Hi guys are the bell housings interchangeable on these two gearboxes?i have a ca18det 5 speed and was going to put a PPG kit through it but they don't do them anymore so the plan now is to get a sr20det gearbox which they do the kit im after and is rated at 600hp,then swap the bell housing with the ca18det one.Is this an easy change?

Yes make sure it's from either a s13 or s14, from experience with a mates car that did this swap the s14 is the better box.

Bellhousings are interchangeable yes. All silvia boxes are externally the same (other than 6 speed)

my understanding is the only difference is the bellhousing between CA and SR 5 speed.
Later 5 speeds had a difference sandwich plate design..... happy to stand corrected though

What 5 speeds are later? S15 is still the same box as s14 im pretty sure

I thought s15 were 6speed and not as strong as the 5

S15 came with 5-speed FS5W71C. Optional was 6-speed FS6R92A or Automatic.

S13/S14 used 71C gearboxes

* S13 SR gearbox
* S13 CA gearbox
* S14 SR gearbox

They don't have a removable bellhousing. Instead its integral with the Front Case. By partially disassembling the gearbox and swapping the front half of the case it can be used on the other engine .

Well guys it looks like that everything that has been discussed is now out off the question,$12,500 for the ppg gearset for the sr20.So now will look into rb25det,can they handle a lot off hp?

Yes, the RB25DE can handle a lot off hp (there was no RB25DET).