Engine Trouble Quick question CNC “Racing Heads”.

[Carl Walichnowski]

That's what I originally thought I needed to use with the MZ "R" head, but the piston hit the ground strap and closed the gap. The NGK BPR7EIX was just that little bit too long.
The E refers to the length of the threaded section.
I think they should have made the head thick enough for the E length plug because a bit extra thickness there would make it easier to unscrew my compression tester.
Anyway as it is, with the BPR6HIX there's some threads inside the head that aren't covered by the plug, so they're bound to hold onto sooty deposits which I hear can glow and be a cause of pre-ignition or whatever it's called.
It's like they made the head just in between the spark plug sizes so that it is impossible to get a plug that's exactly right.

So choose carefully. Before buying a new plug with the E length, check the clearance under the ground strap with your H length plug. Use some soft putty like Play Dough maybe or make some dough? Idk.

As you can probably tell I forgot what the measurements are that the H and the E refer to. I think H is 10mm and E is 14mm long, but I'm not completely sure so you best Google a chart of NGK plug codes.

Anyway I don't think a too short plug would prevent starting. A too long plug would, if the gap is squashed closed by the piston.
If you have difficulty turning over the engine then maybe that's just the high cranking pressure?
If the engine will not turn over even when the plug is out, then maybe you don't have a thick enough head (or base?) gasket and the piston isn't clearing the head at all.

OK after looking into this a bit more the NGK BPR7EIX is 19mm or 3/4'' and NGK BPR6HIX is 12.7 mm or 1/2", I'm going to try the NGK 7333 BP6ES Standard Plug that Street Ryderz referred to, hopefully that works, it's also 3\4" but since it's cheaper I can use it as tester and if it does work then use it to do a plug chop. So your saying there's no spark plug betwwen 12.7-19mm which would work best for us?? And let me ask what plug did you eventually go with??

 

[FurryOnTheInside]

OK after looking into this a bit more the NGK BPR7EIX is 19mm or 3/4'' and NGK BPR6HIX is 12.7 mm or 1/2", I'm going to try the NGK 7333 BP6ES Standard Plug that Street Ryderz referred to, hopefully that works, it's also 3\4" but since it's cheaper I can use it as tester and if it does work then use it to do a plug chop. So your saying there's no spark plug betwwen 12.7-19mm which would work best for us?? And let me ask what plug did you eventually go with??

I think you really should check that you have the clearance over the piston crown for a 1/4" longer plug before you go ahead and buy an "E" plug, it's just as easy to stick some Blu-Tack or whatever you call it in the USA into the head and crank the engine over. As Gary mentioned all the plugs with the P in the code have a projected tip so they're really very long indeed.

We don't even know for certain that plug length is what's causing the problem with your engine but we've focused right in on it for some reason. TBH just from looking at the picture in your original post it doesn't appear to be any different from the heads that take a "H" thread plug perfectly well.

I just used the BPR6HIX (that I had already from before I got the MZ "R" head and the longer plug that I thought I needed), but I might get a BPR7HIX for longer trips in the summer. I have had no actual problems with the H length.

 

[gary55]

The only reason I'm going along with the chance that the plug depth is a poss. reason is from what he has said that it all started when he switched to the new head, and what I have experienced with the different performance of the two heads I show in reply #4. Their chambers are nearly identical and they both sealed well, but with the differing plug depths and same plug on the same engine the one with the recessed plug just ran terrible while the R head ran great. Their is always the chance the head didn't seal well, or that coincidentally something went awry at the same time. At the same time if I recall the head you have is a 6cc head and will have a shallower chamber than the heads I have shown. You can easily mark the top of the piston with a felt tip and place it on the head with the plug installed to check clearance.

SAM_0870

• gary55
• Jun 8, 2018

 

[Carl Walichnowski]

I think you really should check that you have the clearance over the piston crown for a 1/4" longer plug before you go ahead and buy an "E" plug, it's just as easy to stick some Blu-Tack or whatever you call it in the USA into the head and crank the engine over. As Gary mentioned all the plugs with the P in the code have a projected tip so they're really very long indeed.

We don't even know for certain that plug length is what's causing the problem with your engine but we've focused right in on it for some reason. TBH just from looking at the picture in your original post it doesn't appear to be any different from the heads that take a "H" thread plug perfectly well.

I just used the BPR6HIX (that I had already from before I got the MZ "R" head and the longer plug that I thought I needed), but I might get a BPR7HIX for longer trips in the summer. I have had no actual problems with the H length.

Well I'm basically troubleshooting right now and I'm hoping this will resolve my problem. I've switched the carb as I mentioned, double checked all the wiring, and this weekend my plan is to clean out my tank and check the petcock since ity seemed like the gas was flowing poorly. I'll take some pics to see if you guys can tell by the photos if it's the lengh of the plug that's the issue. If it's not the spark plug then I'm at a loss.

 

[Carl Walichnowski]

Another question I have, what is the amount I'm to tighten the Head down using the torque wrench once I put that back on?

 

[gary55]

Another question I have, what is the amount I'm to tighten the Head down using the torque wrench once I put that back on?

I go 168 in. lbs. = 14 ft. lbs.

 

[Carl Walichnowski]

I go 168 in. lbs. = 14 ft. lbs.

Thx that helps.

 

[Carl Walichnowski]

Ok here's another thing I was thinking may be playing a part in the MB not turning over. I wasn't sure the width of gasket to use when I was putting my head on, I think I have three or four different widths of the copper gaskets and I used the middle one. Would this impact the motor not turning over? How do you even determine which width of gasket to use??

 

[gary55]

When you say "not turning over" I assume from what you said before that you mean it won't start. Generally when you say a engine won't turn over it means it is frozen and the piston is somehow stuck not allowing it to move freely. So to select the best gasket size do a forum search on "new cnc head less top speed", and look at reply #60. Use 1/16" rosin core solder and shoot for a squish gap of .8mm

 

[Carl Walichnowski]

When you say "not turning over" I assume from what you said before that you mean it won't start. Generally when you say a engine won't turn over it means it is frozen and the piston is somehow stuck not allowing it to move freely. So to select the best gasket size do a forum search on "new cnc head less top speed", and look at reply #60. Use 1/16" rosin core solder and shoot for a squish gap of .8mm

Sorry for the delay, your correct the bike will simply not start. I’ve tried everything I can think of. The bike has spark, I put a new carb on and went back to the stock head. Can’t figure the problem. Here are pics I took of my spark plug in the head. Thx for any advice.