Mike,
At 09:26 AM 3/20/98 -0600, you wrote:
>I removed the engine from the MGA.
Welcome to the wonderful world of mystery and DPOs.
>It is a 1600 block with an early MGB head. The PO used the aluminum
rocker assembly from the MGA. Shouldn't he have used the MGB rocker
assembly?
Generally the same geometry throughout the whole rocker shaft assembly, so
no problem either way. However, pay special attention to the oil supply
drilling in the rear pedestal and the matching drill hole in the cylinder
head. The MGA and some of the MGB models had this hole on the centerline
of the pedestal. Some MGB models had this hole offset about 1/4" from the
centerline of the pedestal. If the hole in the pedestal does not line up
with the hole on the head, it will block the oil flow to the rocker
assembly. The easy fix for this is to mill or grind a small slot in the
base of the pedestal to connect the two holes.
>The MOSS catalog states that the MGB head originally had a thickness of
3.177 to 3.192. My MGB head thickness measures 3.078.
The MGA 15 head fitted to the 1500 and 1600 engines had 1.500" intake
valves, 1.281" exhaust valves, and a 38.7cc combustion chamber. The MGA 16
head fitted to the 1622 engine, and the MGB 18 head fitted to the 18G
through 18GK engines, had 1.565" intake valves, 1.281" exhaust valves, and
43cc combustion chambers. All of these heads were the same height, 3.187"
nominal dimension. The 1622 engine used flat top pistons to achieve the
same compression ratio as the earlier MGA engines. Other engines used
dished pistions to achieve the desired compression ratio.
Before you jump to any conclusions about your head being shaved down too
far, there is one other possibility. The 18V engines from '71-74 had big
valve heads ... the intake valve was increased to 1.625 and the combustion
chamber was enlarged and reshaped to incorporate the larger valve and to
reduce the 'Westlake Promontory' (the peak between the intake and exhaust
valves(, and the thickness of the head was reduced to maintain the chamber
size. Due to the thinner head the block face was relieved to allow
clearance for the exhaust valves and the tappets/pushrods were shortened to
correct the valve train geometry. This head would be about 1/8" thinner
than the others. If you have one of these heads, I believe it should have
an "L" cast on top at the rear where other heads would have "15", "16", or
"18". Someone else will probably correct something here.
To convert the early MGB head to the MGA compression spec, you need to
shave off about 0.045" to reduce the compression chamber from 43cc to
38.7cc. As a rule of thumb here, 0.010" is aproximately equal to 1cc of
chamber volume.
When calculating compression ratios, you must also include the volume
between the top of the piston and the cylinder head, and also the volume of
the dished area in the piston. For an MGA 1600 with dished pistons, this
is about 15cc, for a total of about 53.8cc final chamber volume, and giving
the 8.3:1 compression ratio. My best guess is about 10cc for the dish in
the piston, and about 5cc for the space above the piston. If you resurface
the block, the space above the piston gets smaller. If you rebore the
block the space above the piston gets larger, but the new pistons probably
have a shallower dish to compensate.
If your head was shaved 1/8", you might expect to find 1/16" shims under
the rocker shaft pedestals. Otherwise you might want to check for use of
the shorter pushrods from the 18V engine.
>The questions are.... How much can you safely shave the head before it
changes it's stock performance characteristics and compression ratio, and
which rocker arm assembly should be used?
There is enough iron in these heads to shave 1/8" off without ruining the
head. The more you shave off, the higher the compression ratio goes. The
final compression ratio is a function of piston displacement, piston dish
volume, space between the piston and the head, and the head chamber volume.
As an aproximation, 0.010" shaved off the head raises the compression
ratio by about 0.2, so for example 8.5:1 becomes 8.7:1.
>The piston tops look pretty flat to me. Weren't the original MGA 1600
pistons dished?
Yes.
>If so, how dished were they and are the visual differences between the
flat tops and dished obvious?
Very obvious. The flat tops were obviously flat on top. The dished
pistons have a flat land around the circumference about 3/16" wide. and a
generally spherical dish in the center about 1/8" deep.
For the purpose of a guess here, I will make a few rash assumptions.
Supposing your 1600 has flat top pistons, and that the block has been
shaved .020" as a cleanup cut. The space above the piston will have been
reduced from about 5 cc to 3 cc. The space in the piston dish area will
have been reduced from about 10cc to 0cc. Further suppose that you really
do have an early MGB head that has been shaved all of 1/8". The head
chamber volume will have been reduced from 43cc to about 30.5cc. Total
size of the combustion chamber is 30.5+2.5+0=33cc.
Swept volume of one cylinder is 1588/4 = 397cc. Adding the chamber volume
of 33cc gets a total of 430cc maximum volume at bottom dead center. And
the compression ratio would be (brace yourself) 430/33 = 13.0:1. Yikes!
You would need at least 104 octane fuel to accomodate that high a ratio
without detonation, and maybe even as high as 110 octane.
On the other hand, a high performance racing engine will quite often have a
rather radical cam, which will keep the valves open longer, which will in
turn reduce the effective volume of the cylinder at low speeds, and in turn
reduce the effective cylinder pressure. In this case you may not see as
high a pressure reading as you would expect from the greatly reduced
chamber volume.
You can make a reasonable measurement of the combustion chamber volume in
the head. One fluid ounce of water (1/8 cup) is 1.8 cubic inches, or 29.6
cubic centimeters. Turn the head upside down with the valves in place.
Start with a know amount of water in a measuring cup, maybe 1/2 cup. Pour
water into one combustion chamber until the water level is flush with the
surface of the head. Good idea to have a straight edge across the top so
you can tell when the water level is right. Then check the amount of water
left in the measuring cup. The difference between the starting and ending
amount in the cup is the volume of the combustion chamber. You can do the
same measurement on dished pistons, and on the space above the piston in
the engine block. These measuremente are much easier if you have a 50cc
laboratory test tube graduate in cc down the side.
And since it appears that we're looking at a modified high performance
engine here, it may also be expected to have a non-stock camshaft. So I
will add a few comments on cams, even though you didn't ask.
For the purpose of measuring lift and duration on a camshaft, you need to
mount the cam between lathe centers, or in a pair of v-blocks, or in the
cam bearings in the engine block, so you can rotate it as it would in
operation. Then you need a dial indicator (or equivalent) to measure the
height of the cam lobe. You should also have a means of measuring the
rotational angle of the cam as you turn it. The height difference between
the lowest point on the cam lobe and the highest point is the lift.
Lift is 0.245" for the stock MGA 1600 cam, more like 0.264 for a mild
performance cam, or 0/277 for a radical race cam. The stock rocker arm
ratio is 1.426:1, so these numbers translate into lift at the valve of
0.350, 0.358, and 0.375 respectively.
As you rotate the cam there will be about 2/3 turn at the low point of the
cam where the indicator stays the same height. This is the dwell of the
cam when the valve is closed. You want to determine the angle of rotation
where the indicator just starts to move upward, and the other point of
rotation where the indicator just settles back down to the dwell position.
The difference between these two angles is the duration of the cam. Dwell
plus duration = 360 degrees on the cam, 720 degrees at the crankshaft.
Duration for the stock 1600 cam is 252 degrees out of 720 degrees of
crankshaft rotation (126 degrees at the cam). Duration is the same for
intake and exhaust valves, but the timing is not symetricalm with the
intake operating 175 degrees of crankshaft rotation after the exhaust.
Duration for a performance cam might be 270 for a street cam, 286 for a
road cam. and 302 for a full race cam. All of these numbers are
traditionally stated as angle of rotation of the crankshaft, which does 2
turns for each turn of the camshaft. So divide these angles by 2 when
measuring rotation of the camshaft.
And if you're not confused enough yet, just ask some more questions.
Advice is often free, and sometimes it can be worth as much as you pay for it.
Cheers,
Barney Gaylord - 1958 MGA with an attitude
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