Valve Train Geometry

[Doug D.]

Ok...

This is something that's always been pretty straight forward for me, at least until now. I've installed a couple different sets of aftermarket gear, which consisted of solid shafts, spacers and whatever rockers I was using at the time, plus the usual ancillary hardware. The whole thing was always pushrods, rocker assemblies with the rockers spaced to center the adjusters on the valve stems and everything torqued and set to specs.

My most recent efforts have involved installing high ratio rockers (1.25:1) on my past 1641 and present 1835. On the 1641, I always set the lash to .006", which worked pretty great. The 1835 long block came with a cam card for the Engle W-100 cam the builder had used, which put the lash at .004". I set it up this way, only to find a light miss and occasional backfire on the 1-2 bank. I readjusted the valves back to the old .006", which cleared up the miss, but left the engine a tad short on muscle, or at least that's what I attributed it to.

Now, I'm reading a vague and ambiguous article that talks about shimming up the rocker pedestals to achieve proper "geometry". The article, which refers to the installation of a set of 1.25:1 HR rockers, states, "We determined that the rocker pedestals needed to be shimmed up .060 to get the proper 'geometry'. We torqued the studs at 18 foot pounds and set the valve lash at .006."

My questions are these:

First, how did "we" determine that the pedestals needed to be shimmed up .060"? If there's some magical equation for this, I can't seem to find it, and the statement in the article suggests that .060" is a fluid variable which could change depending on your set up.

Second, why is it necessary? What are "we" accomplishing by shimming up the pedestals?

If anyone could shed some light on this, I would once again find myself owing a debt of gratitude to the group.

PEACE, and ride safe!

[DeathBySnuSnu]

With a cam that mild you dont have a lot to worry about.

With an engine that is a big pile of aftermarket parts the original dimensions are gone and we end up being just where it works.

Deck height and replacement heads and cams being drastically different. The geometry of the valve train can be way off.

This really comes into play with big cams that lift to the extreme ends of length of valve and travel of the spring will allow.

So you put it together without the puch rod tube so you can see the push rod.

Install an adjustable screw out rod and the rockers.

Put an indicator on the valve and and spin the engine measiring the travel.

Then go back amd spin it to half travel.

The rocker should be in the mid point also.

As in near 90 degree to the valve and the push rod.

If not you extend the adjustable push rod, add/take away shims untill that mid travel point lines up.

Then take that adjustable push rod and measure it and make your permanent push rods all that length.

On a side note....

What push rods do you use?

Less than zero lash would lose power.

A couple thousandths loose is just noisey.

[Doug D.]

Thanks Rex!

The pushrods are stock length for a 1600. The heads and valves are also 1600 stock, the heads being machined for the 92 mm jugs. The article I read was confusing, as it never explained why shimming the assemblies was necessary, nor did it even say what engine they were building. Made me think it was something that had to be done with every engine. After reading your explanation, the whole thing makes perfect sense. My set up being what it is, I'd say shimming the pedestals probably isn't necessary, however, if I needed to do it in the future, I'm certain sure I could after reading your response. As an exercise though, I think I'll measure the travel and see where everything lays anyway, just for my own satisfaction.

Once again, Rex, you save the day! Many thanks!

PEACE, and ride safe!

[DeathBySnuSnu]

The 1.25 rockers that look a lot like stock type, as in have the adjuster at the valve. That kind are usually a near direct bolt on with the geometry and push rod length.

But those foot style with the adjuster on the push rod side usually take a shorter push rod and a mid travel geometry check. The 1.25 being worse than the 1.4's. And when you check actual full lift with your indicator you will be suprised how much over ratio they are.

I have 1.4's on mine now and with a cam made for them.

Stinger got my old 1.25's of the same style that I run on my "little" cam. You want the old little cam, an eagle 2246 roughly equivalent to a w130 engle? I stepped up a bit.

[Doug D.]

Thanks Rex!

I appreciate the offer, but I'd hate to tear down again to change out the cam at this point. I finally have it running pretty decent, and with bit more massaging I think I can get it where I want it without any more drastic changes.

Thanks again, for all your help!

PEACE and ride safe!

[stinger608]

With a cam that mild you dont have a lot to worry about.

With an engine that is a big pile of aftermarket parts the original dimensions are gone and we end up being just where it works.

Deck height and replacement heads and cams being drastically different. The geometry of the valve train can be way off.

This really comes into play with big cams that lift to the extreme ends of length of valve and travel of the spring will allow.

So you put it together without the puch rod tube so you can see the push rod.

Install an adjustable screw out rod and the rockers.

Put an indicator on the valve and and spin the engine measiring the travel.

Then go back amd spin it to half travel.

The rocker should be in the mid point also.

As in near 90 degree to the valve and the push rod.

If not you extend the adjustable push rod, add/take away shims untill that mid travel point lines up.

Then take that adjustable push rod and measure it and make your permanent push rods all that length.

On a side note....

What push rods do you use?

Less than zero lash would lose power.

A couple thousandths loose is just noisey.

This is an excellent explanation of how to set up valve geometry!!!

Great job Rex!

That "little" cam that you have is equivalent to an Engle W130? If that is the case, it is a fairly radical "race" type came. Pretty "lumpy" in fact. I ran a W120 several years ago and it was a fairly lumpy cam.

[DeathBySnuSnu]

Well yeah.....it ain't tiny.....

I have left cam in the w series behind.

It now has an fk10 in it.

It came in a little over spec. If I had known that before hand I would have went with a web cam 86c instead.