How to Use Valve Lift to Determine a Cam’s Base-Circle & Find the Right Pushrod Length for a Chevy LS (or Any Engine)

Longer pushrods are required with non-adjustable rockers when lift is increased. Why?

To increase lift, we need to reduce the base circle and the lifter rides lower in the lifter bore. When this happens, the standard 7.385 pushrod length will not give the lifter the desired .055 to .080 lifter preload (or one turn of the rocker bolt) we target. For this reason, we need longer pushrods to maintain preload.

Summit Racing always recommends using a good pushrod length checker (in case heads have been milled, etc.), but we can make a good approximation. 

Here are a few givens for a Chevy LS engine. (If you have a different engine, you will need the cam journal diameter and standard pushrod lengths.)

1. LS engines have a 2.165 in. cam journal diameter. Half of this equals a cam centerline to journal distance of 1.082 in. 
2. The nose of most cam lobes is usually .010 below the cam journal o.d. This allows the cam to slide in without interruption. 
3. 1.082 – .010 = 1.072 in. This is the distance from cam-centerline-to-lobe-peak of almost any aftermarket LS cam. Knowing this and lobe lift, we can estimate pushrod length. 
4. 1.072 – lobe lift = Cam Base Circle radius.

If a 7.385 in. standard pushrod has the right preload with a .550 lift LS6 cam, how much longer pushrod would you need with a .625 lift performance cam?

First, we need the lobe lift numbers. Here are a couple examples:

• LS6 Lobe lift = .551 lift / 1.7 rocker ratio = .325 lobe lift
• Performance cam lift = .625 lift / 1.7 rocker ratio = .367 lobe lift

Next, we subtract our lobe lifts from our 1.072 cam-centerline-to-lobe-peak to determine base circle radius.

1. 1.072 (centerline to lobe peak) – .325 lobe lift = .747 in. (LS6 base circle radius)
2. 1.072 (centerline to lobe peak) – .367 lobe lift = .705 in. (Performance cam base circle radius).

Next, we subtract the smaller base circle number from the bigger one.

•  .747 – .705 = .042 in. (This is the additional pushrod length required)

Finally, we add this number to our standard pushrod length to determine a new theoretical length.

• 7.385 in. (standard) + .042 in. = 7.427 in. (new theoretical pushrod length).

In this case, a 7.425 length Summit Racing HDR pushrods part number SUM-14917425 would be nearly perfect. Its 11/32 diameter and .120 wall thickness interchanges with the thin-wall factory 5/16 pushrods and is 23% stronger as a bonus.

• Note: If you have a cam with a lower lift exhaust lobe, the pushrod might be a little too long on that side theoretically. This usually does not present a problem with .025 in. or less lift-splits. A lifter with factory plunger travel has more oil compressed out with a slightly long pushrod. This reduces aeration and delays valve float.
• Another Note: If you mill the heads .030, that is a 1:1 relationship to pushrod length. You will not find a .030 shorter pushrod, but you will find a .025 shorter pushrod. (7.400 in the example above.)

Although pushrods are typically found in .050 increments, Summit Racing often has .025 increment pushrods available. If you are having a problem finding them, just give Summit Racing a call and they will help you find something suitable. 

Bonus Track: On a LS, 11/32 pushrods fit in place of O.E. 5/16 without clearance issues. They are far stronger due to the added diameter and will increase rpm through added valvetrain stability.