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Crankshaft Replacements

9E194: Crankshaft

In the current market, there are several channels for the supply of new crankshafts for Rileys. A visitor to Beaulieu Autojumble these days will see displayed new cranks for 9hp, 12/4 and even 6 cylinder models.

RRSL currently stocks nitrided crankshafts made in EN40 steel for all the 9hp models, except for the Merlin that is well served by the outside trade. The RRSL items are intended for use as direct replacements for the originals with the correct crankpin sizes and nose pieces for the various marks of 9hp engine used over the years. They are intended to be used with white metal bearings and if the owner wishes original con rods. If original rods are being used it is however always a good idea to renew the bolts and nuts which the RRSL also have available and of course we all know that old split pins should never be reused!

If the owner wishes to keep a pre Merlin 9hp engine original but would like to use new con rods as well as a new RRSL crank then Messrs Formall’s have advised us that they are more than willing to manufacture and white metal these for our members as needed.

We feel that Register members will find a comparison of the various options useful.

(white metalled rods)
(shell bearing rods)
(trade supplied)
Fully interchangeable Yes No See note
Nitrided for increased durability Yes Yes Normally
Available from stock Yes Yes See note
Oil filter recommended No Yes See note

[Note: The outcome here will depend on what is specified by the supplier.]

Oil & Oil Filters

All the 9hp engines prior to the Merlin had no oil filtering other than gauze in the sump to catch larger debris. They should use an oil designed for non-filtered engines as supplied by Penrite and others. These oils do not hold contaminates in suspension as modern oils do so the contaminates will settle out in the bottom of the sump which needs to be removed and cleaned out every so often.

In the case of the Merlin 9hp, an excellent Tecalamit felt oil filter factory fitted, so a new trade sourced crankshaft C/W new rods and shell bearings can be used. With these filtered engines a good 20/50 multigrade oils as supplied for classic cars from the 60s would be a good option when starting with a clean rebuilt unit. If original crank or rods are being re-used it is always advisable to get them crack tested and if you are regrinding an original crank ensure that good radii are maintained at the big end journals.

We are not currently aware of any suitable shells for fitment to early 9hp big ends. If suitable shells are found it is recommended that the conversion should include going to full flow oil filtration, as unlike white metal, shell bearings cannot absorb debris that continues around the engine in the oil until filtered out. Fitting a suitable filter can be a bit of an awkward job to do on a 9hp engine.


There is another aspect of crankshaft design that often crops up, the subject of crankshaft balance. When you look at the range of crankshaft configurations for Riley cars and indeed other makes of engine, you will notice marked variations in the size and shape of the counter balance weights. The reasoning behind this is that with a four-cylinder in-line engine it is not possible to achieve perfect balance. Therefore, every designer chooses what he or she believes to be the best compromise.

Crankshaft balance is a complex subject but, in this note, we will try to provide a simple view and beg leniency from the experts. There are two elements to balance that designers consider, Primary and Secondary balance.

Primary Balance

Primary balance is all about trying to get the best compromise on balancing the rotating and reciprocating masses in the engine. In simple terms when an engine is running the crankshaft, big end and part of the connecting rod can be considered as rotating and the piston, little end and the rest of the con rod can be considered as reciprocating up and down the bore.

It is relatively simple matter to balance the rotating masses; the more complex element is the degree of balance the designer looks to achieve with the reciprocating masses. If we consider the piston, little end etc, for each revolution of the engine the pistons reverse direction twice and similarly twice a revolution the pistons are accelerated in the bore to a maximum piston speed and then slowed down again ready to reverse direction. This all requires changing vertical forces within the engine. These vertical forces on the piston assembly are also influenced by what stroke the piston is on: firing, compression, exhaust etc. Therefore, what crankshaft designers do is to decide the size of counter balance weights they are going to add to the crankshaft to partially balance out these varying vertical forces.

Now to our Riley car crankshafts, the early 9hp shafts were counterbalanced within themselves, so addressed the rotating forces. The later Riley cranks had further counterbalance weights added to reduce the reciprocating out of balance. The RRSL crankshafts are copies of the original Riley product but use a better material and are Nitride hardened. Replacement crankshafts from other suppliers will often exhibit alternative forms of counterbalance weights as that supplier’s view of a better option for Primary balance for their product.

RRSL crankshafts are dynamically balanced as supplied but this does not necessarily mean that all the rotating mass of the engine will be in balance. For best results, a specialist company should balance the crankshaft, flywheel and clutch components as an assembly. These same specialists will also be able to check that the con rods are a good match for each other.

Secondary Balance

Secondary out of balance results from the piston not moving in simple harmonic motion. With the engine running at a constant speed the varying angularity of the connecting rod produces different displacements of the piston for a given angle of rotation of the crankshaft rotation. The shorter the con-rod the more marked these variations are. Secondary out of balance can only be addressed by the addition of secondary balancers. These comprise two counter rotating shafts running at twice engine speed and they were the brainchild of Frederick Lanchester back in 1904. The Porsche 944 is one vehicle that uses this option of reducing vibration, as do some modern larger capacity 4 cylinder engines.

Some Riley engines have a damper to combat inherent crankshaft torsional vibration. Examples of these are the friction type as seen on Riley 6s and the post war Metalastik rubber insert dampers that were normally part of the crank pulley.