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I've designed my own jig to dual plug my heads (42/40 valves). I'm pretty sure it will work once I weld it up. I have a few spare, damaged heads to practice on. My question is, what size plug should I use in the lower spot. I've read that 10mm would be ideal because more aluminum is left behind to prevent cracks. Obviously there must be a reason for different diameter spark plug threads. Is there a disadvantage to a little skinny 10mm vs a 14mm thread plug? Life span? performance? More liable to strip out? Apart from durability, I'd like to use 10mm, simply because it would be much simpler to machine.
About the issue of cracks caused by 14mm plugs in the bottom. I obtained(for cheap) a trashed set of heads that were dual plugged by someone a long time ago. The heads have stripped plug holes, recessed valve seats, burnt valves, warn out springs and broken fins. The bottom holes are 14mm. No sign of combustion chamber cracks at all.

What size where the valves on that buggered head?

I seem to remeber that the 10mm plugs where impotant on the R100 big valve heads.

Dr Curve's thoughts: http://mailer.fsu.edu/~jroche/road_rod.html

Garnet wrote:Dr Curve's thoughts: http://mailer.fsu.edu/~jroche/road_rod.html

I've been looking for that article. Thanks.

Garnet wrote:What size where the valves on that buggered head?

I seem to remeber that the 10mm plugs where impotant on the R100 big valve heads.

I guess the question is, if 10mm plugs are as good as 14mm plugs, why go larger? The old dual plugged heads are 42/40's. Obviously with big valve heads, the need for extra metal is even more important. 44/40's vs 42/40's is still only 1mm difference in the distance of the seat from the spark plug. Of course, the other benefit of 10mm plugs is that you need to remove a lot less fin area from the outside of the head.

After surfing around a bit I could find no agreement on plug size. Oak and Snobum say that 14mm is fine but add that you should weld up the new plug area so you can run the same long reach plug that is in the top.

MotoBins uses 12mm plugs on R100s and 10mm on R65s.

Couldn't find any info on MI's sight but their pics look like they use 14mm.

I'm sure you've read all this and more.

Garnet wrote:After surfing around a bit I could find no agreement on plug size. Oak and Snobum say that 14mm is fine but add that you should weld up the new plug area so you can run the same long reach plug that is in the top.

MotoBins uses 12mm plugs on R100s and 10mm on R65s.

Couldn't find any info on MI's sight but their pics look like they use 14mm.

I'm sure you've read all this and more.

It doesn't appear to be a science. Bob's BMW locates the second plug slightly lower in the chamber.
If 10mm is equal to 14mm (can't find any data to the contrary) I'll go that route.
I can always bore them to 12mm and add aluminum later if they don't hold up.

I asked the same question to Jim cray in the uk who twin plugs engines as part of his upgrade package, after reading the dr curve article, Jim said that he has never had an engine come back with cracks and he uses 14mm short reach plugs, I suggest that you use a grade hotter on the bottom as it helps reduce oil fouling. Hope this helps. Charles

I have used 10mm in the bottom of a big valve head (lots of material left afterwards) and have found suitable NGK spark plugs. Used to use BPR7ES on the top and D6HA in bottom IIRC.

Most current four valve superbikes have 10 mm plugs, and as they are putting out close to 200 BHP/ liter they must spark all right.

I have D6HAs in the bottom, but it looks like I could use one grade hotter.