The low down on diffs - Blackbourn 421

By: Rob Blackbourn

Date: 01.03.2019

What's going on down the back with your car's diff? Sometimes the answer is one you definitely didn't want to hear

Mick McCrudden’s expert advice in Unique Cars is a must-read for me. The specialist tips you get from an experienced front-line automotive engineer like Mick are great value. His ‘Diff Jockey’ article in issue #420 was a good example. Who knew that grease can be a practical substitute for bearing blue when you’re checking the engagement of pinion and crown-wheel teeth? Makes sense once you know.

If I’m honest though, I’ve generally left finessing diff-centre set-ups to experts like Mick. I’ve swapped a few, mind – only modest skills are needed to pull a diff-centre from the banjo housing before bolting in a fresh unit.

One episode that did get me on intimate terms with diff set-up subtleties was during my troubled relationship with a classic 1929 Austin 7 Meteor. Forget shims and bearing-blue (or grease for that matter), the baby Austin had externally accessible adjusters in the axle-housings each side of the banjo-housing that enabled you to move the crown-wheel assembly laterally, to either load up or back off its engagement with the rigidly supported pinion. The good book’s message was simple: If the diff whines under load, back it off a whisker. If it whines on the over-run, wind it in a bit. Actually the task of finding the sweet spot was far from being that simple – it involved numerous stops along the road where you wriggled under the little Austin, tools in hand, to have another suck-it-and-see go. Thankfully I got there in the end.

With electronic stability and traction-control systems now the norm in mainstream motoring, the once sexy limited-slip diffs and lockers have been pushed toward the margins; these days it’s mainly motorsport folk and serious 4x4 off-roaders who use them. Similarly the basic trick of welding diff spider-gears, not that uncommon a while back, has been almost consigned to the dustbin of car-fettling history. It was an inexpensive DIY-way to consistently get the power down via both back wheels. It had its shortcomings though when used on the road – I recall mates upsetting servo operators by leaving black rubber tracks where they pulled U-turns around the pumps, and occasionally breaking half-shafts, and having to fight for steering response in some situations against the diff’s efforts to push the car straight ahead.

My most memorable diff-centre swap came during a quick HR Holden trip from Melbourne to a Gold Coast wedding. Just before dawn on wedding-day Saturday, not far south of Maclean on the Pacific Highway, a pinion bearing that had been humming happily through the night at around 75mph let go suddenly. In a big way…

Eventually an NRMA tow-truck unloaded us beside an old garage in Maclean. While the mechanic had gone for the weekend, my spirits lifted when the bloke on duty said he knew a cane-farmer nearby who was a backyard Holden wrecker. Not only was the farmer happy to bring me a good diff-centre, he offered a choice of ratios. With the deal done – using borrowed tools, jack and axle stands, and a sheet of gasket paper I found behind the garage workbench – I got stuck in.

With the wedding in mind (and my planned pre-wedding snooze abandoned) there was a touch of Leyland Brothers pressure on my progress: "Will Mal get the sump back on before the tide comes in?"

While I can’t remember whether Mal Leyland did beat the tide, I do remember that we didn’t quite get to Surfers Paradise in time for the wedding. All wasn’t lost though; after thoroughly degreasing myself, and taking a brief kip, I was in top form for the reception. And the cane-farm diff worked like a charm for as long as I had the HR.

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