Changing diff ratio (F-series) - housings machined differently for different ratios?

[Stefan]

Took my 3.5 F-series diff to a diff shop with a set of 3.9 F-series gears to put in (don't have the housing these gears were from), and they rang me back and reckon that diffs with different gears have differently machined housings, and that my 3.9 set (out of an F-series) wouldn't work without machining the housing, which they couldn't do.

The crappy thing is, not only am I back where I started, I most likely will have to pay for taking my diff apart and putting it back together again.

Can anyone else confirm swappability of gears between housings? I've never heard of this and assumed it would be an easy swap...

[Smokey228]

Not sure about the F series, but the G series housings differ slightly between ranges of gear sets... I.e. 3.* - 3.9 fit one housing, and then 3.93-4.8 or something like that are different... Thats prolly not the right numbers, but thats the general idea...

Same thing for LSD and non LSD, some differ some work... IRS and non IRS are different etc...

Worth doing a search and finding out, im pretty sure there is a diff guide in the stickies somehwere! Should give you some more information...

Cheers,
Jase

EDIT: Also, not sure if its the same, but for some people doing the G series gear swaps from a non LSD center, had to use a 3mm spacer to move the crown wheel into a better position...

[thechuckster]

if the 3.9 gears came from an IRS housing and the 3.5 gears are already in an IRS housing then i'd call shenanigans?

[JustCallMeOrlando]

Didn't Norbie get 3.5 guts put in his diff? Or did he just get the Truetrac built into the other housing?

[Stefan]

Well, I got my diff back. They were quite nice about it and didn't end up charging me.

They explained that the 3.9 crown wheel I had provided was a lot thicker than the 3.5 one that was in the diff already. So, to anyone who wants to change diff ratio - be aware that it is much easier to swap your centre into another diff rather than try to change the ratio.

[wilbo666]

Didn't Norbie get 3.5 guts put in his diff? Or did he just get the Truetrac built into the other housing?

Just another housing pretty sure

Cheers
Wilbo

[TheStitt]

thats seems really wierd I run a True trak, 3.5 IRS ratio in a 4.1 live rear end.

[Stefan]

Very strange. Maybe the gears I have are funny.