J Type Overdrives
Which Models have J type overdrive?
MkIV Spitfires from FH60001
All MkIVs were fitted with the 3-rail type gearbox.
All UK spec 1500s were fitted with the single-rail type gearbox. However, because USA specification vehicles had the 1500cc engine sometime before the UK and other export countries. It is possible that there are some re-imported 1500s that have a 3-rail type gearbox fitted as standard.
Are they the same overdrive unit? - No.
Are all the parts interchangeable? - No.
Are the Adaptor Plates the same? - No.
Although outwardly they appear the same (as are the vast majority of components), overdrive units for the 1500 (with single rail gearbox) were fitted with a much improved Oil Pressure Relief Valve system, which cannot be fitted to the original spec. MkIV J type overdrive used on a 3-rail type gearboxes. Also, due to the different gear selector system and gear level mounting, 3-rail type gearboxes and single rail type gearboxes each have their own specific overdrive to gearbox adaptor plate.
Can I fit a 1500 unit to a MkIV?
Should I fit a MkIV unit to a 1500?
Our reconditioned (exchange) J type overdrive units are suitable for both single rail and 3-rail type gearboxes, but you must use the correct gearbox adaptor plate.