MZ heads, which plug (reach)?

FurryOnTheInside

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I was about to buy a BR7EIX when I stopped myself and went to check that the BR8EIX plug was clearing the piston at TDC, just in case.

(I have just fixed another problem with the ignition but haven't been able to test again. I wasn't really expecting to have two problems that prevent starting!)

I have found that the plug has been hitting my piston. :eek::mad:
Squish gap is 0.55mm so it's inside the 0.5-1mm range.
There is a small mark on the piston and the 0.03" plug gap is closed.
I had tested the bike I was able to get the engine to turn over.
So I believe it is only a small amount too long for clearance, but I'm sure it would work best if it didn't reach all the way to the piston so I don't really want to look for a plug indexing washer.

What a waste. Why are MZ Miami recommending such a long plug?:mad:
Or did I get the wrong information from somewhere? I am "sure" that information came from them.

What plug do other members use in their MZ heads? :)

What length/ letter do I really need? o_O

I have a BPR6HIX so I have just put that in. Is that any good? :)
I think H is the nearest alternative. I don't really know about spark plugs. :oops:

Thank you :)
 
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Did you mill the head or decide to use a thinner head gasket?
I just used twisted Teflon tape as a gasket which gave me a squish gap of 0.55mm to maximise the compression.

(This isn't the cause of the plug being hit by the piston, because even if the squish was set to 1.0mm that's only 0.45mm higher, but the piston completely closed the spark plug gap which had previously been 0.030".
0.45mm is 0.0177" so if the squish gap was 1.0mm the plug would still be hit by the piston and its gap would have been closed to 0.0133")

A stock aluminium gasket I tried gave a squish gap of 1.2mm (0.047") which is outside the recommended 0.5-1.0mm for a squish gap. This would allow the plug to clear the piston by only 0.017" but is still the very bottom of the combustion chamber so not really what I want.
 
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Well, if it didnt hit before and it does now id say the teflon failed you..lol
 
Well, if it didnt hit before and it does now id say the teflon failed you..lol
It didn't hit when I tried a stock aluminium gasket because I didn't fit the plug or start the engine, I just fitted the aluminium gasket to see what the squish gap would be (1.2mm).
I can calculate that the piston would barely clear the plug even with the aluminium gasket.

A 1.2mm squish gap is outside the 0.5-1.0mm effective range.
I don't want to drop the compression and lose all my squish just to use a particular plug.
 
It didn't hit when I tried a stock aluminium gasket because I didn't fit the plug or start the engine, I just fitted the aluminium gasket to see what the squish gap would be (1.2mm).
I can calculate that the piston would barely clear the plug even with the aluminium gasket.

A 1.2mm squish gap is outside the 0.5-1.0mm effective range.
I don't want to drop the compression and lose all my squish just to use a particular plug.
NGK B7HS. Iridium is nice if it fits, but the B7HS works fine and the advantages of the irid. plug don't outweigh the advantages of a good squish clearance.
 
NGK B7HS. Iridium is nice if it fits, but the B7HS works fine and the advantages of the irid. plug don't outweigh the advantages of a good squish clearance.
I just fitted an H plug. An NGK BPR6HIX so I have the H thread and the iridium. :)
I just wanted to know if this is the correct thread length for the MZ head. I was worried it's too short now. I guess I was lucky. :)
Thanks :)
 
I just fitted an H plug. An NGK BPR6HIX so I have the H thread and the iridium. :)
I just wanted to know if this is the correct thread length for the MZ head. I was worried it's too short now. I guess I was lucky. :)
Thanks :)
The plugs I use on these heads are about 9/16" from face of washer to end of electrode. You could also use a indexing washer if you really want the irid. plug.. Just curious, Have you done a compression test yet?
 
The plugs I use on these heads are about 9/16" from face of washer to end of electrode. You could also use a indexing washer if you really want the irid. plug.. Just curious, Have you done a compression test yet?
B7HS is the same H thread length as the BPR6HIX I have just fitted after removing the BR8EIX, so I DO still have iridium. I'm sure it will be fine. It's just a bit of a faff to take the head off now to see how much thread is exposed or recessed, easier to just ask! :)
Yes I have done a compression test with the tTt gasket and it is at least 125psi. I stopped pedalling at 125psi as it wasn't increasing much after that, and I was making noise in my living room pedalling the drive chain around and pumping the engine.. I didn't want to annoy the neighbour above me, lol. :oops:
I know that I could get higher compression with the longer E thread plug + an indexing washer but that would leave exposed threads in the combustion chamber. I am happy to use the H if it isn't too short and that's what others are using successfully. :)
 
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