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Valve Adjustment

September 28, 2010 | Troubleshooting & Repair

By Ron LaDow Precision Matters

Adjusting Valves: The First Step in Tuning Up a 356

Because of the interaction of the various engine systems, a tune-up on a 356 (and other engines of similar vintage) must be done in a certain order:

  1. Set valve clearances
  2. Tune the ignition system
  3. Tune the carbs

Each prior portion of the tune-up contributes to the later. At the later-stage tuning of the carbs, there is a certain 'circularity' to the adjustments, but you have to draw the line somewhere. Contributions here will cover most 356s from 1955 and some 912s.

You should have at least the Owners Manual, or Elfrink's tome to identify some of the parts referenced.

Except for the carburetor adjustments, no special tools are required. If you are performing a tune up for the first time, allow a weekend. Order the parts first; there are few things that discourage fiddling with old cars like having to wait a week for a miserable piece of cork that also made you miss that Saturday get-together.

Getting Started - Adjusting Valve Clearances

Tools Required:

  • Feeler Gauge
  • Stubby Flat Blade Screwdriver
  • 13mm Wrench

Parts Required:

  • Valve cover gaskets

What

When the valves are closed and on their seats in the heads, there is a certain looseness in the actuating linkage; that's 'valve clearance'. It's there for a variety of mechanical reasons, some too technical for this discussion, but not least just to make sure the valve is fully closed.

The clearance could be measured anywhere in the individual cam/valve linkage, but is most easily measured at the interface between a valve stem and its mating rocker arm.

Your specific clearance requirements should be determined by the maker of your cam. Also, thermal expansion in aluminum cylinders will affect the numbers; I don't know but hope the makers of them offer guidelines. If you have a totally stock engine, follow the recommendations in the Owner's Manual or Shop Manual.

On a late engine with iron cylinders, cast pistons, 9:1CR, 266* cam, .004" on the intakes and .006" on the exhausts works just fine. Note that exhaust valves are to the outside and intake valves are to the inside.

How

Always adjust the valves when the engine is cold.

To begin, get the rear of the car up in the air safely; you will be under the car (read about how to jack up your car safely). Remove the rear wheels or not. Put paper towels on the exhaust pipes and the carb heater duct, and newspaper or drain pans as you please on the ground.

Pull the bails off the rocker covers. Pry if need be, but don't harm anything; a decent-sized hardwood dowel does well. Pull the rocker covers off the engine; some oil will now run out; clean it up, but leave the paper here and there. It will continue to drip.

Back on top, remove the distributor cap and find TDC (Top Dead Center). The rotor should point to about 5:00 when the OT mark aligns with the case scribe line (or the Timing Kit zero scribe). Now rotate the crank clockwise 180* which will give you TDC for #4 cylinder (the firing order of a 356 is 1-4-3-2).

Each clearance is measured with feeler gauges at the valve stem/rocker interface, so the surface of the rocker needs to be pretty flat; look as closely as you can while you check the clearances. If one or more of the rocker faces is seriously 'grooved', they will need attention before you can do a proper adjustment. Or, as some of us have done, give it your best shot with what you got.

How tight is a matter of experience. A slight tug to release the feeler gauge is proper for a zero-clearance feeler gauge fit on true, lubed surfaces. If the feeler gauge pack falls out slowly of its own weight, you're in good shape. More than finger-tip pulling means you're too tight. If you're engaging in 'what you got', give this a close look; that groove introduces all sorts of friction.

OK, check/adjust both exhaust and intake valves on #4 cylinder (Note: As you face the back of the car, the #1 cylinder is on your right to the front of the car, #2 is on your right to the back of the car, #3 is on your left to the front of the car, and #4 is on your left to the back of the car).

Use an up-side-down 13mm box-end over a favored stubby screw-driver. Loosen the nut, adjust the gap between the rocker and valve stem to the correct tolerance, then tighten the nut while holding the spacing with the screwdriver.



Go back up on top and rotate the crank clockwise 180*. Then adjust #3. When you're done, move the tools to the other side, rotate the crank another 180* clockwise, and set #2. Then one more 180* rotation and adjust #1 (you will now be at TDC). Look while you do the work; if it's only, say, one rocker that looks really bad, maybe you can find a replacement. Check to see that all the adjustment screws are similar in the amount of threads showing. Look for rust, smell for gas, peer intently.

Assuming the engine was running close to properly, you should find no surprises in the actual adjustment or examination, and the adjustment should require minimal changes. But if one (or more) valve will not accept the feeler gauge with the screw all the way out, you likely have a push rod which is not seated in the cam follower. Loosening that rocker from the stand and wiggling the push rod should allow it to 'drop' about 3/16" or so toward the center of the engine. You can now retighten that rocker and find the proper adjustment.

Clean the rocker covers. If you have glued-on gaskets and they have been oil-tight, you can chance the fates and re-use them as is. Otherwise, make sure the covers are clean and the gasket surfaces flat. Check the latter against the heads or other ad hoc references. They can be knocked and twisted back to flat.

If you're using 'stock' gaskets, they should be glued in place to keep them there; 3M trim adhesive works. If you're using the metal-re-enforced gaskets, just put them in and reassemble.

Your next step is Ignition Tuning.

Editor's Note:

This is part of our module on how to tune up your 356. Components include:

2 Comments

Aa493c9f9299cdb0d51a6d201e2b655c 45
Al Zim
June 26, 2011 at 6:59 PM
You will notice that the feeler gauge that is used in curling on the end. That is because the valves are set TOO TIGHT. <br /> At Zim's we add a little extra clearance to account for push rod and cylinder mis-matches. Intake .006 in Exhaust .008 in. al zim
Aa493c9f9299cdb0d51a6d201e2b655c 45
Al Zim
June 26, 2011 at 6:59 PM
You will notice that the feeler gauge that is used in curling on the end. That is because the valves are set TOO TIGHT. <br /> At Zim's we add a little extra clearance to account for push rod and cylinder mis-matches. Intake .006 in Exhaust .008 in. al zim