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

[Josh]

Yes these two oil passages can be a headache when mix matching parts.

Is there anything wrong with the MX3 head? I would suggest running that head as usually people swap the MX3 head onto the miata as it flows a lot better. the intake and exhaust ports are quite a bit larger. All of your electronics from the miata should swap right over to the MX head. even the coolant passages can be adapted with if you have the ports available from the miata.

Either way if you are not using the complete MX3 block with head or miata block with head and are mix matching you will need to plug off one end or the other. many 323 and miata guys used to swap on the MX head, all you would need to do is tap the holes and find a plug that will go into them that is flush. I suspect you should be able to do that to the block as well.

I found out all of this the hard way when I bought a never run built motor off of a guy and they ran a Miata head gasket in it. Oil everyware. I had a 1mm copper head gasket made. I suspect the difference .3mm will not cause enough difference for you would notice.

[clubman]

Exactly Josh, you got it spot on!

Regarding the use of the MX-3 head, there are some issues that need to be solved. First of all the valve cover of the Miata won't fit to the MX-3 head. This makes some trouble. First of all the CAS sensor cannot be fitted. Even if I drill n tap the sensor on the valve cover, then its the cam gears where on the MX-3 do not have marks on them for the CAS to read. Miata cam gears cannot be fitted as they are larger in diameter. Next, I use Toyota cops which do not fit on the MX-3 head as it is too narrow.. I also have done a lot of work on the Miata head and valve cover. So it is unfortunately not an option to swap heads.

The solution I finally followed is to make a 1.2mm head gasket with steel o-rings (didn't make sleeves/grooves on the block or head, they are pressure fitted once you install the head). The remaining part of the head gasket is made from reinforced permanite, something I really don't like as I will be boosting 1.45bar on a GT2560R and I just pray to not having any water/oil leaks.

I really liked the idea of tapping the block oil returns but the mechanic was not a fan of that and insisted on the custom made head gasket.

I have contacted Cometic but they don't look to care at all to make a custom head gasket. They don't even reply. May I ask your opinion on the current head gasket setup and where I can make a custom MLS head gasket or copper like yours?

Thanks for your help, I really appreciate it..

[Josh]

I believe it was a company in Florida who made it. I would have to go digging through my files to find out where exactly.

The problem with using a custom head gasket is that the miata head is missing those ports, SO there will be no material on the head side to seal the gasket around the holes on the block. It is cast iron, You could find someone to weld them up and then resurface it.

[clubman]

The miata head has no holes but has full material. If you mean that because there are no holes on the head the rings of the headgasket will not seal yes that might be the case but I was planning on making a head gasket with no rings, just a sheet of copper or metal. I guess it would seal..

[Josh]

Correct, they are different castings so the miata head does not have the material added to the head, so even if you got a gasket to cover the holes in the block the miata head would not put any pressure over the holes in the block, they would be left half exposed. You have to plug the holes in the block.