COMP Cams 4120 Gear Drive System User Manual

COMP Cams

®

Gear Drive System

#4120 Ford Small Block

Thank you for choosing COMP Cams® products; we are proud to be your manufacturer of choice.
Please read this instruction sheet carefully before beginning installation, and also take a moment to
review the included limited warranty information.

Notes:

1. The installation procedures which follow assume that the engine has been disassembled and

cleaned as it would be for a normal timing set change.

2. The reverse idler gear should have between .005" and .070" in vertical travel when the set is

installed on the engine with the power idler in hard mesh between the crank and cam gears. If
this is not the case excessive, heat can be generated causing damage to the gear set.

3. The purpose of the reverse idler gear is to prevent the power idler from disengaging when the

crankshaft rotates backwards as happens in shutting off the engine.

4. The engine is timed with the timing marks facing one another just as you would with a chain

and sprocket drive.

5. Idler pins will always be in contact with the timing cover during use. Back of idler pins must

have a minimum of .015" clearance to block.

6. On small block engines the rib on the inside of the timing cover must be removed.

7. The Dual Idler Gear Drive is intended for use with a harmonic balancer. It is not for use with

an aluminum hub.

Installation Instructions:

1. With a dab of grease install bronze wear ring, included in kit on the back of cam gear.

2. Install cam gear on engine and install fuel pump eccentric and bolt. We recommend a one

piece eccentric but a two piece eccentric if a spacer is placed behind the eccentric so that the
outer ring can spin freely. Torque camshaft bolt to factory spec. Late model small blocks use
a .251” thrust plate. Early models use a .217" thrust plate. 351C engines use a .156" thrust
plate.

COMP Cams®

3406 Democrat Rd.

Memphis, TN 38118

Phone: [phone] Fax: [phone]

www.compcams.com

INSTRUCTIONS

Part #118

Revised 5/31/13

3. Install crank gear on engine. You can advance or retard the camshaft timing ±2 cam degrees

from factory setting by using the advanced keyway (triangle keyway and timing mark) or the
retarded keyway (rectangle keyway and timing mark).

4. With the #1 piston at TDC line up timing marks on engine centerline as shown in the figure

below, slide idler gears in place with large gear on the passenger side of engine. On 351W and
late model 302 engines the long pin in the idler assembly must be changed with the shorter pin
supplied in the set. On some 351M and 400M blocks most of long pin must be ground off to
allow proper clearance under the timing cover.

5. With the idler gears halfway on push timing, cover with gasket into place. Remove the cover.

The gears should be aligned at least 90% with the cam and crank gear. Use a feeler gage to
check clearance between block and back of idler pins. A minimum of .015" to .060" is
required. If there is not sufficient clearance, remove the idler assembly and grind off material
from the end of the idler pin closest to the block. Repeat step 3 until correct clearance is
achieved.

6. With the idler assembly now in place, turn the crankshaft clockwise so that the power idler

(large gear) is in hard mesh with cam and crank gear. There must be .005"-.070" vertical travel
of the reverse idler (small gear). Replace the idler bracket, install the timing cover.

COMP Cams®

3406 Democrat Rd.

Memphis, TN 38118

Phone: [phone] Fax: [phone]

www.compcams.com

Part #118

Revised 5/31/13