The 3rd and 4th syncros and sleeve want to be Changed.
However my assumption was that it would be the same as the
Hydro box. The diameteer of the Syncro Sleeve is smaller than
The hydro 3rd and 4th. Can anybody shed some light, I am
Confused as the Clutch shaft is the big spline, thus the y1,S1and J1
3rd and 4th gear Syncro set won't work?

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If the transmission has a sticker that has "YS1" on it then the internals will be the same as a hydro unit. Your M/S 3rd gear will have 24 teeth if it is a b16 or B17 GSR and 26 teeth if it is an LS. The LS uses a smaller sleeve and synchros than the B16/GSR. The YS1 will have 3 shift forks on 3 individual poles where the early 88-91 B16 transmissions S1/Y1/A1/J1/Y2 use 3 shift forks with 2 poles. The J1/Y2 use a larger 3/4 sleeve than the 5th and the S1/Y1 have 3/4/5 synchros the same size.