Spark plug reach

V

vilanar

Senior Member
Hi folks,
I did take off my Offy 400 heads from 8BA , and looking the spark plug electrodes they seem to be rather deep in the pocket. Combustion chamber shrouds them quite a bit so that spark don´t have a
straight shot....
I have used NGK 6L and Autolite plugs .
Have you modified the combustion chamber or machined the head so that
plugs goes deeper in the head ?
What is the safety margin from plug to valve ?
I tried to search but didn´t find anything for this subject.
Thanks,

Ari
 
you can use a longer plug or use a powertip plug. I also use a large 45 Degree end mill to clearance down to the first thread. Yes some hi lift cams can hit the spark plug in the older style heads, However Offy and Edelbrock both began moving the plug up. To bad they didn't move it into the transfer area.
 
Excellent drawings. It shows how a power tip plug might fire the chamber with a 45 degree relief
 
I agree with Ron, "excellent drawings", but it troubles me that all of them show the plugs recessed in the heads. That can't be the way the manufacturers designed them can it? I am going with "Edmunds Custom" heads that have the spark plug moved enough so that the electrodes miss the valves. This seems to make a lot of sense to me; why didn't the other manufacturers figure this out? It seems to be another case for using 3/4" reach "surface gap" plugs in this application. There was a thread on this a few days ago; I for one, will be interested in what that poster finds out. I will repeat again that I had a very successful experience using surface gap plugs in a 276" Merc with a Isky Max-1 15 years ago.
 
I did not like the recess of the electrode even though the plug was between the valves on my Eddie Meyer, 21 stud heads. I too used the chamfer but used blank plugs in each hole and found the depth of chamfer does vary from hole to hole. I did not cut all the way to the first thread but to the end of the NGK plug body (minus the electrode), which does not have a thread.
 
vilanar said:
Thanks , guys.
Now I know what to do.
Click to expand...
I don't know what you mean by this, but I think $20 spent on a set of plugs to try beats the hell out of pulling your cylinder heads and doing some irreversible modifications.
 
Is the machine work on the OFFY heads complete crap like my manifold? I opted for Edelbrock heads after seeing the crap quality of the OFFY product. The Edelbrock's are beautiful. The one I got are good to .400 lift. my cam is .375 but the deck was milled a little so I had trouble closing the gaps on Champion rc12yc's . I replaced them with autolites that have the same reach, but the electrode is about 1/16 shorter, then I added Moroso indexing washers to shim the plug out a little more. Now she runs gRRRReeeaaT!!
 
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