2nd plug hole: roll or cut tap?

[tadd]

I've done about three sets of twin plug heads a while back and was setting up for another set. I was wondering if using a roll tap for the plug threads was a smarter idea? I've never used a forming tap but have read they are far produce far superior threads in 'stringy chip' materials like aluminum.
Thanks in advance...

[cgarr]

I use three different taps for the plug hole, 1st is a .25mm under size then I use a custom tap then chase it with a thread chaser to burnish, this keeps the plug snug, a standard tap will leave a loose plug. I only twin plug with 12mm plugs, are you using 12 or 14mm?

[tadd]

cgarr:
12mm plugs. In the 2.x heads there isn't enough meat. I did my first set back in 2006 following the DIY thread. I also add 1.5mm of shift towards the center of the head (in addition to the 0.375" shift back towards the exhaust. Otherwise the seat is 'blemished' (it's on the outside enough not to matter, but I prefer a clean seat fir the plug). I started with a standard class iii tap and ended up using helicoils like the early heads (pre 68, iirc).
Moved to a class four, and that was a much better fit.
I like the idea of cut undersize, then burnish. I bought a class 4 roll tap and tried a few holes in a piece of 6061 (convient, not the same) driven by hand using a center in the chuck for alignment. It was non trival to drive. However it fit the plug well.
I think I will multi step it as you do. Makes alot of sense. Once you have the form cut undersize, do you follow the other two operations while still 'chucked up' or free hand after removal? I use an old BP series one. The slop in the quill (along with the wax trick) keeps me from breaking fins since I feed by hand. I havnt had any luck using the power feed.
Thanks!

[cgarr]

I keep the head chucked in the Mill and do all the tapping. Since your using the 12mm plugs, go out and get a helicoil metric tap, I think its the one for the 10mm helicoils, that tap is perfect for the first cut, its just under-size of 12mm

As far as fin damage, try a ball end mill then finish the hole with an end mill, a 2 flute ball cuts without chatter.

[tadd]

Nice tip with the ball end. One of those slap your head "why didn't I think of that" ideas.

I'll call tapco in the morning on the helicoil tap. I think the roll tap is an H6, so no slop, mid tolerance. From what I've read the roll taps final thread form is greatly dependant on the starting hole size due to the two sides pushing the material into the 'peak'. Following up after an undersize cut tap should eliminate that me thinks.

Thanks again!

[ajwans]

Is there any advantage in using a helicoil like the factory did for the first hole?

Was the factory intent that if the threads were crossed the remedy was to simply use
a new insert instead of taking the head off for a thread repair?

If not, why did they helicoil the plug holes?

andy

[neilca]

tadd,

I too am about to machine some 2.2/2.4 heads. Can you tell me the degrees you move your mill head? Can you further explain the .400" offset to the exhaust port? I have read this in another post and never really understood which direction it was moving. Did you make a jig to hold the head or are you using a vice?

Thanks,
neilca

[cgarr]

Maybe I can help. I use a fixture to position the head on the table then set the mill head to 24 and 12.5 degrees. Here is a short video of how we do it. Set your speed at 1000 RPM

[neilca]

Wow, Thanks Craig for the settings and the video. I will try it on a head tonight and verify I have all the cutters I will need.

[cgarr]

I never used measurements to set the head up. I have two old heads I use as jigs to setup the mill, both early and later because of the size difference but the mill angles I use are the same for both. Here is my 2.7 head which should be the same fixture as yours. I measured from the studs to show where to start the hole and they just so happen to be the same at 1.665 and thats where you put it. If you use the mill angle setup the plug hole should pop out centered right between the two valve seats. Just make sure when you do set it up that the head is square on the mill bed or that changes all the settings. Also you will have to remove the one stud in the head to clear the mill.

[ajwans]

Craig, would you mind posting your measurements for the smaller heads?

Why don't you use a helicoil in the second hole like the factory did with the
first hole?

andy

[cgarr]

Andy, These are 2.7 heads and should be the same for early heads, head stud spacing is the same just the cylinder bore would be smaller for the early heads but the 2nd plug is still in the same place. I have never used coils in the plugs. I don't know why they used them but the threaded head works just fine without them.

[tadd]

Neilca:
Sorry I missed this! I wasn't holding out, honest!

I hang the head from the quill with an 8" rod turned to 3/4 on one end and threaded for the original hole on the other with a plate turned with a lip to fit the cylinder recess. Then I just adjust the quill pitch and yaw till it sits flat on the table and clamp with fingers. Now unbolt the head and rotate 180*. This however will not be mirror symmetry, so one adds a 'left/right' table translation to correct (other wise you'll end up in a seat!).

I used canning wax that I beat between the fins with a rubber mallet to avoid breakage. The ball end mill avoids that crap (thanks cgarr). Bore away!

The tap was the trouble spot for me, but a call to tapco solved that. Cgarr's trick of burnishing works nice if you use a cut rather than roll tap.

I can take some pix if your still interested.

Again, I'm sorry I didn't see this when it was posted!

t

[neilca]

Not a problem Tadd. I found the info I needed in one of your old posts.