Sunday, August 12, 2007

Adjustable Push-rod

One of the trickier bits in building a high performance engine based on after-market VW components is your valve-train geometry. Here's the situation: The lobes of the cam impart about three-tenths of an inch of linear motion to the cam-follower; what most American's refer to as 'tappets.' A rigid push-rod conveys that motion out to the heads where a lever called a rocker arm is used to reverse the direction of the motion, converting the upward push of the cam into the downward shove of the rocker arm. The rocker arm bears against the head of the valve's stem and the downward shove causes the valve to open by some amount, once it has overcome the pressure of the valve spring.

The tricky bits involve the fact that the motion of the rocker-arm is not linear but is an arc, whereas the rocker-arm itself is not symmetrical, with the out-put side being slightly longer than the input. To add to the complexity of the problem the push-rod on the input-side of the rocker-arm is at an angle of about minus three degrees, whereas the valve stem on the output side of the rocker-arm is at an angle of plus 9.5 degrees, both relative to the traverse centerline of the rocker-arm's fulcrum (ie, the rocker shaft).

Which means less than nothing if you are dealing with a bone stock VW engine. So long as you do not alter any of its dimensions the losses in the valve train are but a trifle.

(The above offers some idea as to why most designers of high-output engines use the cam to actuate the valves directly (as in old flat-head Ford V8).)

Shade-tree types prefer to ignore valve train geometry -- another of those 'unimportant' details. But the sad truth of the matter is that it isn't unusual for a big-bore stroker with a hot-rod cam to perform worse than the stock engine.

Fortunately, for a particular engine configuration, full understanding of the topic is not required. For the two engines, the assembly of which I am describing in this blog, I will provide a 'cook-book' approach that should allow the reader to come within a few percentage points of the ideal geometry. But you will need a couple of special tools. One is an adjustable push-rod, which I'll describe below. The other is modified stock adjusting screw, which I'll describe (and illustrate) in a future post.


To make an adjustable push-rod you start with a stock push-rod. I prefer the older style because of the smaller head diameter but the later model will also work. (Fig 2 will give you some idea of the difference in head diameter. Some steel push-rod kits use the smaller diameter heads, leading to an error if you check push-rod length with the larger heads.)

Using a hacksaw, cut the push-rod in two. Make the cut approximately in the middle of the rod. Then cut 5/8" to 3/4" from one of the pieces.

Dress the cut ends square with a file. If using the old style, ream the ID with a drill bit suitable for threading to 1/4-20. That would be a #7 but if you don't have a set of number-sized drill bits, you may use 13/64". The later model push-rod has a slightly larger ID and I believe it will accept a 1/4-20 tap without reaming (but check).

Tap each half of the push-rod tube to a depth of at least one inch.

Prepare a section of 1/4-20 threaded rod about 2-1/2" long being sure to chamfer the ends. Run a pair of 1/4-20 nuts onto the rod. Give the threads a drop of oil and screw the rod into the ends of the modified push-rod. The nuts will be used to lock-in the length once it has been determined.


Fig 3 shows a handful of parts heading for a fellow engine-builder trapped in the Nevada desert. Since I didn't know what type rockers or push-rods he'd be using, I sent one of the old-style adjustables.


Adjustable push-rods are available from after-market retailers but they usually put the thread at the very end of the rod, making them horribly inconvenient to use.