cylinder by a couple of thou or more.  Take this into account
when selecting the proper gasket thicknesses, and make sure both sides are the
same measurement.  If your block has been "decked" use a gauge block set, zero
at the block face, then position the feeler at the piston and rotate the crank
through to the piston top center position noting the gauge does not go
positive.  If it does, add that thickness into your calculation for overall
metal loss from the head skimming and block decking when selecting the
appropriate gaskets.

One of the "usual suspects" ... Yours for speculation, In My Humble Opinion.

Regards,
Glenn  Merrell, VTR Webmaster ProTem
Triumph Stag Register USA VP
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*Member of:
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*73*S*T*A*G****************************************************

Wendy Dron wrote:
> 
> Good Morning All
> 
> Does anyone out there know the correct thickness for Mk I and MkII heads, in
> their original unused state.  We've got one of each at the moment, and have
> just been told that they're beyond planing and are thus knackered, but we'd
> like to know exactly how knackered!
>