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David,

Do you recall what the head angles were on the mill? Also, how much did you shift the head to keep the hole closer to the exhaust valve side of the head like the original? I'll end up coming to my own conclusions here but it would be nice to bump my findings against yours...

Here's another thought on the small plug and smaller 1.0" dia second plug hole. Since my plug ends measures .92 dia. the rubber boot will nearly be touching the heads fins when installed. The question that comes to mind here is will the close proximity of these parts deteriorate the rubber too quickly... Is this plug boot available in silicone???

Thanks
Scott
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75 911S 3.0 to 3.2 now to 3.4 Twin Plug (Inwork...)
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Old 12-14-2004, 09:59 AM
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Scott, I don't remember the exact angle but I do remember adjusting it a bit as I did the first head. I believe the port was 32mm and the largest bore size in the port was 36mm. I wanted the port to be about 41mm, so I set the angle for a taper a little less than from 41mm to 36mm at the required depth. I bored it to the depth of the 36mm bore then moved the mill head up about .040", milled again, raised the mill head milled again, etc. until I was at the 41mm port size. So I ended up with several little steps that I blended with a sanding drum.

I don't remember why I didn't just bore it straight from 41 to 36mm, but I must have had a reason at the time.
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Old 12-14-2004, 04:28 PM
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Dave,

Sorry if I wasn't clear, but my questions were concerning the second spark plug hole.
I do plan to open my ports as well so thanks for that FYI.

Thanks
Scott
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Old 12-15-2004, 09:59 AM
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I didn't write down any of the angles.
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Old 12-15-2004, 08:10 PM
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Dave,
I'm curious about the thread depth for the second spark plug. Many twin plug machining results in no or very slight relief cut in the combustion chamber. Yours seem to have quite a bit. Did you cut to match the length of the plug threads? Also, did you cc the heads and check the comp ratio after the work?
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Old 12-18-2004, 11:02 AM
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I set the plug depth on the 12mm the same as the 14mm. I cc'd the heads and a piston after the work was done so I don't know how much I lost. If you assume the relief is 1cc, then it's about a 0.08 point loss on my engine's compression. After cc'ing the heads I decided to use JE pistons to raise my compression from 7.2 to 7.7.
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Old 12-18-2004, 03:59 PM
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Plug depth

I managed to start on mine over the holidays, one side is done anyway...

Concerning plug depth, in my case it turned out that buy the time the sealing surface cleaned up I was already deeper than the original plugs. No problem though.
I replicated the original angles indexed the head 180 and moved 4 tenths (.4"). I went with the 14mm plug and will likley use the "F" (FR5DTC) series Bosch plug which has a 5/8" hex, that's what's in the pictures anyway. I hope they come in handy...

Scott

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Old 01-04-2005, 08:09 PM
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Scott: great machining work!

I have two suggestions:

1. Don't use those three-pronged spark-plugs, just ordinary Bosch (or NGK) ones.

2. Remove sharp edge where you went trough surface and try to replicate shallow "dent" in combustion chamber roof just like original plug has. You don't want any sharp edges in your combustion chambers as they heat up easily and cause knock, which negates the benefits of twin-plugging somewhat.

Yes, your C/R will drop a little but you can probably fly-cut cylinders to compensate or even raise it a bit (now that you have twin plugs).

Cheers!
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Old 01-05-2005, 02:11 AM
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I use those three-pronged plugs with excellent results...
Old 01-05-2005, 07:03 AM
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Scott, good work!

I cut the holes in the valve covers the other day and I was off by about 3/8". The problem was I used the upper covers to line up for the lowers and I guess since I couldn't actually slid the uppers over the studs I didn't get a good straight shot at the new plug holes. I could probably live with them, but I went ahead and ordered new covers that I'll cut correctly. I've got a rough AutoCAD drawing of the covers if you'd like for lining up for your's.
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Old 01-05-2005, 08:27 AM
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Great work. Thanks for describing the process.

Sherwood
Old 01-05-2005, 09:29 AM
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BeepBeep,
I just haven't got around to working the combustion chamber side yet but will soon. Thanks.

David,
Thanks for the offer on the valve cover spark plug holes, it will be a while before I work them.

I spoke with a Bosch technical adviser the other day while hunting for plugs and apparently they supply plugs to CART, IRL, & NASCAR. I found it interesting that they all use the twin ground electrode type plug. The racing plugs are hand made and the ground electrodes are machined instead of welded. Here's a breakdown on the Bosch spark plug numbering system, but not all of them are available in the U.S.A.

Scott

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Old 01-06-2005, 10:01 AM
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Air Injection Plug

David,

Where did you get the plugs that block the exhaust port holes left from the air injection pump? I guess I could make some but if they’re reasonable…

Thanks
Scott
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Old 10-04-2005, 09:38 AM
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I made them. I just used some aluminum rod. Notice I threaded all the way through to the port. Just to be safe I drilled a hole through the plug and wired it to the exhaust stud. As you can tell from my signiture, I'm still not up and running so I have no info on if they stay tight or not.
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Old 10-04-2005, 11:11 AM
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