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The primary function of the valve gear is to open and close the valves as close as possible to the intended contour dictated by camshaft design. To achieve this through a broad engine speed range rigidity, lightweight components and minimised friction are the target.

In recent years the majority of manufacturers have swung to overhead camshaft (O.H.C.) designs to operate the valves.
Due to pushrods, rockers etc. being eliminated, more efficient cam profiles with higher rates of valve acceleration can be utilised, as this design minimises flex and friction together with less weight of reciprocal components.
The valve opening and closing cycle more closely follows the intended cam contour, resulting in quieter, more efficient and smoother operation.

The BMC Range of ‘A’, ‘B’ and ‘C’ Series engines can benefit from adopting the same principles in the valve train.
Why use roller rockers?

During valve opening and closing cycles the conventional rocker arm ’pushes’ and ‘pulls’ across the valve stem.
These forces are transferred into the valve guide, wearing the stem, guide and ultimately the valve seat with consequent power loss. Evidence can be seen by inspecting used components.
For example check side play on a worn valve/guide - there’s always more wear on these components in line with the rocker arm.

A worn rocker shaft and/or rocker bushes allow more oil to be deposited at the top of the engine, thereby placing more emphasis on the valve stem oil seals to control this unnecessary oil flow.
If this oil is allowed down the valve guides, it contaminates the incoming fuel/air mix.
This contamination lowers the octane level of the charge and reduces the amount of compression that could otherwise be used.
Needle roller bearings in the roller rockers cut friction and require a minimal amount of lubricant, hence the restricted oil flow to the top of the engine is possible.
Why change roller ratio?

By increasing the rocker ratio, the cam follower and pushrod do not have to move as far or as fast (for a given valve lift) compared to a lower ratio rocker.
This is beneficial because the main reciprocating mass in the valve train is usually the camfollower and pushrod.

The advertised ratio on the ’B’ Series engines is 1.4:1 (or more precisely 1.426:1).

However, after numerous tests carried out on engines ranging from MKI’s to the latest 18V - prefix engines, none appear to come up to this ratio. In fact, the majority have measured between 1.327:1 to 1.367:1.

Many hours can be spent in optimising rocker assemblies to gain a ratio closer to the specified goal. Try and try again with different assemblies seems the only way with standard components.


To put, what appears to be, such a small discrepancy into perspective the following figures should assist:

Valve lift can be calculated by multiplying cam lift by the rocker ratio minus valve clearance.

Eg. Standard 1800 ‘S’/MG’B’ cam

Cam lift .250

Clearance .015

Ratio 1.426:1
(.250 x 1.426) - .015" = .3415" Valve Lift

Let’s see what happens! If we take 2 well-known cam grinds, the 1800 ‘S’ (MG’B’) and the popular Camshaft Grind # 113 and work with the various rocker ratio’s in the following chart:
Valve Lift at rocker ratio (incl. changes to valve clearance)
Camshaft Cam lift A B C D
MGB/1800 .250" .3178" .3415" .3592" .3831"
Grind 113 .265" .3376" .3629" .3817" .4071"

Rocker ratio in column A 1.327 :1 Not unusual
B 1.426 :1 Std. spec.
C 1.5:1 Roller rockers
D 1.6 :1 Roller rockers

From the above chart we can see that with the ‘wrong’ rocker gear set-up even the Grind 113 is not able to achieve the lift of a standard cam with the correct geometry - it gets worse!
A ‘loss’ of .026" of valve lift translates into a sizeable loss of airflow potential through the valve ‘window’ - the open area around the circumference of the valve.

Using the above example, we will transpose these figures into the typical MKII ‘B’ Series inlet valve.

Valve Dia. Ratio Valve Lift Area @ full lift Equiv. to Valve Dia.*
1.625" A .3376" 1.724 sq. in.
1.625" B .3629" 1.853 sq. in.
1.625" C .3817" 1.949 sq. in.
1.625" D .4071" 2.079 sq. in.

* at factory spec. 1.426 :1 rocker ratio

Most tuners would agree it would be time well spent to achieve this sort of potential improvement.
From the above comparison it is evident that by using a rocker ratio of 1.327:1 the valve dia. is effectively reduced from 1.625" to 1.507".
Conversely, by fitting 1.5:1 ratio rockers the same 1.625" dia. valve has the same open area as a valve of 1.712" dia.

This extra flow potential is accomplished without the shrouding effect (lost flow potential) usually associated with the fitting of larger diameter valves.

HP Hi-Flow Yella Terra Rocker Pic Image copyright (c) 2011.