5 replies to this topic

[ace01]

The title doesn't really explain the question fully;  So I want to change the diff on my car from it's current 3.9 to a 3.4 and it's in a rod box.

 

I compete in hillclimbs and sprints and have used the 3.9 for a few years and it works well.  I would like to compete at a couple of circuits where a 3.9 would mean using too many rev and therefore I would like to change the diff to something better suited. 

 

In the past I have used a remote box and locking the gears to change the pinion is reasonably straight forward.  However I have been using a rod box in my car for the last couple of years and have no idea if I can lock the box to enable me change the pinion without splitting the unit. 

 

I have rebuilt both types, but am more familiar with the remote type.  Anyone know how?

[Midas Mk1]

Yeah, easy to do, I use two old pot joints with bolts through them to lock the diff, then with the end case of the gearbox off can easily take the pinion nut off with my 1/2" Milwaukee.

Refit alright to do, with the weight of the engine on the box you can torque the pinion nut no bother on your own too. 

(Obviously engine / box out of the car)

[Cooperman]

I once tried a 3.44 in an 'S' with a SCCR gearbox for rallying with a 544 cam and the amount of clutch slipping I had to use to get it off the line on tests was huge, especially on uphill starts. After a couple of rallies, I fitted a 3.9 and it's been fine and I set my rev limit at 7200 rpm which is c.100 mph. When I took it to pieces I was surprised at the amount of 'clutch dust' in the housing! It wouldn't have lasted long.

However, I have another rally Cooper 1275 (a 1990 car) which has standard gear ratios and I do use a 3.44 diff.

I hope this helps and good luck with the competitions.

[Ethel]

Yes you can select 2 gears without stripping more than you need to get the interlock spool out - the diff cover. I like Midas's idea though.

[Spider]

I loosen the Pinion Nut by locking in 2 gears. When the diff is out, there's the ball, spring and steel tube on the lower RH side of the diff for the primary detent. Removing these parts allows you to rotate the Interlock Spool, this also removes the Selector from the Bell-crank Forks, then with a small bent tool, you can manipulate the Bell-crank Forks to select 1st and 4th. The top Fork is for 1st & 2nd, the middle is for 3rd and 4th.

 

I usually start off by engaging 4th, that Fork needs to be pushed in to the Gearbox, then select 1st by pulling the top fork out, the gear train might need rotating a little while doing the latter to allow the dogs to line up.

 

This pic show the Interlock Spool and Selector rotated and disengaged;-

 

[ace01]

Thank you everyone, and especially Spider, for the information and pictures.