Taking a grinder to Britain's motorcycling heritage.
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Wednesday, 11 August 2010

Here we go again

Quote from Draganfly web site ;

Pre-unit A Group When rebuilding the bottom end of an BSA twin or other engine with a plain timing side bearing we strongly recommend that the bushes are fitted to the crankcases and then 'line bored'. This will ensure that they are in perfect alignment with the drive side main bearings, resulting in a perfect bottom end that will last many thousands of miles. Most automotive machine shops should be able to do this for you. Our timing side bushes are manufactured in the UK to a high standard and are made slightly undersize for finish machining once fitted.

The Timing Side bush has long been considered the Achille's Heel of the BSA twin range. There is a conversion that was pioneered by Devimead in Tamworth, and now SRM in Wales. This conversion is well known and consists of machining the cases to accept a roller bearing, and altering the crank to accept an end feed to the big ends. This conversion is good for all twins, except the early long stroke motors, you know just like the one fitted in the A7.
The condition and running fit of this bush to the crank journal is critical as the oil feed to the big ends is transferred through this interface. Too much play and the oil pisses out around the end of the crank rather than taking the more difficult route of forcing it's way past the big end shells.
Because of this and the lack of precision boring equipment in Johnny's garage, the bottom end will be taken to Draganfly for a complete overhaul of the bottom end. Speaking to them today revealed a turnaround time of 8 weeks!.....sounds familiar.
There's a spare set of crankcases under the bench, so they will be pressed into service for a while to finish off jobs that require the engine to be in situ.
It would be nice to think that in the eight or so week interim many of the rolling chassis jobs will be completed,........... but don't hold your breath.

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