S1 Publishing(Oxford)©2007

World of motorcycleS
S1 Publishing(Oxford)© 1999-07
 Maintenance - Engines
Cranks. Big-end Journals

What is a big-end or mains journal?
In the white box you can see the mains journal, it runs central in the crank. Their are two mains shells which are contained by push fit in the two crank cases. The one in the photo is in good condition

In the red box you can see the big-end journals

In the blue box you can see the oilway, this supplies lubrication between the shells and journals, if it gets blocked, your crank with heat seize.

Note: this crank comes from a ZX10a1 which has the crank cases top and bottom split. Bearings from engines which have a sideways crank case split are differrent, I'll do a page for those when I get time.

What do I look for in a big-end or mains journal?
 You can see the big-end journal,  has some scoring on it, and when I ran my fingernail accress it it felt ridged, that is a bad sign it means a re grind is neccessary
A quick check with a vernier guage will let you know if the journal has gone ovoid, instead of it being perfectly circular.
 Here I placed the teeth of the guage over the journal and tightened the brass screw making sure ther was no gap between them
How far it has gone ovoid let's you know if you have it to be just re ground and thicker or oversize shells, or if you have to add weld then have it re-ground and new shells. Your engineer will advise you on which. 
When I turned the guage by 90degrees, I found a gap, a sure sign that the journal had gone ovoid.
Adding weld is ok to a point, but cranks like this are forged in one piece, which is better, the chances of a welded jounal failing is quite high.

have found a person who reverse engineer car engine cranks and drop forges them, whether or not he'll do a bike crank of not I'll try to find out.

A re-grind is a little cheaper than buying a second hand engine sometimes, if you do the work yourself that is. 

This is just a general guide.
If you are about to do your own maintenance, using these pages, I take no responsibility if you get things wrong!
Modified for 2009