CRG Research Report - 302ci Engine OEM Valve Adjustment

C R G	CRG Reports	Exterior	Engine	1967 Model ID
Numbers Decode	General Info	Interior	Transmission	1968 Model ID
Drivetrain Decode	Options	Underhood	Chassis	1969 Model ID

CRG Research Report

302ci Engine OEM Valvetrain Adjustment

(for "30-30" and other OEM small-block solid lifter cams)

Reviewed by the CRG
Last Edit: 29-Sep-2005
Previous Edits: 09-Sep-2005, 09-Jul-2004
Original Release: 09-Sep-2005

Introduction

The traditional method of adjusting cylinder head valves one or more cylinders at a time, with each cylinder at TDC (top dead center), is fine with hydraulic valve lifters and with most solid-lifter camshaft designs. But this method cannot be used with the factory "30-30" solid-lifter cam (PN 3849346) used in 1967-69 Camaro 302/290hp Z/28 engines, as well as in 1964-65 Corvette L-76 327/365hp and L-84 327/375hp (F.I.) engines. This cam derives its 30-30 nickname from the factory rocker arm clearance of 30 mils (0.030 inch) for both intake and exhaust. It has VERY long clearance ramps that are 0.020-inch high, and at TDC for any cylinder, both the intake and exhaust valves for that cylinder are still on their ramps rather than on the cam's base circle, which is why the Service Manual for all cars so equipped specifically says to set them "hot and running".

There is, however, a better way to adjust the valves with a "30-30" - you can set them "cold and not running" by setting the intakes at 90 degrees ATDC (after TDC) and the exhausts at 90 degrees BTDC (before TDC) - at these points the lifters are on the base circle rather than the ramps. This has been confirmed with cam-lift/crank-angle diagrams, and results in a nice mechanical "singing" sound without "clacking"; it runs better, sounds better, idle is more stable, and throttle response is improved. Many other Corvette "30-30" and Z/28 owners have followed this procedure since we developed it and all have seen the same positive results.

Clearance Compensation

The difference between "hot" clearance (i.e., at engine idle speed) and cold clearance on a cast iron pushrod engine is negligible, so clearances can be set cold, which is much more convenient. When running hard, such as at sustained WOT (wide open throttle), the exhaust valve head heats up considerably. About 80% of exhaust valve cooling is through the seat, but the stem temperature will increase also, which causes the stem to grow and decrease running clearance. This is why exhaust ramps are typically higher than inlet ramps - to allow for more stem growth while maintaining some running clearance to ensure that the valves seat fully. Since the intake valve is cooled by the incoming fresh intake charge, its temperature and clearance will remain more consistent over the entire engine operating spectrum.

The factory 0.030-inch/0.030-inch (intake/exhaust) clearances are derived from multiplying the 0.020-inch maximum height of the ramp above the base circle by the theoretical max lift ratio of 1.5. However, the actual as-measured lift ratio for the stamped OEM rocker arm at the lash points is actually about 1.37:1 (not the design ratio of 1.5:1, which is a theoretical max lift measurement, or the actual as-measured max lift with factory stamped rocker arms, which is about 1.44:1). The desired clearances in this procedure are therefore derived by "factoring" the OEM recommended clearances by the ratio 1.37/1.5 to compensate for the actual as-measured rocker arm ratio of 1.37 at the lash point. That number is then rounded down slightly to result in the cold clearance number. The clearance ramp, which is exactly 0.020-inch high on the lobe, is all taken up at 1.37 x 0.020 = 0.0274-inch clearance. Rounding down yields the desired 30-30 cam clearances of 0.026 inch/0.026 inch for this cold procedure.

Using the 0.030-inch clearance with the valve closed is too loose - the ramp ends/begins before the 0.030-inch clearance is taken up, resulting in the valve being lifted off and returned to the seat at greater than ramp velocity. This will contribute to valve seat recession, and can cause valve bounce at the seats at high revs - it will also be noisy.

The "Cold" Adjustment Procedure for the 30-30 Cam

You can adjust two valves at each 90-degree rotation point, starting at cylinder #1 TDC and turning the crank 90 degrees at a time, seven times (first measure and mark your balancer at 90-degree intervals from TDC). Removing the plugs simplifies rotating the crank, but you were going to change them anyway, right?

Table 1: Cold Adjustment Procedure for 30-30 Cam


Step   Rotate        Adjust exhaust Adjust intake
Number Crank         to 0.026-inch  to 0.026-inch
------ -----------   -------------- -------------
1      Rotate cylinder #1 to TDC
2      ---           cylinder #8    cylinder #2
3       90 degrees   cylinder #4    cylinder #1
4      180 degrees   cylinder #3    cylinder #8
5      270 degrees   cylinder #6    cylinder #4
6        0 degrees   cylinder #5    cylinder #3
7       90 degrees   cylinder #7    cylinder #6
8      180 degrees   cylinder #2    cylinder #5
9      270 degrees   cylinder #1    cylinder #7

The rocker arm nut should be tightened until a light drag is felt on a feeler of the same thickness as the recommended clearance (0.026-inch for the 30-30 cam). Then the clearance can be verified by inserting a feeler gauge that is +0.001-inch thicker; if it won't go, the clearance is between the two gages, which is just right. (Normal engine service will usually result in slight loosening of clearances over time and Chevrolet service recommendations from the 60's recommend a lash check every 12,000 miles as part of a normal tune-up.)

If you like, you can then go back after you're done to each cylinder's TDC position and check clearance on that cylinder's two valves, and you'll find that they've closed up to 0.024-inch, indicating that both valves are still on the ramps at TDC, as I pointed out in the beginning.

Trivia - the point of max inlet lift on the "30-30" cam is at 112 degrees, with a lobe separation angle of 114 degrees (angle between points of max lift, not the geometric center of the lobe - the lobes on the "30-30" cam are asymmetrical).

Using This Procedure on Other SB Cams

The above progressive procedure will ensure that you are on the base circle on any OEM Chevrolet cam for adjustment. For other OEM Chevrolet small-block solid-lifter cams the factory-recommended clearance should be factored by the 1.37:1-vs.-1.5:1 ratio difference at the lash point and rounded-down to the next-nearest thousandth or two to ensure that the valves are picked up and seated at ramp velocity. Again, a fundamental assumption is that you are using OEM stamped rocker arms.

"097" Duntov Cam

The original "097" Duntov was used from late 1956 to 1963 and designed for 0.012-0.018-inch clearance with 1.5:1 rocker arms. But in 1963 the intake clearance was revised to 0.008-inch and this change was also recommended in Corvette News in the late 1950's for "weekend competition events"; the tighter intake clearance gave a bit more effective inlet duration. Factoring the original 0.012-0.018-inch by the 1.37/1.50 rocker ratio correction yields 0.01096-0.01644-inch, call it 0.011-0.016-inch (hot), so if you're already running the 0.008-inch intake clearance it shouldn't be tightened any further. We recommend a (cold) setting of 0.010-inch intake and 0.015-inch exhaust for 283s, and 0.008-inch intake and 0.015-inch exhaust for 327s. A complete crank angle/cam lift diagram in Corvette News, Vol.8, No.4, indicates that neither the exhaust nor the inlet is on the clearance ramp at TDC of the firing stroke, so you can use the indexing procedure outlined earlier, or it's also OK to adjust both the inlet and exhaust at TDC #1 and TDC #6 as outlined in the 1963 Corvette Shop Manual with the "097" Duntov cam; it has much shorter ramps than the "30-30" or the LT-1 cam.

Early-70's "LT-1" Solid-Lifter Cam

What about the later early-70's "LT-1" solid-lifter cam? In the case of the LT-1, it's OK to adjust the intakes at TDC (its closing ramp ends about 10 degrees BTDC, so it's off the ramp at TDC), but the exhaust is still on its ramp at TDC, so the exhausts should be set at 90 degrees BTDC using the indexing procedure. Both intake and exhaust recommended clearances should be factored down by the 1.37/1.50 rocker ratio correction here as well, from the factory spec of 0.024-0.030-inch (hot) to a (cold) setting of 0.021-inch for the intakes and 0.026-inch for the exhausts.

Using This Procedure on Big Blocks or With Other SB Rocker Arms

While this 8-step indexing procedure for adjusting the lifter on the base circle of the cam can be applied - in principle - to any solid-lifter cam, the actual lash dimension required to implement the procedure varies with the height and length of the cam clearance ramps and the as-measured, functional rocker arm ratio (not the nominal value).

The lash dimensions published with this procedure only apply to the small-block engine with above noted cams and stock, stamped rockers; do not use the lash dimensions in this Research Report on any other motor/cam/rocker combination.

Any aftermarket SB rocker or any BB rocker will require the same type of rocker arm ratio measurements to be completed. This is normally done by creating a crank angle/cam lift diagram using dial indicators and a degree wheel. However, an alternate method of obtaining the required lash dimension for a given solid-lifter setup is to accurately hot-lash the valves to the cam manufacturer's specifications, allow the motor to cool, immediately re-measure the lash while cold, then use that cold lash measurement for subsequent cold lash setting using the 8-step indexing procedure.

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