Head Thoughts
As far as meat left in the head, it depends on what
head you have. The early non extended head bolt boss
heads are thin. Believe these were Series 2 heads and
prone to cracking so doubtful that many are still
around. Also, these were the only ones with the longer
pushrods. Think this was the head that was sawed in
two in the attached post fro Hank Rutherford. I've got
one of these as my emergency backup. It has cracked
and been welded.
The 2a 7-1 heads have more meat and can be milled
without a problem and still have enough meat left for
additional machining. These are the ones that have an
extended head bolt boss next to where the carb goes.
AFAIK, these heads all came with the shorter pushrods
and work just fine when the head is milled to 8-1.
These heads were the same casting for 8-1 or 7-1, just
the milling was different. We're talking 1962 or so
for the introduction of this casting.
Have heard there is an even later head casting that
has even more meat. These are the best choice for
milling beyond 8-1. I think these were introduced
after Rover discontinued importing into the North
American Market so doubtful that there are many of
these around. Believe this is the casting that ACR
uses to build their Stage II heads.
If you do have a 7-1 head, from any casting, you will
have to be sure there is clearance between the
thermostat housing and the water pump after milling.
Lay the head on the block without a head gasket and
with a piece of paper between the head and the water
pump. Grind off the top of the water pump housing till
the paper no longer gets stuck between the two.
As far as what you get for hp out of 8-1 over 7-1,
it's a bit up in the air. Probably not a whole lot
like maybe 3-4hp. Of course, 3-4hp in a grossly
underpowered vehicle makes a noticable, though not
breath taking difference. For the cost, it's more than
worthwhile to mill the head.
The below is a very helpful post from "H.Rutherford"
that I picked up off the LRO list some years ago.
Keep in mind that when he says 7-1 head, he's probably
talking the old non extended boss head.
A while back I modified a 7:1 head to 8:1 specs. In
doing so, I collected some interesting info. Here
goes:
7:1 head is 3.700 in thick, chamber volume is 91 cc.
8:1 head is 3.600 in. thick, chamber volume is 75.2 cc
7:1 pushrod length overall, 7.300 in
8:1 pushrod length 7.175 in overall (some parts
suppliers are not able to tell you the difference
between pushrods, just that they are different)
Within the range of our interest, there is about 1.2
cc change in volume per .010 in. change in thickness
Milling .100 in. makes a 7:1 head 8:1. Almost exactly.
Head gasket volume and volume above the piston crown
is 4.25 cc. Swept volume of one cylinder is 571.5 cc.
I sawed a 7:1 head into slices to measure casting
thickness. I discovered that there is little material
left between the waterways and the head face after
milling the required .100 in. IMHO, I would NOT advise
going any further as I doubt the head would hold up
for long, I imagine cracks would develop on the head
face. If you want a 9:1 CR, use an 8:1 head. The
castings are definitely different, a stock 8:1 head
has the same metal thickness in critical areas as a
stock 7:1 before machining. I would advise obtaining
a burette for measuring combustion chamber volume to
be sure you don't go too far with CR. I can provide
more info on CC'ing if you need it.
If you modify a 7:1 to 8:1, use 8:1 pushrods.
Otherwise the rocker geometry is off, and the
possibility of fouling the rocker exists. By rocker
geometry, I mean the angle that the rocker arm
contacts the tip of the valve stem. If the longer
pushrods are used in place of the shorter (correct)
ones, the rocker arm depresses the valve with it's
very tip, concentrating wear and forces at a place
where the rocker was not designed to be operated.
This can cause premature wear of the valve stem,
unusual wear of the rocker arm, and failure (breakage)
in extreme cases.
H. Rutherford
'62 SerIIa SWB
Aloha
Peter O.
--- Robert Palmer <rbpalmer_nrt@hotmail.com> wrote:
>
> Finished pulling the cylinder head - and found the
> exhaust valve in cylinder
> #2 had a couple of pie-shaped gouges in it. Beyond
> that, all of the valves
> look pretty sooty - which coincides with my sooty
> spark plugs (I had them in
> there for less than 30 minutes running time and they
> came out coated in
> black soot) - so I guess I get to lean out the
> mixture screw on the carb
> once I get everything back together.
>
> So a couple of questions :) First, I'll be ordering
> all of the bits to
> replace the exhaust valve, plus the replacement head
> gasket. Rest of them
> look fine - so is there anything else I should be
> ordering while I have this
> torn open?
>
> Second - How can I tell if my head has already been
> milled to 8:1 or not? I
> checked for the stamp and it looks like a 7:1 head.
> Can I go by the push
> rod length, or should I get the thickness measured,
> and mill down to 3.590"
> (v. 3.690")
>
> Thanks!
>
> Bob Palmer
> '69 Series IIA 88
> '72 Series III 109
> '76 101FC (70FL00)
> '03 Discovery SII
>
>
>
>
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