Re: housing bore for main bearings on 7th series 8

Posted by 32model901 On 2010/8/12 18:06:41

Some additions comments:

If possible find out if the mains in the crankcase were line bored while it was bolted to the block.

To do the line boring operation correctly the crankcase must be bolted securely to the block. If it is not, there is a good chance the alignment of the mains will not be on the same axis when it is attached to the block. End result, a tight crank and improper or non-existent oil film, which will destroy the babbitt bearings quickly.

Tightening the crankcase to the block will distort the crankcase slightly, this is why it must be bored with the block attached.

I've seen work where the crankcase was line bored without the block attached and the engine re-builder attempted to correct a tight crank by selective tightening of the block to crankcase nuts, trying to bow the crankcase to correct improper main bearing alignment.

If the line boring operation was performed with the block attached I would attach the block securely to the crankcase before checking clearance in the mains with Plastigauge.

If the crankshaft has been stored horizontal for two years, check it for straightness before you check clearance in the mains. You should always store a crankshaft vertical, if it is stored horizontal for a length of time it could have bowed.

Regards,

Dave