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6.5" To 7" Diff Swap


brumster
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Question for you.

 

I've sourced a 3.92 tripoid diff to swap out my 3.62 tripoid but the old 3.62 is the earlier 6.5" diff with the smaller shafts (early drum braked model Sierra).

 

My understanding is the larger 3.62 shafts (26 spline as opposed to 23) are 30mm+ longer, so while the obvious option is to just source some appropriate shafts, will this additional length cause a problem or do they compress up to the equivalent old length, or is the additional length in the splines and so no issue with the length between gearbox and hub flange?

 

I don't want to drop the drum brakes, of course.

 

The other longer-winded option would I guess be to swap the crown wheel and pinion over into my old diff but that's going to be a right ballache!

 

Just wanted to check I'm not going to have plunge issues if I just source the later driveshafts to match the new diff and "throw 'em in"....

 

Cheers,

Dan

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Yup, positive, just measured it. 6.5" diameter crown wheel. Either way, it's not an issue with the diameter - it's the inner driveshaft splines that are the problem. Just want to make sure the length on the later shafts won't cause me a problem.

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That's my other option (if all the dimensions are the same?) - the Haynes doesn't detail stripping the diff down, but is there anything awkward about it? I presume it's just remove the two lock stops on the side, remove the lugged ring and then the bearings come out, allowing removal of the gear assembly complete so I can swap them over?

 

Would rather not go that route unless someone can advise they're definately the same - I have read on t'interweb that the 6.5 and 7" diffs are otherwise identical apart from the crown wheel size...?

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If it is indeed a 6 1/2" diff then you can't swap the guts as they are designed for a 6 1/2" crown wheel

Therfore the differential bit will be to small for a 7" crown wheel

And setting up a diff is not easy

So back to where you are now

You have a 6 1/2 diff

You want to fit 7"

Best buy the shafts as well it'll make the whole fitting process easier.

Are you fitting to a chassis frame or to a Sierra sub frame?

Will your wider diff fit the Sierra frame?

I say this because the Sierra 7 1/2 diff won't fit a Sierra 7" frame, the other way round you can pack it out

 

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again, don't know if the diff is physically a different size, but you would either need to make a mix and match driveshaft setup which might not be physically possible, or just replace the whole rear subframe, diff and trailing arms. The latter sounds easier to me!

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No, the diff casing is the same external size, the 6.5/7" diffs actually measure just under 7" across the casing. So the new diff has dropped straight into the subframe (it's an Exmo).

 

I'll get the driveshafts ordered tomorrow. I hope the additional length isn't something to worry about. Otherwise maybe I'll have to transplant the new inner CVs onto the old shafts, but I seriously doubt the tripod bearings will be the same size so that may not be possible.

 

Snapperpaul - "the setting up of a diff is not easy" - can you qualify that a little? Do you mean shimming it up? What's not easy!?

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There is a pinion crush washer that needs correctly setting and the drive shaft bearings have a required pre-load

The pinion to crown wheel needs setting probably by the input crush bearing

Crown wheel and pinion are a matched pair and can't be swapped separately

To me it's cheaper and easier to just buy the correct diff and shafts

 

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Aye, will ring up my chappy tomorrow and see what the deal is with the shafts.

 

Does anyone know how the later model Sierras took up the extra 30 or so mm on the longer shafts? Does anyone have any to hand that they can measure the splined ends and/or the compressed/extended lengths so I can compare with my early shafts?

 

I'm just wondering whether I need to worry about the 30mm or if, for example, 15mm of it was just on the diff end and 15mm at the other end, and the actual shaft length was pretty much equivalent between the inner CV and outer CV. Or, worst case, maybe I just need to swap the outer hub carriers to match.

 

If the new shafts are not that expensive I might just suck-it-and-see.

 

Cheers,

Dan

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From Seventh Seven web site

You learn something new every day

More drive shaft info and will just add to the confusion

 

http://www.super7thheaven.co.uk/Sierra_Rear_Drive_Shafts

 

I get the feeling that your 23 spline diff hub combination is going to be the issue here, looks to me like you need a new diff, shafts and hubs

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Aye, looks like it, doesn't it.... :( oh well!

 

Just realised there is another option, which is to make up a hybrid of the Lobro inner with the push-in outers, but I think getting the right overall length might be an interesting challenge when buying off ebay :-S

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Shaft diameter is what you need

Now are you talking about keeping the drum hubs with 23 spline tripod ends and fitting to different diff ends

Gets complex lobro shafts are one thickness only the desiel push fits are the same diameter so hybrid lobro to push ends up with more splines than your drum end, then you would have to break the tripod open replace the desiel spline end with your 23 spline end

It's easier if you had standard push fit splines but still the shaft diameter is what gets you, I'm still looking for desiel push shafts

You have got me worried now as although I have a 3.9 push fit rear inc drums I have a Quaife ATB in a diff to fit

Note to self, check driveshaft fit before dropping the rear off

Edited by Snapperpaul
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My initial thought was keep the old shafts (since they have the correct length and outer - hub - fitment) and just swap the inner CVs for new ones; but obviously, as you point out, I wasn't overly hopeful that all the parts are interchangeable. That site linked above talks about lots of differing shaft thicknesses so I figure that's unlikely to work. And I doubt the tripod bearings are the perfect fit into the CV cups between the two (now that would be lucky!).

 

The splines on the end of the actual middle shaft (that the tripod bearing assemblies fit onto) - are they always the same spec as the splines on the actual CV stub? I appreciate the shaft diameter can change, but is the spline count something I don't have to worry about...

 

Waiting for a call back from my man on the shafts.... :) I can see some hassle coming my way :) scouring ebay for lobro's!

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