The MGA With An Attitude
A question about PUSHROD LENGTH and the way it affects valve motion.

At 05:25 PM 8/30/2000 -0700, Kelvin Dodd 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 difference 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 germane 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; high lift 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 accommodate 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 tolerable 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 guide causing increased friction and accelerated 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 desirable 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 important point is that it makes the motion of the rocker arm more symmetrical 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 horizontal, 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 symmetrical. Now the rocker mounting height has not only been lowered, but the input end of the rocker arm also sees an amplification 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 symmetrical 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.

Barney Gaylord
1958 MGA with an attitude

Thank you for your comments -- Send e-mail to <Barney Gaylord>
© 2000 Barney Gaylord -- Copyright and reprint information