Pushrods and valve train geometry

[Tom Wilson]

I am putting one of Len Fanelli's cam and roller lifter setups in an XPAG and think I have misunderstood Len's directions on pushrod length. Can the experts help me clarify info?

I think the valve train geometry is important and pushrod length affects this. Len sent a new set of nice pushrods that need to be cut to length and the tip installed. No directions were provided.

This is how I think I should set the geometry and valve action up:
* Valve stem height to match the standard spec, and all stems at the same height.
* Rocker arms are stock, though tips have been correctly redone.
* Ideal rocker pedestal height the same as a stock engine that hasn't had the block or head surface milled down.
* At zero lash, a straight line should go through both contact points of the rocker arm and its center pivot point on the shaft, and should be at a 90 degree angle to the valve stem.
* Movement (scrubbing) of the rocker tip across the top of the valve stem should start on the inside (center of engine side), then move towards the outside edge as the valve is opening, and return past center towards the inside edge when the valve is fully opened.

This requires correct pushrods length and rocker pedestal height.
Do I have this correct?

Tom Wilson
TC0272, TC0273, TC0279

[Rick Waters]

From time to time, I've seen cross section drawings of the XPAG, looking straight down the length of the engine. Perhaps taking a good look at such a drawing will show you the geometry?

This assumes zero lash was present in the drawings...

-Rick, TC 7881

[Gene Gillam]

Tom, no answer to your question but a question of my own...who did your rocker arms?

[John Libbert]

Hi Gene,

Try Rocker Arm Specialists in Anderson CA.

Right now I can't connect to their web site (stupid computers!) or I would provide a link for you. I haven't used them but I'm planning to. Let me know what you think of them if you use them.

Best regards,

John Libbert

[Gene Gillam]

John,

They've changed their name and are now Rocker Arms Unlimited and are at: http://www.rockerarms.com/

I had emailed them a week back and was given the following for prices: complete rebuild including re-doing the tips, new bushes and hard chroming the rocker arm shaft was $189, redoing just the tips was $96 ($12 ea). I sent mine in and now I'm waiting.

Would still like an answer to Tom's original question though.

[Gene Gillam]

John,
Got my rocker assembly back from RAU and I'm please with the work...everything looks like new. Unless it fails under use I'd recommend them - took about a three weeks for the turnaround.

I would still love an answer to Tom's original question though. Since the rocker arms have adjustments to set the valve lash what difference does it make if the pushrods are exactly the correct length or not? Won't the adjusters make up any difference as long as they're close?

[joe guttridge]

These are my thoughts , having faced this problem. I would have thought that the straight line drawn across the top of each valve (or the median valve in a real world where valve heights vary from valve to valve)at right angles to the valve stem axis at the point of 50% opening of the valve should pass through the tip of the male ball end on the rocker & hence also the bottom of the female spherical seat in the pushrod when the pushrod is also at 50% opening.
Practically the valves in the XPAG are inclined slightly so what I did was to measure the height of a straight edge clamped at right angles to the head of the valve (ie. by clamping it directly to the valve cap) at the mid point of each side of the machined face that the rocker cover sits on and calculate from this the height that the bottom point of the female recess in the pushrod should be at. (the mid points of these side faces are about 115mm apart & the distance from mid points of the rocker male ball end & the rocker valve bearing pad are about 70mm, so the difference in height between rocker ends is 70/115 X height difference measured at rocker box seat face mid points) I then calculated the length of pushrod I needed. Basically I calculated that the bottom face of the push rod recess should be 10mm higher than the centre point of the valve tip when both are measured from the rocker box seating face at the point of 50% lift for your camshaft. I then choose a push rod a bit smaller than indicated to allow for erosion/wear of the valve seat, by cobbling up a test pushrod.
I cobbled up a test pushrod by butchering an old XPAG pushrod for its spigot ends & then sliding the ends into a bit of 8mm OD X1mm wall thickness hydraulic pipe (which is common as muck in UK). Make the tube a bit long & then file it down as you try it in the engine. But DON’T use it as a pushrod – it’s normalised &, although not far from the chemical spec of the original, its too soft for use on the engine – anyway chrome moly is better!
Good luck

[Steve Simmons]

Another argument is that the push rods should be as long as possible while still allowing adjustment. The shorter the rods get, the longer the adjusters protrude from the bottom of the rocker arm. So as the rocker tip comes up and the adjuster goes down, the push rod is moving more and more laterally. Running a longer push rod will help to keep the push rod vertical during its travel, reducing the tendancy to bend.

Using a measurement for push rod length is dependent on the position of the adjuster. I think most people put the adjuster in the center of its range, but I feel it may be best to raise the adjuster higher and use a longer rod to compensate. Of course the most important thing is that the rocker arm tip swipes the valve as centered as possible.