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How to set differential backlash

This tech tip will speak specifically to setting up the differential, and specifically the back lash, in a Midget converting to 3.55:1 ring and pinion but can be used as a general guide for all differential set ups.

The part number should be STR520 which is a British Leyland special tuning part number. Originally the 3.55:1 were produced as limited run and not fitted to any standard road cars.

The original crown wheel would of had the backlash figure etched into the back of it. I have noticed that the latest reproduction CW&P’s (Crown Wheel & Pinion) do not have any set up information, either in the box or etched onto the parts. As a result, I have tended to set the backlash at between 0.007” and 0.009” on all differentials built with these CW&P.

I do not know what the figure BL10/11 means but I would assume the BL stands for backlash. If it does, then 0.10” seems quite excessive. I have only seen one or two original crown wheels etched 0.010” and these were on very early Sprites running a 4.55:1 ratio.

In order to set up the diff properly, you need to set the pin first. The way to do this with a new CW&P is to use the same spacer shim which is sandwiched between the bearing and pinion head. The pinion head height would of been originally set by the factory and the size of the shim would been determined at that point. Plus these shims are no longer available new. As the pinion uses a collapsible spacer, the nut should be tightened to the torque set out in the manual. Once tightened, there should be sufficient preload on the 2 bearings to enable you to put a breaker bar on the nut and hang a 1lb weight off the end before it starts to move the flange. Always use a bit of oil on the bearings before fitting as this will enable an accurate set up. When the bearings are dry, they tend to not want to rotate freely but when fitted and full of oil, the bearings lose their preload quickly.

The carrier can then be fitted complete with crown wheel and the bearing shimmed up to set the backlash. It is best to use engineers blue on the teeth to show where the teeth are engaging. The contact pattern should be as – (a) tooth contact – in the attached.

The proper tool for setting up is no longer available.

Comments (1)

  • Harry Mac Lean

    If you have an inch/pound torque wrench this can be set by keeping the drag on the bearings to 10-15 inch/pounds and not more. On used bearings it should no higher than 5 inch/pounds. You can also use paint instead of blueing compound to set up gear lash. Gear lash on these can be from .004-.012 BUT .007-.008 IS BEST.

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