Sorry folks, I have to requote all of this as being relavent.
At 05:25 PM 8/30/2000 -0700, Dodd, Kelvin wrote:
>.... I have always taken for granted that the pushrod and tappet swap was
a no-brainer. One thing that has always niggled has been understanding
that there was a change in head thickness along the way.
This is going to get a little long, so before going any farther I would
like to mention that the 18V engines from 1972 to 1974 are slight
exceptions to the following discussion. With these engines the head has
larger valves, which in itself is not important to this, but they also have
the "appearance" of having the head shaved considerably on the bottom
surface to reduce the height of the combustion chamber. This head is
indeed thinner than other B-series heads, with the entire difference in
height being below the valve heads, and everything above the valve heads
being about the same geometry. The significance here is that the valves
are positioned closer to the top of the block, and that these engines came
from the factory with eyebrows cut on the block for the necessary valve
head clearance. Just as a side note, if I am correctly informed, the
eyebrows in the block may have been continued for some time after the
discontinuance of the use of this head (in the USA). In the case that one
is swapping this particular head onto a different engine, one should keep
in mind this differnece in valve position.
>Eyeballing the tappets and pushrods that are on hand, the later setup is
about 1/4" shorter than the early set. I was wondering if anyone had run
into any geometry or adjustment problems with this conversion.
Uh-oh. I hope you're not telling me there are two different length
pushrods being sold under the same part number, or that one pushrod is
being sold for two different applications. Consulting my trusty MGB parts
catalog I find only two part numbers for (original) pushrods. These are
8-3/4" long for the early engines and 10-1/2" long for the 18V engines
(with bucket tappets). So what do you have in hand that is 1/4" different?
I am going to venture an educated guess and say possibly 10-1/4" long for
the other part, reasoning to follow.
> Did you notice any problems with your setup in the 1500, as this would
probably be the most critical application.
The short answer is yes, but I was making more than one change from stock.
With all B-series MG engines the exhaust valve is positioned partially over
the side of the cylinder wall above the engine block. The significant
difference with the 1500 engine is that the intake valve also overlaps
above the block a bit, where the larger bores don't have this problem.
However, installing a large valve cylinder head on the 1600 engine can
cause the intake valve to sit above the block, and using greatly increased
intake valve size can get there in the 1800 engine as well. Otherwise all
B-series engines share similar geometry traits for the valves, so this
discussion is germain to everything from 1.5 to 2 liter.
> It is a concern, as I am seeing a number of inexperienced people
>building
1500 engines using untried combinations of parts. It suddenly becomes the
vendor's fault when a combination such as; highlift cam, high ratio rockers
and crossflow head cause valve interference with the 1500 block.
No kidding! However, "suddenly becomes the vendor's fault" is surely this
customer's expectation, but not truly the case. I wouldn't blame the
vendor unless the catalog specifically lists such parts as being
compatible. One such statement IN THE CATALOG (at least in a sales flyer)
that used to bother me about a dozen years ago was the assertion that the
early MGB cylinder head could be considered to be a performance upgrade for
the MGA. Yes the 18 head has larger valves, but it also has a larger
combustion chamber that will lower the compression ratio considerably. So
without any additional explanation, that statement in the catalog WAS
somewhat misleading. Just a thought, and that was quite a while back.
Now per my meager (but significant) experience, there is enough clearance
for the valve heads that one can install an upgrade street cam, such as the
one Moss calls "222-270 Camshaft, high performance", without getting the
valves too close to the block. Alternately one could install the high
ratio rocker arms, also without getting the valves too close to the block.
Or one could shave the head considerably, like possibly up to 1/16 inch,
without causing any interference problems. And in any case I'm talking
about maintaining at least 1/16 inch of valve head clearance at point of
closest approach. If you are doing any of this, and you are the least bit
uncertain of the valve head clearance you may end up with, you had best
measure it to be sure.
The problem comes when you do a combination of these things that combines
reduction of clearance. For instance, when I shave an MGB 18 head by .045
inch to get the proper (stock) 38cc chamber size for the MGA 1500/1600
engine, and also install a cam with a higher than stock lift, then I have
to grind the eyebrows in the top of the block to maintain the minimal
required valve head clearance.
So far, none of this has anything to do with changing the length of the
pushrods, because the adjuster screw on the rocker arm has sufficient
travel to accomodate these modifications. Shaving the head just lowers
everything upstairs a bit, and the ball on the adjuster screw just ends up
a tad higher in relation to the rocker shaft, but the rocker arm still
rests at the same position and moves the same in relation to the valve. A
little higher lift on the cam moves the rocker arm through a larger angle
of motion, but with mild changes this is still tollerable with the standard
rockers and pushrods.
Now getting closer to problems, there are two different types of high ratio
rockers. The cheap and dirty type mounts on the original rocker shaft, has
the same length of arm on the valve end, but a shorter arm on the pushrod
end. Among other things this causes the pushrods to stand at a less than
vertical angle, and may require enlarging the clearance holes in the head.
Using the standard rocker shaft with more rocker motion will start the
rocker at the same original position and move it farther down the angle to
attain increased valve lift. This is approximately the same affect whether
you use this type of high ratio rocker, or with using a high lift cam, and
either by itself would be acceptable.
However, if you were to install this type of high ratio rocker arm AND a
high lift cam, then both of these items will increase the angular motion of
the rocker, and you will end up with some very nasty rocker angle at the
fully depressed end of the stroke, such that the output end of the rocker
is wiping excessively across the tip of the valve stem. This is not a
desirable condition, as it applies considerable side load to the valve stem
in the valve guiide causing increased friction and accellerated wear of the
guides. In some unfortunate cases it may result in eventual fatigue
failure of the tip of the valve stem. Ouch! But still we haven't changed
the length of the pushrods.
The second and more desireable type of high ratio rocker arm has a shorter
than original input arm and a longer than original output arm, and the
rocker pivot point is repositioned a little farther from the valves and
also just a little lower. For one thing this allows the pushrods to stand
up straight. But the more inportant point is that it makes the motion of
the rocker arm more symetrical relative to the valve stem. The rocker will
then start at a little higher angle, proceed across horizontal, and finish
at a lesser angle, such that the start and finish angles are about equal
relative to horizointal, thereby minimizing the wiping motion across the
tip of the valve and reducing stress and wear on the valve stem and guide.
After all this explanation, this is where the shorter pushrod can come into
play. Suppose the combination of high ratio rocker and high lift cam have
increased the valve lift from .350 to .500 (.150 difference), and the
rocker pivot point has been lowered by .075 to keep the rocker travel
angles symetrical. Now the rocker mounting height has not only been
lowered, but the input end of the rocker arm also sees an amplifiction of
motion similar to the output end, so it is nice if the input end of the
rocker is also lowered some to keep the rocker travel angles symetrical at
that end as well. Now the pushrod indeed should be about 1/8 inch shorter
than original. Also, if the head is being shaved to increase the
compression ratio at the same time (especially if shaved a lot), this also
lowers the rocker shaft, and a commensurate reduction of the length of the
pushrod would be in order.
Following all this, for a full out racing engine application the pushrod
might indeed need to be about 1/4" shorter. But for only a little head
shaving or a little higher lift cam the stock rocker adjuster screw will
take up the difference. What I have just described as the better design of
high ratio rocker arm will by itself (usually) allow for the use of the
standard length pushrod by leaving the tail position of the rocker a bit on
the high side, or by having a long travel adjuster screw. As far as I know
the only reason for going to shorter pushrods is if you use a combination
of these tinker toys together.
Now if that didn't cover it all, the floor is still open to discusison.
For the moment, I'm tired of typing.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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