Originally Posted by RGregor
I've been running botton 14mm plugs in my modified 90/6 engine a long time ago. No problems with cracked heads. Heard of but never saw one.
With the 1000ccm and 1070ccm engines I always ran 12mm bottom plugs (heard of cracked heads and didn't want to see mine crack).
Long plugs with an aluminium spacer.
Always NGK BP7ES for the top 14mm plug and NGK D7EA for the bottom plug.
No pinging/fouling or whatsovever problem at CRs of 11.7 for the 1000ccm engines (tested up to 12:1) and 11.2 for the 1070.
Thanks for the NFO!.
I also ran 14mm lower plugs in my /6 heads with no problem. These are drilled 12mm so I'll work with that.
I got NGK BR8ES for the top. Doesn't have the extended nose of of the BP~. See how it works. Then DR8HS for the bottom. Local auto parts places had them and I figure if I can get them in this hickhole I can get them anywhere. Cheap enough to try out and not cry if I want something else.
if they work well I'll go to Platinums or Irridiums. I'm reconsidering spacers for the lowers to make that happen. I'd feel comfortable with threaded steel spacers installed on the plugs.
I have some GR5 plugs I just took out of the old heads and I think I'll take a hacksaw to them. The lower body may be just right to fabricate a set of spacers. I have neither a lathe nor a surface grinder so I would have to have the work done by a machine shop.