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LSD rebuild


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Does anyone know the pre-load that needs to be applied to the main output bearings on a Sierra diff.

I have an SEiW which currently has a 3.9 Sierra diff. I obtained a Sierra 4X4 LSD diff, so I now want to rebuild the 2 diffs into one to put the 3.9 ratios on the LSD. My preference is to keep my current housing with the 3.9 pinion and change the crown wheel onto the LSD diff unit. Then install this whole unit including the bearings (as a matched set) into my current housing, so any information on how to set up these bearings would be welcome.

I can then get it back on the road  !!

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Are you sure you want a 3.9 final drive? What 'box ratios are you using? A standard ratio'd type 9 or MT75 will have an all but useless first gear, and it's even worse if you plan on any sprinting or hill-climbing.

SPC or Road and Race transmissions can carry out the swap for you for a very reasonable sum. Diff pre-loads are something I would leave to a professional, unless you are one of course......... :blush:

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3.9 ??? seems the right ratio to me, with a type 9 box...well it has been driving OK for the last 2 years, 1st gear fine! peaks out in top at just below max revs nicely :D  just need some guidance on the best way to set up the main diff bearing. I am sure loads of people have rebuilt Sierra diffs.

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Setting the preload is not technical and is just done by feel. With the crownwheel removed assemble the pinion to the diff casing using a new collapsible spacer. You will need a special tool which holds the nut static relative to the diff casing and then turn the pinion until the bearing starts to tighten. If you've done plenty of wheel bearings before then you'll know when the preload is about right. The difficult bit is setting the meshing pattern of the gears to eachother using engineers blue. This is done by moving the diff/ crownwheel assembly bearing housings side / side intil the desired backlash is achieved (on a dial gauge on one crownwheel tooth) and the meshing pattern is satisfactory. The preload on the main diff housing bearings is also done by feel (but obviously by now you will be turning both pinion and crownwheel preloaded bearings together). If you are patient like me then you'll have no problems - if not ask Phil Stewart 01959 525105 to do it for you.

Regards

Graham

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Thanks for the info Graham, I should be able to set that up OK.  Yeah, did plan on having to set up the messing of the gears, don't suppose you know what the backlash should be (got dial gauge etc.). I was going to set it to detect some backlash (checking for any tight spots of course), as there was in my current diff and the one I am stripping down.

Dave

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0.1 to 0.17mm is the std spec but some racers recommend opening it up to 0.3 mm to allow better lubrication. The wider gap will cause a little more noise though. You'll probably find the backlash will vary between teeth so take the spec as limits. If you've not been careful to heat up the crownwheel in an oven before bolting it to the diff you might find the variation is greater.

Regards

Graham

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