How can you tell if a 200SX S15 diff is Helical or plate type (with out pulling it appart)

If by 'plate type' you mean a clutch pack LSD, they didn't come with one. Oz spec manuals came with the helical, but autos came with viscous centre. If the diff is from an Oz spec car, one way to tell would be by counting the number of turns of the pinion (tailshaft) relative to the crownwheel (halfshaft) - manuals had 3.7 CWP, but autos had 3.9.

OTOH, if the diff your looking at is an import (Silvia), then it may not even have an LSD centre as some models in Japan didn't have one (I think).

All K's King model S13 silvias have an LSD standard. A K series basicly means Tubo model. S14 and S15 turbo series should also have LSD as standard items. Be careful of the dodgy bro's S13 conversion to turbo with
crap brakes and a single spinner.

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1928 Model A CC Pickup

Be careful too that some tight arse has not stuck a phantom grip in an open diff and is now saying that its fulee sik limited slip

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Richard... http://www.youtube.com/watch?v=rqM9_Wq-TKc

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USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

Phantom Grip.......the name says it all.