air injection plugs in cyclinder heads keep falling out

[fredmeister]

Guys,
I have a 930 engine where I have plugs installed in the cylinder head exhaust ports that block off the M10-1 threaded holes for the air injectors (emissions stuff) that I have removed.

The plugs are the same items sold on Pelican for 914 rear brake caliper plugs. They are made of steel and from thermal cycling they keep coming lose and falling out on me.

I have tried high temp Loctite and even JB welding them in with no joy.

Now I am looking to replace them with aluminum plugs so the thermal expansion rates are the same and then have them tack welded in as well.

Anyone know where I can find aluminum plugs with the fine pitch M10-1 thread?

Also anyone know if a non structural tack weld can be made to work with the existing stainless steel plugs I have in there now? it just has to prevent rotation or the plugs from falling out of the threaded hole in the head. As far as I know you cant tack weld dissimilar metals. But I am no welding expert.

Its too hard a material to try and drill a lock wire hole in the existing plugs or I might try to lock wire them as in aircraft bolts.

thanks for any info,
Fred

[Spenny_b]

Fred, I had a similar challenge before I pulled my engine, one fell out whilst driving, sounded bloody awful (duh).

I was going to do exactly that, lock-wire them by drilling a v small hole in each of the neighbouring fins then wiring them so they can't unwind.

I got the holes fully welded up when the heads were removed for flowing, but that takes some serious heat in the heads to be able to fill the 10mm holes. How about buying a length of 10mm ally stock and an M10 fine thread die, and just making some plugs that you can then tack weld in, no special tools required. Again, you'll need some serious amps on the TIG to get it done, but more realistic than filling a hole.

I don't have an M10x1 die but if you want to go that route but don't want to make them, PayPal me $20 to cover the die and postage and and I'll go get one and turn some up on the lathe for you?

[fredmeister]

Originally Posted by Spenny_b

Fred, I had a similar challenge before I pulled my engine, one fell out whilst driving, sounded bloody awful (duh).

I was going to do exactly that, lock-wire them by drilling a v small hole in each of the neighbouring fins then wiring them so they can't unwind.

I got the holes fully welded up when the heads were removed for flowing, but that takes some serious heat in the heads to be able to fill the 10mm holes. How about buying a length of 10mm ally stock and an M10 fine thread die, and just making some plugs that you can then tack weld in, no special tools required. Again, you'll need some serious amps on the TIG to get it done, but more realistic than filling a hole.

I don't have an M10x1 die but if you want to go that route but don't want to make them, PayPal me $20 to cover the die and postage and and I'll go get one and turn some up on the lathe for you?

Hey man I do appreciate the offer. I mean seriously that is really generous of you to offer.

I will tell you what happened in the meantime. After I posted this message I started up back on the internet and found a place that sells aluminum pipe plugs. As I have a pipe plug in the hole in cylinder 5 already from previous falling outs whilst at the track for a DE day and having to McCyver a repair, I ordered 6 of them and will tap the remaining five M10 holes with an 1/8 inch NPT thread and install all 6 plugs. Then go out and find a shop that can TIG weld them in place.

Its funny but I ask other guys with old air cooled 911's whether they experience problems with these plugs always falling out and they say no. Makes me feel like a special case. Such a huge PITA especially when it happens at a track event.

thanks again,
Fred

[JFairman]

I installed the stainless steel ones with copper sealing washers that install with a 3/8" drive allen head socket wrench a long time ago. I got them from Pelican.
I put them in real tight with plenty of red loctite on clean threads and they've never come out on their own.
This is in 911 turbo heads.

I think you just need to put them in tighter and with red loctite on clean threads. You can use brake clean, q-tips, and compressed air to clean the female 10x1mm threads in the heads before installing them.
If you ever need to remove them, getting them very hot with a MAP gas torch will soften the loctite and let you screw them out.

Hope that helps.

[Spenny_b]

Hey no worries Fred, just ping me if needed

Yeah I've not used the red Loctite that JF mentions above, but I've got a bottle of the dark green 648....jesus, that stuff really does grab. It's a bearing retaining compound, if you go to install something that's normally a sliding fit (note, not an interference fit, as you must have some clearance space for the compound to remain and not be wiped back out), you'd better be quick about it when you slide them together, I've had it stick halfway, and it's an absolute pig to sort out.

Not sure what the temp rating is, but I'm sure Loctite will have something in their catalogue that's suitable.

(lol@ "McGyver a repair"...noooo, I've never had to do that before!)

[john walker's workshop]

The fix is 1/8" HEX head brass pipe plugs. They're tapered and pretty much the same thread as in the heads. Crank them in there tight and they rarely come loose.

[KTL]

Good tip by JW as usual. The M10x1.0 and 1/8 NPT (1/8-27 tpi) are very close to being the same thread pitch. So it appears using the 1/8 plugs in the existing hole (no rethreading needed) creates the thread interference to keep them in place.

Correct me if i'm wrong on this one, but isn't the tap/threaded hole the tapered part of the fastening? Usually the plugs are straight from what i've seen. I've only worked on a handful of tapered thread installations (915 trans plugs, cam housing spray bar plug replacement (used 1/8 NPT plugs), brake booster vacuum fitting on PMO manifold) and I thought the tap/hole was the tapered part.

We once had a friend lose one of his air injection plugs on a 2.7 at a DE event. I always bring a small bin of misc nuts & bolts & stuff in case somebody needs something. Sure enough that brake line tube nut (with steel brake line) came in handy as a temporary plug.

[RedCoupe]

With National Pipe Tapered Thread (NPT) which is commonly used in the US, both the male and female threads are tapered. There are other pipe threads in use that only have one surface tapered, so its likely that the parts you described aren't NPT. Don't know what thread the 915 plug is.

[KTL]

Thanks for the clarification on tapered stuff. The 915 is M24x1.5 and tapered, so it must be metric pipe thread? It has to be tapered because it's got no seat/flange on the plug, nor is there a bottom to the drain/fill hole.

There used to be this nice little website magneticdrainplugs.com that had some good information on metric pipe plugs. But it seems they went out of business. damaged transmission drain plug out, fluid changed. Wow!

[fredmeister]

Well an update on the aluminum pipe threaded plugs.

I got them from JEGS and cleaned out the thread after I tapped it with a pipe tap. This tap is tapered to match the taper on the 1/8 NPT pipe plug.

So I used a lot of red loctite and screwed it in as tight as i felt comfortable without stripping the thread in the head. It is holding so far after 3 days on track and local driving. So I will monitor it and hope for the best.

I had tried both cast iron and brass pipe thread plugs and they all fell out over time. I am sure it was due to the different thermal expansion rates of those materials versus aluminum heads.

The aluminum plugs are the best match obviously and so far so good.

Appreciate the responses from everyone on this thread.

Fred

[Spenny_b]

Cool! Fingers crossed for you mate.

S