ok...did a bit more diggin and found the following...
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I have compared a JDM AE86 Kouki housing to an RA40 one just yesterday. The results are as follows:
Upper trailing arm spacing Lower trailing arm spacing Spring hat spacing.
RA40 519mm 977mm 950mm
ae86 494mm 980mm 930mm
Note: the springs and upper trailing arms are not in ideal positions. Springs are quite elastic and should bridge and flex. The upper arms are very short and sit on a bit of a dodgy angle when flexed 13mm sideways. Id suggest an upper arm cut and weld mod to get it right.
As for the springs:
RA40 springs have a tight wound tapered end whereas ae86 ones have two fat ends. You will need RA40 springs for the diff end as that is the end where the spring tapers. Note coronas etc were very similar and springs will interchange.
As for the trailing arms:
The RA40 trailing arms are identical length units to AE86. Different bushes though. I have taken rough measurements and the cup diameter looks much the same.
As for the forwards or backwards offset of the mounts and how they compare to one another:
The RA has mounts 10mm further forward on the housing for all the suspension arms including the panhard rod. Their relative positions to each other are all much the same, but the RA diff will sit further back using the AE86 floorpan mounts and suspension arms.
As for the panhard rod issue:
Yes the ra40 panhard rod is a different length to an ae86. The pin to mount the rod is in a different position on the RA40 diff, making the rod about 50mm shorter than the AE86 one. Im not sure how the mount on the floorpan will compare from the RA to AE86. I have not checked. Use an adjustable rod.
As for the tailshaft:
Im not sure at this stage about whether you can use the RA40 rear section or not. I measured the length from face to face on the ra40 one, it is 760mm. In my opinion if it is about 10mm longer than a t18 rear section then the RA40 rear tailshaft section will be fine, and will effectively lengthen your wheelbase by 10mm, when used in an AE86. This could be why you have to force it together when using a t18 rear section, since the trailing arms are all the same length from RA40 to AE86, but the RA40 diff is sitting further back due to its mounts being further forwards on the housing.
SOMEONE WITH A T18/JDM TAILSHAFT SITTING AROUND PLEASE MEASURE THE REAR SECTION. My guess is 750mm.
As for the bushes:
The centre bolt is larger on the ra40 bushes/diff. The outer bush cup size is the same on both cars, according to my measurements. Note, I have not actually fitted it yet, but my assumption is I would use an RA40 bush one end, and aa AE86 one the other. Look to the info above for real practical advice.
As for the Swaybar:
The RA40 diff I looked at had no mounts on it. The JDM one does. You will need a welder.
As for the Brakes.
Most RA40 diffs have drums on them. Not sure of their size but I suspect they are slightly larger drums than AE86. Only RA40GTs came out with factory lsd and disc brakes. That diff pictured is very rare and expensive. Don't get your hopes up of finding better than a drum braked, open centre 4.1 ratio.
The width from one mounting face to another is 1410mm on the RA40 diff. Not sure if a JDM diff is the same as I was comparing a housing to a complete diff. Hope this settles a few bets.