Lotus 5 speed gearbox in an S4 and concentric clutch
Posted: Tue Nov 04, 2014 11:06 pm
Hello everyone,
First of, thank you all for making this forum such an invaluable source of information : without it I would not have embarked in any of the following.
I have been working on and off for some time on a ?68 S4se FHC brought back from the US, and eventually after completion of body overhaul with a new paint, the first road test uncovered that one cylinder had low compression. Rather than just fixing it, I decided I should take time to overhaul the original block yet enjoy the car in the meantime, so went for swapping engines, and while at it I got tempted by fitting a 5 speed gearbox and a concentric hydraulic clutch.
So in case it is of interest to anyone, here is a summary of these steps :
Fitting a lotus 5 speed gearbox (from a +2) : many forum posts and the Lotus conversion article from the wiki were very useful, so that the 5 speed mount holder could be easily modified to reuse the chassis 4 tapped holes. The mount holder needs to go back about 21mm, so the front corners were flattened for allowing new holes. The clutch bell opening was used as the reference for longitudinal position, and when dropping the engine back the mounts fell right in place.
Propshaft : one was recut/fabricated to 29? compressed length (that plus 1 ?? for the extended length ? I was told it would be +2? but that was enough) ; one word of caution about the flanges, as for the 5 speed gearbox I actually needed a different flange than the diff one (and in spite of my warning, the propshaft place did not check and first used the wrong flange type for the front rather than the one supplied with the gearbox).
Gearbox lever : the top part of the original 5 speed lever (attached to the bottom via a cylinder filled with rubber) was removed, the remaining rod being about 10mm thus tapped M10, and a new, modified 4 speed lever was attached to it. NB: the original long nut sleeving into the rubber bush cannot be used, and the play on the lever needs to be reduced by reusing the thick washer that fits inside the chromed cylinder). As expected from posts on the forum, the axis position was a little rearward with respect to the position of the 4 speed gearbox lever, so to be on the safe side the fiberglass tunnel was filed to make sure vibrations would not cut into the rubber spring.
Concentric hydraulic clutch slave cylinder from Saab 900 : an alloy holding plate was machined on a lathe, concentric with the guiding tube (pressed out) to hold the slave cylinder, then machined around the attaching holes to add lightness. Thickness was selected to allow for 6-8mm motion margin to the rear when the pressure bearing is in contact with the clutch plate springs (about 30mm from the gearbox face): that leaves plenty of forward operation motion (more than 10mm). Stainless 3/16? pipes were bent to connect the clutch (M10 fittings) to the master cylinder (3/8? fittings) via a braided hose, purge at the top.
Last, a new fast road TTR exhaust : as seen on the forum, the 5 speed gearbox requires a little fettling of the exhaust, and the down pipes were cut after the first ?Y? then rotated about 20? to clear the gearbox before welding them back on. The long central pipe did not need any adjustment aside from cutting to length at the rear. To make room for wrapping (and life a little easier) the frame was widened rear of the engine mount (20mm cut, bent as a triangle 15cm long and welded back like a gusset).
The central pipe is attached next to the rear of the gearbox
First try on the road did not show any problem so far, now I need to sort out details that were remaining from the restoration (replacing dashboard?) and tuning the webers, hopefully to be ready for next spring.
First of, thank you all for making this forum such an invaluable source of information : without it I would not have embarked in any of the following.
I have been working on and off for some time on a ?68 S4se FHC brought back from the US, and eventually after completion of body overhaul with a new paint, the first road test uncovered that one cylinder had low compression. Rather than just fixing it, I decided I should take time to overhaul the original block yet enjoy the car in the meantime, so went for swapping engines, and while at it I got tempted by fitting a 5 speed gearbox and a concentric hydraulic clutch.
So in case it is of interest to anyone, here is a summary of these steps :
Fitting a lotus 5 speed gearbox (from a +2) : many forum posts and the Lotus conversion article from the wiki were very useful, so that the 5 speed mount holder could be easily modified to reuse the chassis 4 tapped holes. The mount holder needs to go back about 21mm, so the front corners were flattened for allowing new holes. The clutch bell opening was used as the reference for longitudinal position, and when dropping the engine back the mounts fell right in place.
Propshaft : one was recut/fabricated to 29? compressed length (that plus 1 ?? for the extended length ? I was told it would be +2? but that was enough) ; one word of caution about the flanges, as for the 5 speed gearbox I actually needed a different flange than the diff one (and in spite of my warning, the propshaft place did not check and first used the wrong flange type for the front rather than the one supplied with the gearbox).
Gearbox lever : the top part of the original 5 speed lever (attached to the bottom via a cylinder filled with rubber) was removed, the remaining rod being about 10mm thus tapped M10, and a new, modified 4 speed lever was attached to it. NB: the original long nut sleeving into the rubber bush cannot be used, and the play on the lever needs to be reduced by reusing the thick washer that fits inside the chromed cylinder). As expected from posts on the forum, the axis position was a little rearward with respect to the position of the 4 speed gearbox lever, so to be on the safe side the fiberglass tunnel was filed to make sure vibrations would not cut into the rubber spring.
Concentric hydraulic clutch slave cylinder from Saab 900 : an alloy holding plate was machined on a lathe, concentric with the guiding tube (pressed out) to hold the slave cylinder, then machined around the attaching holes to add lightness. Thickness was selected to allow for 6-8mm motion margin to the rear when the pressure bearing is in contact with the clutch plate springs (about 30mm from the gearbox face): that leaves plenty of forward operation motion (more than 10mm). Stainless 3/16? pipes were bent to connect the clutch (M10 fittings) to the master cylinder (3/8? fittings) via a braided hose, purge at the top.
Last, a new fast road TTR exhaust : as seen on the forum, the 5 speed gearbox requires a little fettling of the exhaust, and the down pipes were cut after the first ?Y? then rotated about 20? to clear the gearbox before welding them back on. The long central pipe did not need any adjustment aside from cutting to length at the rear. To make room for wrapping (and life a little easier) the frame was widened rear of the engine mount (20mm cut, bent as a triangle 15cm long and welded back like a gusset).
The central pipe is attached next to the rear of the gearbox
First try on the road did not show any problem so far, now I need to sort out details that were remaining from the restoration (replacing dashboard?) and tuning the webers, hopefully to be ready for next spring.