Is this a viable swap? I found a great deal on a running 115 carbed engine. I see that the motor mounts are on the side of th eblock, not the front, but that's not a big problem.

what kind of Ponton do you have? The 190b or 190Db cars are very swappable with M115 engines, find the twin carbed version because Stromberg 175CD equipped M115s can be a headache because it's wider, can interfere with side rail frame, or, you can have the long block M115 and install the intake and exhaust manifold from M121 single carbed 190 or twin carbed 200 along with air cleaner assembly respectively. M121 and M115 have the same intake/exhaust designs.

I did swap on my W110 200 w/ the original(M121) stick shift on the column. M115s are durable(14 head bolts vs. 10 for M121), engine parts are plenty in the Philippines and some of the parts are interchangeable with M121. here's the pics.

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I have a 1955 180. It has the sidevalve four

the one with 4 engine mounts right? that will be difficult, you have to fabricate the bracket to mount the M115 engine.

no, two in the front and a transmission mount.

really, then it is very swappable. is your car had stick shift on the column? hydraulic clutch?

So long as the M115 engine-block has threaded holes at the front to attach the Ponton motor-mount brackets.
I'd try for one of the downdraft Solex carb setups, as the side-draft Stromberg arrangement may foul the Ponton heater box.
I don't know if a M115 will bolt up to the Ponton tranny. If you also need to swap in a compatible W115 tranny/hydraulic-clutch, that will complicate things.

Happy Motoring, Mark

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DrDKW