Head button machining for turbo glow plug

[Mike Rappold]

A friend of mine asked if I could machine him a .21 size head button for a turbo glow plug for an RC drag car application. I can machine the chamber side with no issues. I am trying to figure out how to machine the threaded hole and mating seat for the turbo glow plug.

Was hoping someone on forum has machined a turbo type head button and had some guideance on the method/procedure used. Any help would be appreciated.

Mike Rappold

 

[John Beardslee]

Hi Mike,

I used a # 4 combination countersink & center drill and ground (with my tool post grinder) the O.D. down to .272 the tap drill size for 5/16-24, the thread size of the turbo plugs. The countersink angle on the combination is already 60 degrees. I haven't used it yet but I know I'll probably have to open the pilot hole but haven't determined the size yet. Do a search on here, I think there was a thread a while back that may have detailed the process.

Thanks, John

 

[Ian Inverarity]

Novarossi, O'Donnell etc turbo plugs have a M8 x 0.75mm thread.

 

[John Beardslee]

Hi Ian,

You may be right, when I checked the thread with a pitch gage it looked like 24 threads per inch and the diameter is close enough to 5/16 to be the same and with standard plugs being 1/4-32 I just figured it was a SAE thread. I do know from my heli engines my O.S .91's have a SAE thread and my YS.91's have an M8. I have been known to run an M8 tap thru the 5/16-24 flywheel and use it on the YS and visa versa.

So Mike, check your machinery handbook and check the diameter of the tap drill for an M8 x 0.75 thread and grind your countersink and center drill to that diameter.

Thanks, John

 

[dwilfong]

There was a post on this before and I think Jack O'Donnell said a .110 hole for the opening to the chamber.

I have bin tossing this around for some time now just have not had the time to grind the center drill yet.

 

[Ian Inverarity]

7.25mm is the nominal tapping size, I have used a bit smaller and it worked okay. I aim for 4.6mm opening into the chamber. I find it a bit fiddly to do with a modified centre drill tho, I would like to be able to cut the sealing face with a cutter on a CNC lathe. If the seat face doesnt have a good finish etc the plug wont seal.

 

[John Beardslee]

David, I like the turbo plugs mainly because I'm paranoid about stripping threads and those on the turbos are really stout a plus is not having a compression washer to keep track of.

Ian, wish I still had access to a cnc. The college I retired from is a Haas training center and we had a lot of nice stuff, plus now it's a 2-1/2 hour drive so it's not possible to spend much time there anymore. For the few heads I plan to do I have to live with the center drill option. I used a new one but will now have to pick up an M8 plug tap and bottoming tap. If I don't get a good seat surface I may remove one of the cutting faces on the center drill to make it into a single edge cutter as those usually produce less chatter.

Thanks, John

 

[Ian Inverarity]

John, I have access to a CNC lathe at work, so am fortunate! I dont have a tool small enough to cut the sealing face however. I have been using a specially sharpened drill to cut the sealing faces but it is too blunt now and doesnt do a god job. Cutting the sealing face is the 2nd last operation prior to tapping the plug thread, I do it in a mill, but it takes a while and the only way to know if it worked is to fit to an engine with a plug and a bit of light oil and see if it leaks! If it does the head button is pretty much useless, I have tried a couple of times to re-cut but haven't managed to get it working.

The single cutting edge is a good idea I think.

 

[dwilfong]

I like the small foot print the plug will leave in chamber. Would be killer for the new chambers I am trying.

Would be a pain in the you know what if you go threw all that work and the head was junk in the end.

 

[John Beardslee]

Hi Ian,

The right aluminum or what ever material you are using is also really important I would think in getting a good seal. What cutting fluid is also a factor in the surface as well.

David I think the turbo plug is a perfect match for the torodial head design - can you imagine doing one for a .21 or .12!

Thanks, John

 

[dwilfong]

Yes that is the plan but the time to make the tooling and do some test cuts is the problem right now.

If there was only more time on my hands.'

Just one of those things on the back burner right now.