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Using 731 head compared to standard 200 head
Posted: Mon Aug 11, 2025 8:30 pm
by Atle_323i
I´m having issues with probably blown head gasket on my 323i. So I was thinking about upgrading to 731 head instead of using the standard 200.
As I have googled the 731 is bolt on like the 200 head. Since the head has larger inlet ports, can I use the inlet manifold from my 200 head, or do I have get a new?
I read that the gasket for the inlet is larger, so I need to use from E30 320i, is that correct?
Does this sound like a good idea, or is it just wasted money using 731 head?
Re: Using 731 head compared to standard 200 head
Posted: Mon Aug 11, 2025 9:10 pm
by uwbuurman
It will fit, but i doubt it will run. 731 head sits on a block with larger pistons, hence a bigger bore, largers chambers in the 731 head. So you'll have a compressionproblem at least, but thre is a chance that the valves will hit the block with the smaller e21 bore. 731 head is often used for the 2.7 upgrade with the normal ETA pistons.
Re: Using 731 head compared to standard 200 head
Posted: Mon Aug 11, 2025 9:26 pm
by Atle_323i
uwbuurman wrote: Mon Aug 11, 2025 9:10 pm
It will fit, but i doubt it will run. 731 head sits on a block with larger pistons, hence a bigger bore, largers chambers in the 731 head. So you'll have a compressionproblem at least, but thre is a chance that the valves will hit the block with the smaller e21 bore. 731 head is often used for the 2.7 upgrade with the normal ETA pistons.
Ok, the I can just scrap that idea. Damn Chatgpt, telling me not true things
Are 200 head from 84 525e same as the 200 head on E21?
Re: Using 731 head compared to standard 200 head
Posted: Mon Aug 11, 2025 9:52 pm
by Jeroen
Nope, less camshaft bearings
Re: Using 731 head compared to standard 200 head
Posted: Mon Aug 11, 2025 10:49 pm
by Atle_323i
Jeroen wrote: Mon Aug 11, 2025 9:52 pm
Nope, less camshaft bearings
Thanks for the reply.
So if my head is gond, then I need to source a genuine 323i 200 head, or go for an E30 conversion.
Re: Using 731 head compared to standard 200 head
Posted: Mon Aug 11, 2025 10:57 pm
by Jeroen
If I'm not mistaken the E21 323i head is the same as the E12 520/6, E21 320/6 and early E28 520i.
Re: Using 731 head compared to standard 200 head
Posted: Tue Aug 12, 2025 8:55 am
by Atle_323i
uwbuurman wrote: Mon Aug 11, 2025 9:10 pm
It will fit, but i doubt it will run. 731 head sits on a block with larger pistons, hence a bigger bore, largers chambers in the 731 head. So you'll have a compressionproblem at least, but thre is a chance that the valves will hit the block with the smaller e21 bore. 731 head is often used for the 2.7 upgrade with the normal ETA pistons.
But the 731 head is used on 323i for E30, isn´t that the same engine as E21 323i? And the difference is the intake manifold, as E30 has electronic injectors.
Re: Using 731 head compared to standard 200 head
Posted: Tue Aug 12, 2025 9:26 am
by uwbuurman
Not sure what all the differences are, but the e30 323i head is according to BMW not a straight fit for the e21 323i. For one thing, the pistons are different. E21 and e30 323i share the crankshaft and rods, but have different pistons.
The top of the piston (the deck) has to match with the valve chamber in the head. The space between head and block determin the compression. So for one thing, your pistons and a 731 head don't seem to match. Hence your compression is likely going to be a problem.
I'd source a 200 head
Re: Using 731 head compared to standard 200 head
Posted: Tue Aug 12, 2025 9:43 am
by Atle_323i
uwbuurman wrote: Tue Aug 12, 2025 9:26 am
Not sure what all the differences are, but the e30 323i head is according to BMW not a straight fit for the e21 323i. For one thing, the pistons are different. E21 and e30 323i share the crankshaft and rods, but have different pistons.
The top of the piston (the deck) has to match with the valve chamber in the head. The space between head and block determin the compression. So for one thing, your pistons and a 731 head don't seem to match. Hence your compression is likely going to be a problem.
I'd source a 200 head
OK, thanks for the reply.
I will have to take apart my engine, and have the head checked. It might be all good.
The reason for cheking out 731 head, is that there is someone locally selling one cheap. A 200 head might go for alot more I assume.
But as Jeroen said, it can check for E12 520i and E21 320.
Re: Using 731 head compared to standard 200 head
Posted: Wed Aug 13, 2025 10:19 am
by Duracel79
Hi all,
Combustion chamber in the 200 and 731 heads are identical the valves are the same size as well. Differences are the ports and angle of the ports and the transition to the inlet manifold. 323i inlet fits onto a 731 no problem. Opening up the ports to match the 731 ports would help. You'll need the larger 731 inlet gaskets (although I think they are nla now),so using the larger 885 325i inlet gaskets on a 731 head is required. Image below shows the 3 types of inlet gaskets, top 200, middle 731, bottom 885.
The piston difference is E30 has a small crown (~0.7mm) which ups the CR to 9.8 from 9.5. the e21 323i pistons are flat top. both have half moon cutouts for the inlet valves.
Both E21 9.5 and e30 9.8 pistons will fit the 731 and the 200 heads.
As Jeroen mentioned the eta 200 heads do not have all the cam bearings drilled for oiling and single weak valve springs as they are not required to rev high.
hope that helps
Ben
Re: Using 731 head compared to standard 200 head
Posted: Wed Aug 13, 2025 2:56 pm
by Jeroen
Re: Using 731 head compared to standard 200 head
Posted: Wed Aug 13, 2025 8:37 pm
by Atle_323i
Duracel79 wrote: Wed Aug 13, 2025 10:19 am
Hi all,
Combustion chamber in the 200 and 731 heads are identical the valves are the same size as well. Differences are the ports and angle of the ports and the transition to the inlet manifold. 323i inlet fits onto a 731 no problem. Opening up the ports to match the 731 ports would help. You'll need the larger 731 inlet gaskets (although I think they are nla now),so using the larger 885 325i inlet gaskets on a 731 head is required. Image below shows the 3 types of inlet gaskets, top 200, middle 731, bottom 885.
The piston difference is E30 has a small crown (~0.7mm) which ups the CR to 9.8 from 9.5. the e21 323i pistons are flat top. both have half moon cutouts for the inlet valves.
Both E21 9.5 and e30 9.8 pistons will fit the 731 and the 200 heads.
As Jeroen mentioned the eta 200 heads do not have all the cam bearings drilled for oiling and single weak valve springs as they are not required to rev high.
hope that helps
Ben
Thanks alot for the details. So actually I can use the 731 head, with a minor tweaks and other gaskets on the inlet manifold. I will see how my old 200 head is when I have it out. Need to make a conclusion from there. Still very nice information
Re: Using 731 head compared to standard 200 head
Posted: Wed Aug 13, 2025 10:31 pm
by Jeroen
Agree, better have the head pressure tested first, you may be lucky and only have a blown head gasket even though M20 heads do tent to crack near the cam bearing tunnels.
Re: Using 731 head compared to standard 200 head
Posted: Sat Aug 16, 2025 10:32 am
by e21Keith
After having 200 heads fail on two occasions due to cracks under the cams I gave up and fitted a 731 head. There were no problems at all with this install other than the inlet ports being larger, so I used the e30 gaskets.
I had 3x 200 heads in my stock, two had visible cracks under the camshaft, the third looked good, I even had it crack tested but it also failed, I believe the action of just torquing down the head can cause it to crack from my experience.
Incidentally when building a 2.7 its the 855 head which is preferred.
Re: Using 731 head compared to standard 200 head
Posted: Sun Aug 17, 2025 8:42 am
by uwbuurman
885 head is only possible for 2.7 builds when you use the super eta or e30 325i pistons. And the 325 pistons have to be the the kat version. The non kat version pistons have to be grinded down a bit on the skirt. I also took 2 mm of the top deck of my ETA block and fitted an adjustable camsprocket
