Miata B6 and MX-3 B6 differences

[clubman]

Hello guys,

New member here. I have a Miata 2004 EU spec with a B6 on it. The B6 found on it has the exact same block with the 89-93 miata 1.6 sold in the US. Just has a different head and intake and produces 110hp instead of 115 of the first generation.

A friend of mine had an MX-3 with a B6 (DOHC), MY 1997 and had it fully forged. So when he sold the car he gave me the engine block which I wanted to put in my miata along with the miata head as it has a different intake, has a CAS instead of a distributor etc. Now the engine is out of the car and I tried to install the Miata head in the MX-3 block. I had bought a Miata/MX-5,323 etc Cometic headgasket to use.

To my surprise I found out that the MX-3 block has 2 more oil returns between cyl 3-4 and 1-2 that the B6 miata block doesn't have. So then I started searching for a headgasket for MX-3 and I saw that there are also some other minor differences in the coolant and oil routes to the head. So the problem now is that the 2 oil returns are in the way the cometic gasket ends and probably won't seal properly. Please check the differences:

MX-3 head gasket


Miata Cometic Headgasket:


I searched a lot and found out that all B6s don't have these 2 oil returns except for the MX-3 and Xedos 1.6..

If I was to use the MX-3 head gasket I would loose compression ratio as it is 1.3mm as opposed to the 1mm of the miata.. There doesn't seem to be any 1mm mx3 1.6 dohc head gasket in the market.. I was then thinking of having one made here, or even fill those 2 oil returns with cement-epoxy and use the Cometic one as it would be able to seal after filling them.. I really don't want to remove ant material from the head because I don't wanna damage the cylinder chamber shape..

Any help appreciated,

Thank you

[KAinjection]

Why can't you use the mx3 B6 head then and save yourself the trouble? If it's like the BP your CAS and intake/exhaust manifolds will bolt up fine.

[clubman]

The intake manifold of the Miata won't fit.. Also the cam cover of the miata 90% doesn't fit and it has a CAS installed on it which is needed..

[CODY004WD]

I believe the '04 b6's are very much like the bp4w. Higher intake ports, and a CAS on the front of the cam cover, correct?

You could build a bracket to house/locate the CAS. Basically build a Jig to locate it while in the cam cover/on the engine. Then remove the CAS from the cam cover and remove the cover from the engine, and attach the CAS to the Jig, to see where/how you'd have to build a new bracket. (building the JIG you must be careful of tolerances, as you want the final position to be as close to stock as possible.)

Then you'd need an NA6 intake manifold, with the old style intake ports to match the MX3 head.

That will allow the use of the MX3 cam cover, on the MX3 head, while retaining 04 engine management.

[clubman]

I will keep the mx-5 head no matter what. I don't want to mess with relocating stuff, don't want to change the intake as then there will be idling problems etc. I just asked if you had any idea how to surpass the headgasket problem but I don't see there is a way instead of making a custom headgasket..

[shorestiva]

Tap the extra oil drains and put plugs in them to block them off. Then use miata headgasket.

[clubman]

That's what I thought at the beginning but the problem is that the Miata head gasket edge passes through these holes. So even if I put conical billet aluminum plugs the mechanic is sure that it won't seal properly over the years.. I tried hard to convince him but with no luck.. If you have a strong argument about that please state it..

Thanks for your interest!

[shorestiva]

A guy on festiva forums did that with steel pipe plugs and didn't have a problem but I dunno how long he ran the motor for.

Look into Z series headgaskets. I know the Zm-de head gasket has those drain holes but its .4mm thick. The coolant holes will be different (fwd coolant flow is different than rwd) but you could do a coolant reroute while you're at it.

[clubman]

By reroute what do you mean? Will the FWD passages work for RWD?

Thank you

[shorestiva]

Look around for info on the reroute on miataturbo.net.

All the coolant passages line up in the headgaskets its just that the holes are different sizes in different places. The holes are different so the coolant will circulate in different directions based on where the thermostat is (front of the head in rwd and back of the head in fwd). At least thats what I read when converting a 4g63 from fwd to rwd.

[clubman]

If you mean the Miata "coolant reroute" mod I've already done it. I thought you were talking about the head gasket passages. I don't know if I am correct but I am a bit concerned about installing a headgasket whose passages were designed for a different head, to the Miata head. I'm afraid that it may get hot in some areas and cold in some other..

[shorestiva]

I think it would make the reroute work better since it makes the coolant flow like in a fwd version.

[clubman]

I see your point.. Makes sense, only thing I don't know is if internally the MX-3 head's coolant-oil passages have different routing than the Miata's and don't know if this would cause any hot spots on the head etc..