Jonathan,
although the nominal power output of a 435 engine is 26 bhp SAE which is the same as some versions of the M4 602, the smaller engine needs to be revved to 6,750 to achieve that figure, whereas the M4 only needs 5,000 rpm.

My guess is that this renders the 435 bottom end somewhat vulnerable, especially since the engines don't have an oil filter, so particular attention needs to be paid to regular oil changes.

That's probably why I've got the remains of a 1978 2CV4 engine here, which I plan to rebuild as and when I track down a serviceable crankshaft.
( Must remind Will to have a look in his parts bin before the Summer... )

Russell,
I'd thought about retaining the 435 heat exchangers and crossbox when I replaced it with a 602, but there wasn't any chance that they would fit.
Using the same reasoning, I'm guessing that 602 heat exchangers would not fit a 435 engine, although I've never tried to do this...

ken.
( My 1971 Dyane was originally a 435, but it had been 'upgraded' long before I bought it. I've now got a couple of 435 gearboxes sitting spare, so it could be fun to put the Dyane back to how it should be.)

Thanks Ken

When its daylight I'll check the heat exchangers. I have a couple of complete 435's and a 602 sitting next to each other. Never noticed any difference but the 435 does have longer dump tubes so I guess there must be a subtle difference.

Cheers, that would be great if you have some spare - I will be having a look out at the French National this year as well.
Graham