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.
'How to set differential backlash' have 5 comments
May 15, 2012 @ 6:32 am 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.
November 8, 2023 @ 7:57 am James C Capehart
Harry, I wish you were still with us, miss my life long friend.
January 13, 2017 @ 8:13 am Larry Mosteller
If one had no in lb torque wrench one would not be able to gauge the setting of the collapsible spacer as your instructions do not state how long of a breaker bar one should use. ” 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.” So how long should it be for your 1lb weight?
January 13, 2017 @ 8:18 am Larry Mosteller
BTW, Harry, Great article I heeded this information to rework my differential and could not have set up the thing properly without it. THANKS.
October 19, 2022 @ 1:59 am David Home
I have some issues with this article, firstly, using original pinion shim is a big assumpsion that a new cw & p is exactly the same as a bl part from years ago, not likely. So you should set pinion depth again and fit whatever shim is required for correct contact. Secondly, a breaker bar and 1 pound weight? How long is bar? How heavy is bar? Best use a low ft/lb torque wrench.
Backlash on new gears should really be between 0.003 and 0.005″ to allow oil to stay between gear faces. 0 007 to 0.010 will work but not ideal. And i just do not get why shims are no longer available, they are so easy to manufacture?