Technical Information (MGB 26)
Last modified - 04/15/06
There are two different lengths of pushrod fitted to the MGB engine and, there is no confusion as to mistaking one from the other.
I would presume that most MGB owners who are rebuilding their engines will be using the longer version pushrod with the matching short lifter. These short style lifters (part 460-605) can be found in all 18V engines. This combination can be fitted to all MGB and MGA engines.
The most important questions that a potential engine rebuilder will want to ask are: Should I update to a stronger pushrod? Will the original OEM length work for me? Well, let's discuss second question first.
What you see when you are looking at a pushrod is its overall length. What you can not see is its effective length. Finding the effective l ength (EL) requires that we do one of two things. First, place a 11/32” ball bearing within the cup end and measure the entire length of the pushrod with the ball bearing in position, subtract the ball bearing diameter from this total. Alternatively, measure the entire pushrod length, from this total we subtract the adjuster ball cup depth.
The 11/32” ball bearing measurement method substitutes for the rocker arm adjusting screw, which, has the same diameter ball for alignment within the pushrod cup.
During the engine rebuild process items that contribute towards having to decrease the required pushrod effective length are as follows:
Items that contribute towards having to increase the effective pushrod length are as follows:
Therefore, if upon final assembly, you are unable to adjust your valves without running out of adjustment screw thread, one of two situations will need to be rectified.
Over the years we have measured many of these longer style pushrods and, we have found small variations in their effective length from pushrod to pushrod, which, I guess, depends upon the particular manufacturer. The effective length should be approximately 10.375”.
If you need to increase the pushrod length then you have 2 practical choices. These are as follows:
Since, these longer rocker arm adjustment screws are as expensive as the 5/16” tubular pushrods, it makes more sense to install the longer pushrods.
If you need to decrease the pushrod length then you have one practical choice. And, that is, install shorter made to order 5/16” tubular pushrods.
It is practically impossible to determine the exact length of pushrod you are going to need until the final stages of the engine rebuild process. We will supply you with the OEM length, should you be unable to adjust your valves to the correct clearance, without running out of thread or running short of thread, return the entire set back to us and we will supply you with a longer or shorter pushrod set. We will not accept returned pushrods that shows signs of being run in the engine.
Pushrod stiffness is defined by its cross sectional area, strength and rigidity.
Pushrod materials commonly used are:
Pushrods are mainly subject to compression loads. The ideal position would thus be vertical. However, bending moments, to some degree, are applied as this angle changes.
On the MGB engine, with any particular exhaust or inlet valve closed, you will notice that the pushrod is not perpendicular to the center line of the rocker arm adjusting screw ball. This inward lean contributes to pushrod deflection which, increases as the valve opens. This is especially true of the OEM pushrod when using high valve spring #s and or when operating at high engine RPMS. The use of higher rocker arm ratios, such as roller rockers, only compounds this problem further.
The diameter of the OEM pushrod is approximately 0.280” with a MOSZ (modulus of section Z) of 0.00215in 3.
The diameters of Moss Motors Special Tuning 460-616 are 0.314”OD & 0.189”ID with a MOSZ of .00264in 3.
The strength of the pushrod lies in it’s material, length and cross sectional area.
When using 5/16” tubular pushrods interference with the cylinder head pushrod guide tube holes must be avoided. Enlarging or elongating these holes will be necessary, especially when using higher than stock OEM rocker arm ratios. We prefer to bore out these guide tube holes to 0.660”, starting from the underside of the cylinder head. However, this may not be practical on cylinder heads with modified (enlarged) intake ports. In this case elongate (towards the engine center line) these guide tube holes.
If you chose to ignore these recommendations and install the longer lifter and shorter pushrod combination be aware that lifter failure rates with this combination are more pronounced due to increased pushrod angles which result in greater lifter side loads. In our opinion this component pairing, combined with a heavier lifter weight, may be a contributing reason for higher lifter failure rates.