Fitting an overdrive gearbox to an AC Aceca Bristol - gearbox tunnel width

[Glen Smytheman]

Hi all from NZ,

I am pleased to say that my new AC Aceca Bristol chassis BE655 is making great progress towards getting its Warrant of Fitness so it can be driven on the roads in NZ.  This weekend I am picking up one of the Derek Hughes overdrive units (new and still in its box - you would be amazed what bits find their way down under) with a view to replacing the non-overdrive tailshaft with this overdrive one.  A Derek Hughes overdrive uses existing gearbox mounts so no need to alter the gearbox mounts as I understand would be the case if you were fitting a Bristol 405 type overdrive gearbox (BWCR12). 

My question is whether in fitting an overdrive gearbox where a non overdrive was previously fitted whether the fibreglass gearbox tunnel needs to be widened where the overdrive unit bolts on.  Any information greatly appreciated.  Many thanks, Glen Smytheman

[Flyinghorse]

Glen,
I fitted a Derek Hughes's  o/d kit to my Bristol 400.
I had to get a fibreglass tunnel from Mike Robinson at the time.
Speaking to others recently you could also fabricate one in alloy.
I have the fitting book somewhere in case you did not get it with the kit you found.
With the 400 the gear stick just comes straight up. On the Ace I'm not sure if you have a remote.
Either way I would expect having to fabricate something.
Graham

[Glen Smytheman]

Hi Graham,

Many thanks for your response below.  I have delayed the collection of the overdrive unit a few days as I ran out of time before Christmas - I think the unit that I am looking at is still in the original box but I will certainly reach out if the instructions are now in the box.  Wishing you all the best for 2026.  Glen