valve train geometry, first time, help needed

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valve train geometry, first time, help needed

Post by colinD » Thu Mar 13, 2008 6:35 pm

decided to pull apart a 2110 i had built for a van
low CR, 110 cam stock ratio rockers and it would appear stock pushrods

i ant to rebuild wuth an fk8 and 1.4/1 scat pro comp rockers and up the CR a bit

ive done a bit of reading on geometry and am trying to achieve the adjusting screw in line wth the pushrod at half lift with the srew at half adjustment

i put the cylinders and head on , bolted up, the rockers and adjustable pushrod

i had removed the cylinder spacers used in the initial build giving a deck height of 1.25 mm, with the 044s 56cm volume gives about 8.8/1 CR

problem is the adjustable pushrod is too long when the parts are bolted up
i replaced a 1mm spacer at the base of the cylinder and 2 shims( unsure of thier thickness, the battery died in my vernier)
the pushrod is still to long, I tried 3 shims , and i dont seem to get a whole lot of compression in the valve spring and so i guess valve lift, and i still dont achieve my rod/screw alignment

there are valve lash caps included with the scat rockers, which arent helpin in the situation too, but scat recommend them

is there any problem with me shortening cut to length pushrods shorter than the adjustable one? id shorten it first to find the length required

is there a limit to w short the pushrod should be?

how do i measure the amount of compression left before the (dual) springs bind, ive heard of that, and how do i set that??

sorry for the long post, i thought it would be a bolt up & adjust

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Post by david58 » Thu Mar 13, 2008 6:39 pm

What is the length of the adjustable push rod you are using?
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Post by colinD » Thu Mar 13, 2008 6:46 pm

its 10 mile away by now, LOL
its a few mm shorter than a stock pushrod

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Post by Daimo » Fri Mar 14, 2008 7:00 am

answers in here somewhere......... ... =printable

and you think the question was long :lol:

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Post by colinD » Fri Mar 14, 2008 7:29 am

thanx guys

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Post by FJCamper » Fri Mar 14, 2008 12:12 pm

Hi Colin,

Here's how to do an "at the track" rocker arm geometry setup. This means without the time or tools to do it exactly right, but get close enough to get back in the race. Imagine you'd broken a rocker or shaft and had to slap on a set of rocker parts you'd never set up for, or someone gave you.

I've always wondered why some enterprising company doesn't sell precut and pretipped pushrods in millimeter steps! I'd carry a few sets of those to the track.

First, with whatever push rods you have, bolt on the replacement parts with no shims, but with the valve adjusting screws backed all the way out.

You'll probably find that even with the adjusting screws backed all the way out, you can't get the rocker arms seated all the way down on the cylinders with valves open.

At least you tried. Unbolt the rockers and shim the rockers. If you can't get proper shims, scrounge up some machined-flat washers that are all the same thickness.

Usually, shimmed up, the rocker arms will fully bolt down. If you're very lucky, you can even adjust the valves. The warning signs are running out of threads as you back out the adjuster screws. Be careful to notice on swivel feet adjusters that you're not trying to pop off the swivel foot itself as you unscrew the adjuster. When you hit resistance, stop.

Some swivel foot adjusters allow for more back-out space than others. I've learned to carry a couple of sets of adjusters made by different manufacturers.

If you can't make a swivel foot work, try a standard adjuster. If no adjusters work, shim the rockers out a little more until they do.

Okay. Adjust the valves slowly, watching and feeling for valve spring bind as you do. You'll feel it if it happens. The rocker arm will be pushing so far down on the valve spring that the whole spring is compressed, the coil turns impacted down on each other. The crankshaft will resist your effort to rotate it.

If you force the turning, you'll either break or bend a rocker arm, or damage the spring and/or valve retainer.

Binding means that even with some adjustment having been possible, you still don't have enough slack space in the valve train, and you have to reshim.

If you had no binding, and the valves adjusted out okay, get a mirror so you can see the angle at which each valve adjuster is striking the valve stem.

If the adjuster is just barely touching on one side of the stem (meaning the extreme here, since adjusters should strike the stem off center), you have really bad alignment and have to relocate the rocker arm lateral position on the shaft by shimming it.

Porsche (in the 4-cyl 356 days) had the valve adjusting screw opposite of our VW arrangement. Porsche put the screw against the pushrod rather than the valve stem itself. Scat makes a set of rockers like this, and they are much better because that arrangement better tolerates valve train geometry errors.

Swivel feet themselves, introduced on the Porsche 911, allow for a certain amount of error. VW took the purist route and made us work harder, presuming VW mechanics would always show up for work sober and fully dressed.

If you were able to get to the point that you've slapped on a different set of rockers, and they adjusted okay, and the engine runs, you probably still have valve train geometry error, but obviously not so much to keep you off the road.

At this point, the most common form of error is not getting enough valve lift. You're going to be down on power from a properly set up valve train.

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