Consequences of little end alignment faults
Alignment should be within 0.001” measured both sides of the rod little end, in two axis to check both parallelism and twist.
Rod with “blue” little end measured
Little end has tried to run at an angle and will wag sideways in use along the gudgeon pin.
The rod big end bearing has been rocking sideways in use making the rollers barrel shaped and wearing the rod track convex.
Rod with “blue” big end measured
Twist causes the piston to try to travel up and down its bore like a corkscrew.
In trying to do this alternating right hand then left hand helical action, it is reacting against the big end bearing.
The big end is being subjected to an alternating twisting load which causes heavy friction between the rod sides and the big end roller plates.
Result, power loss, rapid wear to sides of rod big end, heat tempering and softening the hardness of the BE track. Convex track, blue and barrel shaped rollers.
A blue appearance denotes heavy friction due to inaccuracy.
Blue at the little end denotes errors in parallelism in the little end bush
Blue at the big end denotes errors in twist in the little end bush
Blue both ends means you have the set! Lucky You!
There are some who have had circlips come adrift and score the bore. They blame the safety of using circlips.
This is not correct.
To displace a circlip requires very significant sideways force by the gudgeon pin against the circlip.
If there is an error in parallelism of the little end bush when using circlips, then the considerable side forces generated can displace the circlip.
Just consider the heat generated to make the little end go blue!
If you look at the traditional Scott gudgeon pin with end pads, this will stand more abuse of this kind, but the wear on the pads and bore will clearly indicate that errors are present.
The alignment of the rod little end bush is perhaps the most critical single item in the engine.
Errors in this area account for more distruction than all other potential problems combined.
Where it is within the financial comfort of the customer, we recommend the shimming of the rod little end within the piston.
This, together with accurate boring of the little end bush, will ensure that the rod stands vertically controlled at all times.
If you want the best, then fit cages in the big end bearing and raise the durability and life expectancy of the engine to new levels.
Inaccuracy causes rapid wear, loss of power and smoothness.
This engine had done only 1000 miles since being reconditioned
Please note that the photo was taken AFTER big end bearing tracks had been restored by honing.