Overheated 730

[thepitscars]

Hi all - a friend has a 1992 730 that was overheated pretty badly - the head is junk (cracked) and I have seen advertised a recon head from an E28 535 - (for a very good price) -- can I bolt it on there ? Cheers

[shogun]

something must be different, but I do not know the details, the M30B30 head of the 1992 E32 730 has this number 11121706569 Cylinder Head From: 09/01/1986 To : - ,used on E34 and E32 https://www.realoem.com/bmw/enUS/par...BMW-730i&mg=11 the head of the E28 525 has p/n 11121278702 https://www.realoem.com/bmw/enUS/sho...diagId=11_0188
Supersedes: 11121284291(03/01/1982 — 07/19/1985), Exchangeable retrospectively 11121277707 (03/01/1982 — 11/16/1981)
Part 11121278702 was found on the following vehicles: 5' E12 (07/1974 — 07/1981), 5' E28 (09/1980 — 08/1990), 6' E24 (10/1975 — 04/1989), 7' E23   (10/1976 — 11/1989)

[RobertRO]

Hi all - a friend has a 1992 730 that was overheated pretty badly - the head is junk (cracked) and I have seen advertised a recon head from an E28 535 - (for a very good price) -- can I bolt it on there ? Cheers

The M30 cylinder head of a 1992 E32 730i should have the casting# 1708497.
The M30 cylinder head of an E28 535i (manufactured post '85, I believe) should have the casting # 1277358.
You can find the casting number on the intake side of the cylinder head, between intake ports of cylinder #4 and cylinder #5.

Now, these two heads are almost identical: same intake and exhaust port size and shape (and therefore same gaskets), same intake and exhaust valves, same camshaft, etc.
The difference between the two may be:
- it seems (but it's still to be confirmed!) that the 1708497 head got some of the intake runner shape and combustion chamber improvements of the M30B35 head (casting# 1708843);
- the combustion chamber of the 1708497 is allegedly larger. Based on the information you can find in the Internet, the combustion chamber of 1277358 head has about 58 ccm. I measured the combustion chamber of a 1708497 head and got about 63 - 64 ccm (disclaimer: since I used a syringe for that purpose, the measurement may not have been very precise). Unfortunately, I never got the chance to measure a 1277358 head.

In my opinion, the 1277358 E28 535i head should work without any problems on the E32 730i M30. Everything that is connected to the cylinder head just bolts on.
If the combustion chamber is indeed smaller that the one of 1708497 head, then you will end up with an engine having a compression ratio of about 9.7 - 9.8 :1. That's actually great news, just make sure you use high octane gasoline (98 RON, or more).
There's a very tiny risk that the M30B30 pistons will not clear the 1277358 head, considering that its combustion chamber is smaller. However, by looking at the shape of the combustion chambers of both 1277358 and 1708497 head, I believe that it will work. Nevertheless, you may want to test it before assembling the engine.

If you do this change, then please take some time and measure the combustion chambers of the two heads, and also compare the shape of the intake runners.
Then please report back. It would be really useful to clear this subject once for good.

[thepitscars]

Thanks very much for the replies - I did some internet research and from what I could gather the combustion chamber would be bigger ? Which I guess would make the motor happier on RON91 fuel - $1.60 litre here - I was wondering about the head gasket to use - I suppose the one to suit the E28 - another potential issue is the nose that goes on the front of the head and chaincase - the recon head has been faced but does not seem to come with this part - the original is ok - may have to resort to a piece of glass and some sandpaper - so I guess it will work !!

[RobertRO]

I did some internet research and from what I could gather the combustion chamber would be bigger ? Which I guess would make the motor happier on RON91 fuel - $1.60 litre here - I was wondering about the head gasket to use - I suppose the one to suit the E28

If I understood correctly, you intend to install an E28 535i head (casting# 1277358) on an E32 M30B30 engine. In that case, the combustion chamber will be smaller than the one of the original M30B30 head casting# 1708497, which means that the compression ratio will increase to about 9.8:1 (factoring in the resurfacing of the 1277358 cylinder head). Therefore, you will need higher octane rating (recommended: 98 RON).

[...] another potential issue is the nose that goes on the front of the head and chaincase - the recon head has been faced but does not seem to come with this part - the original is ok - may have to resort to a piece of glass and some sandpaper - so I guess it will work !!

You mean that the front timing case was not installed on the head during resurfacing. Trimming it with sandpaper should be your last option. You may be lucky and get it good enough to work, or you may end up with a permanent oil leak and/or a misaligned distributor shaft versus timing case shaft seal.
The best solution is to fit the timing case on the head and go back to the machinist, to shave it correctly. It is an easy job that should not take too much time, so it should be cheap.