Switching engine in 1990 850 to that from a 1988 750?

[exalted]

Been lurking in the forum since I bought my run down but lovely 850 a few years ago and wondering about the advisability of transferring an engine from a 1988 750 into a 1990 850.

The m70 in the e31 currently probably have headstud or head gasket problems that is minor unless you push (5k+ rpm) the car when it loose water. No water in the oil however. The engine in the e32 has a few thousand more kilometers but no bad behavior so far except that it is connected to a worn out gearbox.

Is there any technical issues to moving the m70 from e32 to e31?

Is it a bad idea for some other reason?

If it isn't a stupid idea something beyond the obvious replacement of water pumps, spark plugs and so on that should be done while the engine is out?

[cartoonz]

early M70 from E32 has smaller exhaust valves.
Along with the valve covers, the front timing cover must be swapped and correct water pump installed.
While you are in there... at a minimum
Banjo bolts.
New upper & lower pan gaskets, rear seal.

Might actually be better off pulling the original engine and sending the heads to a machine shop for a full valve job and surfacing... as long as they are not cracked, and the bottom end is good (takes a LOT to hurt those), you'll end up with a better engine for the cost of the head refresh and gaskets.

[Belgiumbarry]

here i am with Cartoonz, better to repair ( machine/new parts) the broken engine than to hope a similar old one will be the solution... if it compares in costs.
Worn parts and risks for break down are the same in all M70's with approx the same mileage unless you find a recently rebuild one .

On the other hand , if you can buy a good running M70 for a "bargain" , you can do the swap and have time to see if repairing the original one is cost advisable ….

[exalted]

On the other hand , if you can buy a good running M70 for a "bargain" , you can do the swap and have time to see if repairing the original one is cost advisable ….

That was the plan since the complete e32 is less then the head gaskets so swap and then see how bad the original engine is was the idea.

Not sure if its worth it if its not a simple swap.

Now I guess the e31 will simply have to sit still in the garage while I do it the proper way.

[dragon850]

That was the plan since the complete e32 is less then the head gaskets so swap and then see how bad the original engine is was the idea.

Not sure if its worth it if its not a simple swap.

Now I guess the e31 will simply have to sit still in the garage while I do it the proper way.

It is a pretty simple swap, yeah the valves are slightly smaller but the early heads actually flow better according to some, so you will not notice much difference.
You will need to transfer valve covers, front upper timing cover and water pump.
You actually do not have to transfer the upper timing cover or valve cover gaskets if you do not mind removing the air cleaner box when you need to change the engine oil.
So very straight forward.

[pollack138]

Something else to consider (financially) if you are thinking of buying a cheap e32 for this. Lots of useable/sellable spare parts especially the differential which is very likely a 3:15 posi which 6 speed owners are looking for.

[exalted]

You will need to transfer valve covers, front upper timing cover and water pump.

Sounds easy enough, perhaps the project is still on. Just got to find another e32 the one I was looking at was gone before I had time to call the seller.

Something else to consider (financially) if you are thinking of buying a cheap e32 for this. Lots of useable/sellable spare parts especially the differential which is very likely a 3:15 posi which 6 speed owners are looking for.

I was kind of thinking in those ways cheap enough an e32 and the parts could actually make the engine swap free except for a few work hours.