The 450SLC has been away for some rust repairs and a paint job for over 18 months now. I’m still in regular contact with the panel beater, but it’s one false promise after another … that hasn’t stopped me from thinking about the possibilities.
The current engine does appear to have an issue with what I am told is the piston rings – it huffs and puffs a considerable stream of blue smoke when running. I do however think it may just be valve seals (my new mechanical speciality – How I changed the valve seals on my 380SEC and have it running the best it ever has). Ignoring that, it’s also the wrong motor. At some point in its past someone swapped out the M117 4.5L motor for a M116 3.5L motor. I believe the motor is out of a W116 as it’s running K-Jet Fuel Injection and not the D-Jet that a 3.5L from a 107 would have been running.
Several years ago, I had the opportunity to buy a 5.0L M117 and automatic transmission from a W126 at a very good price. Seeing as my engine isn’t the correct matching numbers engine and has the above mentioned smoke issues, I thought it would be great upgrade.
Even though I have an automatic transmission ready to go, I’ve always thought it would be cool to have a 5 or even 6 speed manual gearbox behind the 5.0, to make it a bit more engaging to drive. The question is, can I do it on a budget? The internet seems to think not, but are they right?
Manual Gearbox Conversion Options
SL Shop 6 Speed Manual Conversion
The SL Shop over in the UK offers perhaps what is the most well thought out, plug & play option for a manual conversion. They offer a ready to go kit with all brand new parts. The only problem is the price – AUD$25,000!!!
There’s just no way I could afford this in any way. I would love to have a kit like this that I could just bolt in, but it’s in no way financially viable for me. Here’s the link to the kit: https://parts.theslshop.com/mercedes-benz-r107-4-speed-automatic-to-six-speed-manual-gearbox-conversion/
The Kangaroos Team Adapter Plate and Associated Parts
The Kangaroos Team (an odd name for a company operating out of Bulgaria) offer an adapter plate that works with the M117/M119 and can supply most of the parts needed to make the dream real. Their prices don’t seem unreasonable for what they offer. It looks like somewhere around AUD$5,000 with all the bits and pieces.
Here’s the breakdown:
It comes out to €2509.00. I have previously reached out to them for a used gearbox to go with this and was told it would be around €900. There would also be around €300 in shipping of these parts to Australia. At the current exchange rate that comes out to AUD$6,137. Still a fair chunk of money that I don’t have.
I’ve been researching this kit online and there’s a YouTuber who has done the conversion on a W114. It didn’t look like a painless exercise: https://www.youtube.com/watch?v=
I have however found this link on a local forum where someone has done the conversion on a 500SL and said it was mostly plug and play: https://www.topklasse.org/forums/threads/1983-500sl.7605/
The DIY Route
Would it be possible to do this conversion sourcing parts locally? I really don’t know a lot about this type over conversion, but it does look to me as though I would be able to fit an adapter plate between the M117 and an off-the-shelf 6 speed manual gearbox. So what if i looked at what manual gearboxes I could find locally?
A quick search has found that I can pick up an Aisin AY6 6 speed manual out of a VE commodore for around AUD$500. This isn’t the strongest gearbox in the world but is rated up to around 450nm. The 5.0 has around 405nm which would be well within this limit. Even if I did manage to squeeze a little more power out of this engine in the future I think I’d still be in this range.
This gearbox was used in quite a few Japanese cars as well such as the Lexus IS300. So it’s going to be a reasonably smooth shifter. An online search reveals no major reliability issues with this gearbox or complaints about shift quality. Short shift kits also seem to be available for around $300 should I choose to do this.
I would then need to work out how to acquire a 1-off adapter plate for this use case. The only way I could see that I would be able to do this is to design it myself and get it CNC cut. To me it doesn’t look like an especially difficult process – just a matter of tracing and aligning the bolt holes and I do have some experience in creating these sort of 3D models from my 3D printing experience. I could even theoretically 3D print a mock up to test my design out.
The clutch and friction plate would then be VE commodore items. Currently these aren’t particularly difficult to get in Australia, but given that Holden is now defunct and the last RWD commodore came off the production line 7 years ago, I don’t know if this will be the case in the future.
Of course I would then need to fabricate custom gearbox mounts and a get a new tailshaft. These 2 items would be needed regardless of which option I went with.
My gut is telling me this would be the least expensive option, but also the most difficult.
Gear Ratios
What would a manual 450SLC with a 5.0 and 6 speed manual be like to drive. Getting a gearbox with the wrong ratios would change this from a car that’s engaging to drive to something akin to rowing a boat having to constantly seek the right gear.
Here are the gear ratios from the Kangaroos Team (I’m assuming a gearbox out of an SLK350 or similar) and an Aisin AY6
| Gear | SLK350 | SL Shop Kit | Aisin AY6 |
| 1 | 4.459 | 4.002 | 4.17 |
| 2 | 2.614 | 2.108 | 2.19 |
| 3 | 1.723 | 1.380 | 1.49 |
| 4 | 1.245 | 1.000 | 1.19 |
| 5 | 1.000 | 0.780 | 1.00 |
| 6 | 0.838 | 0.645 | 0.84 |
Interestingly, the SL Shop calls there kit a 5+1 gearbox, although to me given that it has 2 overdriven gears to me it seems more like a 4+2.
So what does all this mean? Well, using these ratios we can work out what the top speed would be in each gear using a simple mathematical formula:
Top Speed (wheel) = (Redline RPM * Gear Ratio) / Final Drive Ratio
Top Speed (car) / Minute = Top Speed Wheel * Pi * Wheel RadiusHere is a list of final drive (diff) ratios for 450SL / SLC’s
| Model / Year | Final Drive Ratio |
| 450SL (107.044) 450SLC (107.024) From Jan 1974 | 3.06 |
| 450SLC 5.0 (107.026) | 2.72 |
| 450SL (107.044) 450SLC (107.024) USA 1980 | 2.65 |
And the top speeds in each gear using the 3 gearbox options assuming a redline of 6000rpm and a wheel radius of 67.31cm (based off 225/50R16 tyres on my 450SLC)
| Gear | SLK350 | SL Shop Kit | Aisin AY6 |
| 1 | 52 | 59 | 56 |
| 2 | 90 | 111 | 107 |
| 3 | 136 | 170 | 157 |
| 4 | 188 | 235 | 197 |
| 5 | 235 | 301 | 235 |
| 6 | 280 | 364 | 279 |
It’s quite clear looking from this that the SL Shop kit is geared heavily toward fuel economy. The car obviously won’t be able to reach the redline in the top 2 gears to reach those speeds, but at 110km/h (the highest speed limit in Australia), the car would be revving at 2200rpm in 5th and 1800rpm in 6th. This actually seems like it wouldn’t be all that engaging to drive and would be more of a cruiser. Interestingly if the car was running the 2.65:1 ratio diff of the US cars, the gearing would be even longer!
In comparison the Mercedes-Benz gearbox and the Aisin which seem to be similarly geared would have better acceleration and the tighter ratios would be better around town.
This is all of course just theoretical for now, but it’s something I’ll keep investigating.
