S1 Publishing(Oxford)©2007

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

 
What is the big end?
The big end is where the connecting rod fits over the shell and the grank journal. there is one for every pots you have.
Top, con rod and big end shells

Below, a crank with two of the crank journals showing


What do I look for if I think you have blown a big end?
If you have heard a loud knocking from your engine when under load (accelerating) it is likely to be a big end to have blown, 
 If you try to ride or run the engine after that sort of noise, you will make things worse. the knock is the con rod smashing up and down firstly into the shells, then into the crank itself if you are unlucky.
 You will have to take off the head and barrels to check these (this), but first check your sump, if you find phospher bronze shards in the sump, it is a good indicator. 
Grabbing the con rod, and trying to move it up and down, if there is any movement, you'll know for sure.
 To check the crank for damage you can use a vernier guage or a micrometer, set it to the suspected journal and turn it around if it get's stiff, or if it gets loose, then the crank can be saved with a re grind and new shells, or you can replace the crank, and shells, but sometimes it can be easier to replace the whole engine. Click on the photo if you want more information or check the journals page in the engine section

You can do this before you split the crank cases.
If the con rod is stiff to move on the crank, when you move it backwards and forward on the crank, it is a sign of a semi heat seize. If you run the machine with that will, it won't be long before the shells disentegrate, and the big end journal will get damaged.
 New shells will sort that, but don't forget, you'll have to run in the engine again until the new shells settle in (about 500miles), replace the whole set of shells if you have more than one.

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
©