There are a number of threads that talk about this conversion but I haven’t seen one that covers the swap from start to finish. I just completed the job and think some and decided to post the particulars.
The car converted is my 300D Turbo automatic. It’s a fairly low mile car in very good condition that I use as a daily driver. The automatic worked as designed but I enjoy a manual transmission and decided to undertake the project.
I sourced the 39lb. 300D manual flywheel on Ebay. It wasn’t cheap at $1000.00 but it’s one of the keys to having a smooth running car with the manual. I’d read posts where the where the 240D flywheel was used and they generally talked about vibration. Mercedes used this heavy flywheel for a reason.
The rest of the parts came from a salvage 1978 240D manual at a cost of $600. I chose to buy a whole car as sourcing the parts ala carte is for sure more time consuming and probably costlier in the long run.
Below lists the broad steps.
• I removed the automatic trans.
• I marked the cast flex plate location to the crank before removal. I had a local balancing shop compare the flex plate to my flywheel after resurfacing. They were both neutrally balanced and quite accurate. I did have the flywheel matched and marked so whatever slight imbalances were present were in the same orientation as the flex plate casting that was removed.
• I had a set of used flywheel bolts from an old project and re-used them. This is discussed a lot on the forum. Mercedes has a minimum diameter for the neck on these stretch bolts and mine were well over the minimum for re-use. I did torque using the torque angle method.
• The pilot bearing posed a problem. There was over 0.003” interference between the bore of the crank and the bearing OD. I assume this is because Mercedes skipped the grinding operation for the bore on cranks for Turbo engines. I considered making a bronze pilot bushing but chose to open the crank bore instead. With a lot of patience, a flap wheel, a modified brake hone and a bore gauge I opened the bore 0.0025”. It took some time but the result was excellent and the pilot bearing fit with the right amount of interference. If I wasn’t a machinist in a well-equipped shop this would have been a big problem.
• I installed all new clutch components.
• I installed a new slave cylinder with the manual transmission on the bench along with a new flex disc and measured the overall length compared to the automatic for the required drive shaft modifications, then installed the transmission.
• The transmission crossmember was the next obstacle. I ended up using a 240D manual crossmember. I read that a 240 automatic crossmember would fit but it was way off. The modification was simple on the 240D manual part I pulled from the donor car. I cut ¾ inch from both sides, welded it back together and slotted its mount holes to the chassis and the mount slightly. Finally, I lowered the flange on the crossmember to provide a little clearance against the bottom of the transmission. The fit was perfect. It does require the smaller 240D transmission mount but the result was perfect, no vibration at all.
• I had the 300D drive shaft lengthened and balanced at a local shop for about $300 and installed it with a new center bearing and rear flex disc.
• Installing the pedal cluster is frankly difficult. There is very little room to move under the car but it will fit with patience. I installed a new clutch master cylinder on the bench before installing the pedal cluster.
• I was able to locate a new clutch hydraulic hose assembly from Pelican Parts. The required holes are in the chassis. Again the fit was perfect.
• The shifter assembly from the donor car was in poor condition so I changed all the bushings before installation.
• I had intended to shorten the shift rods from the donor car but located a set of correct 300D manual rods on eBay and used them instead.
• The wiring harness connecting the reverse light switch on the shifter assembly plugged into the main chassis harness without modification. I have reverse lights but obviously no NSS.
• I used the speedometer cable from the donor car as the ends on the cable for the automatic don’t fit the manual transmission. It is slightly longer but not a material difference.
• The last obstacle was the cruise control amplifier. The manual pedal cluster occupied space it used under the dash. As I considered options, I noticed molded-in screw holes on the under dash panel the exact spacing as the amplifier bracket. The wiring harness didn't leave me enough room to mount the amplifier and plug into the harness. I ended up making a spacer to rotate the amplifier slightly so the wiring connection could be made. The result was good.
The swap is not terribly difficult but I didn’t cut corners and it wasn’t cheap. The crank modification might be a little beyond the weekend mechanic and the crossmember modification would be a challenge. Other than that, it’s really a bolt together process. I couldn’t be happier with the results. It’s much quicker and comfortable to drive with none of the disconnectedness of ‘80’s automatic transmissions.
The car converted is my 300D Turbo automatic. It’s a fairly low mile car in very good condition that I use as a daily driver. The automatic worked as designed but I enjoy a manual transmission and decided to undertake the project.
I sourced the 39lb. 300D manual flywheel on Ebay. It wasn’t cheap at $1000.00 but it’s one of the keys to having a smooth running car with the manual. I’d read posts where the where the 240D flywheel was used and they generally talked about vibration. Mercedes used this heavy flywheel for a reason.
The rest of the parts came from a salvage 1978 240D manual at a cost of $600. I chose to buy a whole car as sourcing the parts ala carte is for sure more time consuming and probably costlier in the long run.
Below lists the broad steps.
• I removed the automatic trans.
• I marked the cast flex plate location to the crank before removal. I had a local balancing shop compare the flex plate to my flywheel after resurfacing. They were both neutrally balanced and quite accurate. I did have the flywheel matched and marked so whatever slight imbalances were present were in the same orientation as the flex plate casting that was removed.
• I had a set of used flywheel bolts from an old project and re-used them. This is discussed a lot on the forum. Mercedes has a minimum diameter for the neck on these stretch bolts and mine were well over the minimum for re-use. I did torque using the torque angle method.
• The pilot bearing posed a problem. There was over 0.003” interference between the bore of the crank and the bearing OD. I assume this is because Mercedes skipped the grinding operation for the bore on cranks for Turbo engines. I considered making a bronze pilot bushing but chose to open the crank bore instead. With a lot of patience, a flap wheel, a modified brake hone and a bore gauge I opened the bore 0.0025”. It took some time but the result was excellent and the pilot bearing fit with the right amount of interference. If I wasn’t a machinist in a well-equipped shop this would have been a big problem.
• I installed all new clutch components.
• I installed a new slave cylinder with the manual transmission on the bench along with a new flex disc and measured the overall length compared to the automatic for the required drive shaft modifications, then installed the transmission.
• The transmission crossmember was the next obstacle. I ended up using a 240D manual crossmember. I read that a 240 automatic crossmember would fit but it was way off. The modification was simple on the 240D manual part I pulled from the donor car. I cut ¾ inch from both sides, welded it back together and slotted its mount holes to the chassis and the mount slightly. Finally, I lowered the flange on the crossmember to provide a little clearance against the bottom of the transmission. The fit was perfect. It does require the smaller 240D transmission mount but the result was perfect, no vibration at all.
• I had the 300D drive shaft lengthened and balanced at a local shop for about $300 and installed it with a new center bearing and rear flex disc.
• Installing the pedal cluster is frankly difficult. There is very little room to move under the car but it will fit with patience. I installed a new clutch master cylinder on the bench before installing the pedal cluster.
• I was able to locate a new clutch hydraulic hose assembly from Pelican Parts. The required holes are in the chassis. Again the fit was perfect.
• The shifter assembly from the donor car was in poor condition so I changed all the bushings before installation.
• I had intended to shorten the shift rods from the donor car but located a set of correct 300D manual rods on eBay and used them instead.
• The wiring harness connecting the reverse light switch on the shifter assembly plugged into the main chassis harness without modification. I have reverse lights but obviously no NSS.
• I used the speedometer cable from the donor car as the ends on the cable for the automatic don’t fit the manual transmission. It is slightly longer but not a material difference.
• The last obstacle was the cruise control amplifier. The manual pedal cluster occupied space it used under the dash. As I considered options, I noticed molded-in screw holes on the under dash panel the exact spacing as the amplifier bracket. The wiring harness didn't leave me enough room to mount the amplifier and plug into the harness. I ended up making a spacer to rotate the amplifier slightly so the wiring connection could be made. The result was good.
The swap is not terribly difficult but I didn’t cut corners and it wasn’t cheap. The crank modification might be a little beyond the weekend mechanic and the crossmember modification would be a challenge. Other than that, it’s really a bolt together process. I couldn’t be happier with the results. It’s much quicker and comfortable to drive with none of the disconnectedness of ‘80’s automatic transmissions.
