I'm thinking of converting my 91 Prelude Si with the b21 to a manual. I'm having a hard time finding a D2J4 transmission and was wondering if a transmission from a b20 would bolt up the same and if the shift cables are the proper length to reach the shift assembly? how different are the mounts?

The d2j4 is rare, I'm not surprised you're having a hard time finding one. For a direct bolt on with no VSS modifications you'll want a d2a4. Those were used for all manual USDM 90 and 91 Preludes regardless of whether it had a b21a1 or a b20a5.

You could use a d2j5 or d2j3 from an 88 or 89 but you'd need to make sure you also use the clutch kit and flywheel from an 88/89 model because the transmission input shaft has a different spline, and the clutch is physically smaller (which is why you need the matching flywheel as well). You would also need to modify the VSS in order to have a working speedometer.

Whichever transmission you decide to use, you MUST match the clutch kit and flywheel to the transmission you are using.

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1991 Prelude Si ALB 4WS
2017 Ram Rebel