Difference between '75 and '76 block and heads other than fuel systems
 

I have a '78 450sl with a rod through the block. Last year I parted out a '75 450sl and kept the motor and transmission.

Can I use the block and heads from the '75 on the '78? I have heard that the heads may be different. If that's the case, can I use the block from the '75 with the heads and upper motor from the '78?

Any ideas would be helpful

The 78 and 75 engines were different, with 75 being the last D-Jet and 78 being K-Jet. That having been said you can either make your 78 a D-Jet and use the whole engine, or you can swap the whole upper half from your 78 engine on to the 75 engine.

The 75 heads are different because 75 has mechanical ball studs for valve adjustments vs the hydraulic ones on the 76+ engines. So the heads from the 78 will fit & work on the 75 block, but you will need to move cams etc - but like I said, the whole 78 upper half can & will work on the 75 block.

"So the heads from the 78 will fit & work on the 75 block, but you will need to move cams etc"

I was following you until you mentioned the "move cams."

From what I'm taking I can pull the motor from the '78 and take everything off the top (heads, intake, fuel system) and drop the block from the '75 in using the parts from the '78 to put it back together.

Do I have this correct, or do I need the cams from the '75 for some reason?

Sorry, I'm not the mechanic, just the guy who knows the internet well enough to get the info and relay it to him. Want to make sure I explain it correctly.

Thanks!!

Why did the '78 throw a rod? Many times this is due to broken timing chain guide(s). If so the heads on the '78 may also be destroyed.

Other than seeing the hole in the block, we haven't done much exploration. I assumed the rid got sick of being in a tight enclosed space and made a break for it. But now that you've thrown that scenario, makes me wonder...

so if the timing is good, will the '78 heads work in the '75 block with no modifications? If they are bad, I might just have to find another set of heads.

If you jumped time and that's why the rod is bad, the heads will be bad as well, and the cams may even be cracked too.

The reason when doing the swap you need to move cams too is because the hydraulic cams are NOT compatible with manual ball studs and vice-versa.