changing my diff, need help.

[HLC]

Ok, ive asked this before but i always get a billion different answers.

i have a 3.45 LSD here at home and i want to swap the centres with the centres in the diff in my car.

Now, the problem im having, is will the change in ratio affect anything with the Auto box, like shift points? and how do i correct that? by getting a tune? could i just pick up an AU XR6 Auto ecu since the diff use to live in an AU XR6. and lastly, will this affect my speedo as well?

also, is the job basic? jack the car up, whack it on stands, pull everyting apart, slide the axles out, swap it all over then put back together? (thats the gist of it right? im getting my mechanic mate to help me lol)

cheers.

[SLO AU XR8]

It isn't easy. If your changing gears from one housing to another, they need to be lapped. if the 3.45's are in a housing, just switch the housings, much easier.

[bodes-sh]

yeah as above, is the centre in a live axle diff? or is it in an irs centre? if it is just a centre you are better off taking it to a shop and getting them to change it over. as for the speedo, you can just go to ford and get a speedo gear for an xr6 and it will be the same as what it is now, reading wise, as will trans shift points.

[HLC]

yeah they are both live axle's.

i'd swap the whole assembly, axles and all as ive got the diff disc-to-disc but its out of a ute, so that wont be a swap with my sedan lol

what is lappning? im guessing something you cant do in a workshop yourself?

thanks guys!
-Henri

[The Monty]

It wont affect your shift points at all. I was scared off this when I went to 3.9's, I didnt even get the speedo gear changed yet, its easy enough to just make all my speed corrections to 130%. Only thing is the cluster is going up at 30% extra so my speedo wont reflect how many K's the car has actually done.

[LG]

Go to Ford Parts, and get a Black Speedo Gear from an EF/EL. Cost is about $9, and will fix your speedo.

I would do the same, and just swap the housings. Although while it is out, I would suggest you change the bearings, seals and oil.

Goodluck

[The Monty]

Would ford have the one for 3.9's?

[EvilChief]

i am interested to see what everyones explanation for lapping is...

[ronwest]

Originally Posted by EvilChief

i am interested to see what everyones explanation for lapping is...

I first lapped valves as a teenager (and under my dad's supervision) in a Cooper S motor we'd rebuilt.

Consisted of a suction cup on the end of a stick attached to the flat of an inlet/exhaust valve and with course -> fine grinding paste, "rotate, lift", "rotate, lift" against the valve seat until a nice, even, grey surface was evident.

I've also lapped something flat (can't for the life of me remember what) using figure of eight pattern on a sheet of glass + grinding compound.

I remember my dad lapping diff gears using a blue marker of some sort. Can't remember exactly how he did it - too much water under the bridge...

edit: scratch the last bit. It was for checking lash that the blue marker was used.

[HLC]

Josh raised a point there, would changing the diff also cause the odo to go up more? or would the speedo gear fix this?

[EvilChief]

Originally Posted by ronwest

I first lapped valves as a teenager (and under my dad's supervision) in a Cooper S motor we'd rebuilt.

Consisted of a suction cup on the end of a stick attached to the flat of an inlet/exhaust valve and with course -> fine grinding paste, "rotate, lift", "rotate, lift" against the valve seat until a nice, even, grey surface was evident.

I've also lapped something flat (can't for the life of me remember what) using figure of eight pattern on a sheet of glass + grinding compound.

I remember my dad lapping diff gears using a blue marker of some sort. Can't remember exactly how he did it - too much water under the bridge...

edit: scratch the last bit. It was for checking lash that the blue marker was used.

he ok


monty u r right, but thats normally only done once at the factory. Once done, there is normally need to be redone. there is also the offset market on the diff gears to ensure the correct wear and meshing of the gears. the blue stuff ronwest is talking about is called bearing blue and is used to check the wear pattern on the diff gears (ie heal and toe etc), once the gearset is installed in the new housing. the shimming under the pinion is what determines the mesh with the crown. the two spacers on each side of the carrier set the backlash, which is measured with a dial gauge (FYI 7 +/- 2 thou).

[The Monty]

Gear lapping is the process of imparting a very fine finish and high degree of accuracy to gear teeth. Lapping typically improves the wear properties of gear teeth.

To ensure smooth and quiet running, the Gears and Pinions are lapped after hardening. Lapping is accomplished by running mating pairs together in a gear lapping machine and feeding a liquid abrasive compound under pressure into the gear pair. The compound removes small amounts of metal as the gears rotate, thus refining the tooth surface and achive desired contact pattern.

One of the most common applications for gear lapping many are familiar with is the process of diff lapping. Diff lapping is the lapping of the gear teeth in a vehicles differential to improve gear life and increase strength by removing minor surface imperfections from the tooth profile.

[The Monty]

It will go back to normal once the right speedo gear is used.

[66 coupe]

Originally Posted by The Monty

It will go back to normal once the right speedo gear is used.

However changing speedo gear will make the shift points incorrect.
You will need to use a speedo corrector, and keep the existing gear on the pickup.

[HLC]

what if i do the diff, then get a VDO mob to reprogram my cluster?

[The Monty]

I just copied and pasted my answer, lol, just like when I was at school. I had no idea actually, I just went off what Ive always been told, "Make sure they are lapped in properly!".

[HLC]

so from all reccommendations here, this isnt a backyard job? lol

[EvilChief]

Originally Posted by HLC

so from all reccommendations here, this isnt a backyard job? lol

lol i have done a few in my back yard, even out bush, but you will need to know exactly what to do and what to measure. i just finished building two sets last week at work

[66 coupe]

yeah you could do that, or use a corrector, whichever is easier for you / more cost effective.