'81 W126 500SEL motor into '79 W116 280SEL?

[fat stig]

Hey, I have a 1979 280SEL and have found a cheap W126 500SEL H.K import. I was thinking about transplanting the motor and gearbox from the 500 into the 280. The 500 is a complete running car so everything needed should be there, but was wondering if anyone had any advice or details relating to the transplant. Any foreseable problems with the idea? Also, the 500 has a different rear end to my 280, should I bother to change it also? Is that possible? It looks the same as a W116 450 rear axle but I'm not 100% sure without seeing the two side-by-side. Thanks in advance. Any comments appreciated.

[Bandolero]

The 280 diff ratio is WAY too low for a V8.
You may be better off selling the 126 and buying a rusty 450 wreck and transplant everything into the 280. 

[Big_Richard]

pretty sure the w126 diff will bolt straight into the w116 rear subframe.

but even if not, who cares, the 280 diff with the 5 litre will make the car fly!

in theory anything is possible with the engine swap, i think you may need to find the engine brackets off of a 450 engine and possibly get a custom tail shaft. the rear transmission mounting is different too. and ofcorse the 4 speed auto uses an electronic speedo versus cable operated in the 116

[fat stig]

Thanks guys. I've got v8 116 diff but thought the 280's 3.69 would be a bit of fun with the 500. I had forgotten about the electronic speed though. That might be a bit of a hurdle. Any ideas how to solve that problem? I would rather use the 500 over buying a 450 wreck because it is a mates car which makes things very cheap and easy. Thanks again for the input.

[jbrasile]

Fat Stig,

Interesting project.... if you leave the 3.69 rear end in the 280 you will probably have acceleration equal or better as that of a 6.9

You will need new front springs out of a 450, the engine mounts of course and all of the cooling system from a V8 116.

There might be a drive shaft length problem as well because of the longer 4 speed tranny.

As for the speedo issue, if you use one out of a late 107 they are electronic just like the 126's and fit perfectly since the instrument cluster on 107's and 116's are the same. If you swap rear ends, get a speedo for a 500SL or SLC otherwise just have the new speedo re-calibrated.

I am sure it will not an easy or inexpensive project, but it is doable.

Good luck,

Joe

[fat stig]

Thanks for all the great info guys. I've already got v8 springs and some adjustable koni's in the car. A v8 radiator should be easy to find if the w126 one doesn't fit. Getting a tailshaft modified is pretty easy, we're always getting them shortened/modified at work. I don't like the sound of a 107 cluster...That sounds difficult and expensive to find. Good to know it will work though. I might give the v8 a try with the 3.69 diff and if its undrivable I'll swap a w116 v8 diff in. This is all sounding very possible. I've got some 4 pot brakes of a late model w126 (I think) that I might put while I'm at it.  I know the master cylinder will be different but I can work around that. Thanks again for the help

[jbrasile]

Fat Sitg,

The rad from the 126 will not work, and thinking about it now,  the 280 diff may not be able to take the power, I am not sure if they are smaller in the 280's.

As for the speedo, a 107 used cluster is not that hard to find or too expensive.

Tks,

Joe

[Big_Richard]

a short term speedo solution can be a generic lcd speedo velcro stuck to the dash with a magnet on the tail shaft and a sensor screwed to the floor.

untill you find a late 107 speedo that is..

[fat stig]

I'll give the 280 diff a go I reckon. I'm not sure about you guys but I haven't ever seen an old merc diff blow apart. They are generally over engineered for the application in my opinion. If the worst happens though and the v8 fries it I'm not really any worse off. A digital tacho and a magnet would work I guess but it would look pretty dodgy wouldn't it. I would've thought the 107 speed would be non existent so its good to know they are about. The 500 m117 is an alloy block isn't it? I wonder what the weight difference will between that and the m110. I will have to remember to compare just out of curiosity.

[jbrasile]

Hi Fat Stig,

6.3 and 6.9 diffs do get fried.... hehehe

If you have some patience and check e-bay etc... I am sure you will find a late 107 cluster at a decent price.

Yes the M117 500 is an all alloy engine, not sure about the weight difference but most likely you will need the proper springs.

Tks,

Joe