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Spark Plug Adapter Fabrication

I have a motorcycle with a tiny piston and engine; it's an Aprilia SR50 scooter but it goes like hell. I have a gage and have read compression from small engines before. For an accurate reading the volume added by the tester must be minimized, or measured for compensation with calculations.

This scoooter uses the small spark plug, too, M10x1.0p. Like a model airplane or weed-whacker. I don't have the adapter to 1/8 NPT for my gage. Rather than continue to search to buy one, I thought I'd make one. So here we are.

The thing about spark plugs is the steel threads are heated up, red- hot I think, and as they cool they compress the tough ceramic core, so that thing never breaks because it's pre-stressed like a car windshield or my shop glasses.

I tried hammering it outl there is, like, *no way*. I tried heating it red hot and dropping it in the toilet to quench it. No luck, Chuck. :(

A(n) M10x1.0p tap is cheaper and much easier to use than a die, so I can tap a fitting to take the plug end for the air quench, then tap the plug end to 1/8 NPT for the gage boss instead of trying to thread a piece of steel, brass, or aluminum with the die on one end and the pipe tap on the other to make the adapter. That remains an option.

I was going to drill out the copper core that conducts the spark energy, heat it up and run water through it, but you can't seal that joint and so water would spray everywhere, so I think I'll start with a fresh (used) plug, connect it to an air supply with a long tube, all parts metal, set up some fire bricks to contain the heat, get it red hot through and through, then quench it with the air line. Air cools as it expands. If the quench is fast enough, the ceramic will spall and shatter from the inside, leaving the steel I want just sitting there.

I think it'll do it, although it may send a spray of razor-sharp ceramic fragments along the air jet. I can deal with that hazard with screening and fiberglass and such like sundry items.

Do you think this would work?

Did I get it right about the assembling and prestress, and the geometry?

Do you use and repair small engine? Where do you buy *your* M10x1.0p spark plug to 1/8 NPT male adapters?

Douglas (Dana) Goncz Replikon Research Seven Corners, VA 22044-0394

Reply to
The Dougster
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I think you are working too hard. Break the ceramic top, and chip out the remainder of the ceramic with a punch, then braze a piece of 1/8 pipe to the remaining metal part

Reply to
Bill Noble

From bitter experience of a snapped off spark plug in an engine head - chipping out the ceramic, followed up by a SQUARE easy-out will do it. When I tried a spiral easy-out, it compressed the remnants of the metal shell into the threads.

Reply to
Louis Ohland

Try heating and quenching, or just heating if you have an OA torch. I've found the ceramic comes out pretty easily.\

As far as compensating for the volume of the tester goes, the better ones have a tire valve in the adapter itself. If the adapter was at the gauge you'd have to compensate. At the adapter, there's no volume to compensate for.

John Martin

Reply to
jmartin957

A little rust and the ceramic in my boats sparkplug came flying out on it's own. And you are correct, getting the remains out is hard. Without the ceramic insert, the nut portion collapsed.

Reply to
Calif Bill

I had one, ceramic was snapped off, and then it was drilled out, and a metal tube was epoxied in the full length, tube was steel, I think. no leaks

Reply to
green

Carbide-tipped drill won't do it?

Reply to
Joe Pfeiffer

If it's like most spark plugs you cut the rolled shoulder on the top of the base that seals the insulator and the insulator will fall out. I've even done it with a hacksaw.

Reply to
Rick

I have never had success with those spiral easy-outs - for exactly the reason you state - avoid them

Reply to
Bill Noble

Too bad for you that they stopped making plugs like this one:

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Jeff

Reply to
jeff_wisnia

I would drill the threaded part of an M10 bolt lengthwise and screw it into an M10 tapped hole in a 1/8" pipe cap. Perhaps the seating surface could be turned on the nut that locks them together.

jsw

Reply to
Jim Wilkins

Snap-on has "short" and "long" reach M10 x 1 adaptors:

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Snap-on adaptors have the Schrader valve located down at the spark plugend of the adaptor, so you don't need to do any "calculations" or"compensation" to get accurate readings.

Reply to
David Courtney

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My tester has both size threads. Stepped fitting. But a cheap press in tester with the rubber end will do both.

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10 bucks with all kinds of adapters.

Reply to
Calif Bill

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Why work from an existing spark plug. Turn the plug thread on a lathe, and drill and tap the other end for the pipe thread. Use your choice of materials. It does not need to be the steel that a spark plug is made from, since it is not going to be exposed to the heat of actual combustion. For that matter, I would probably make one from brass or

12L14 steel hex stock to provide wrench flats as needed.

Granted (given the cross-posting), not many of the sci.physics readers may have lathes, but most of the rec.crafts.metalworking readers will. And rec.crafts.metalworking is where Doug usually posts. (Or at least, where I usually see him.)

Enjoy, DoN.

Reply to
DoN. Nichols

Chuch the plug in a lathe and turn off the part that is "crimped" over the ceramic - the ceramic will just about fall out - then braze or hard-solder the correct pipe fitting to the remaining shell.

Reply to
clare

Clarence and Rich win for the identical suggestions that the rolled edge may be turned to shavings on the lathe, revealing something like Jeff's Maytag plug. I wish I had one that disassembles! Mine is currenlty beige; perfect in every way from the fuel injection assist air filter and tube cleaning I did.

DoN wins for his suggestion to buy something at a reasonable price. But you'd expect that from DoN.

I used to have the neatest tire tool. Not only would it remove cores, it would ream the seat that the little tapered plastic seal sits into when you tighten that core head. I've installed a core into M5x0.8p threads in the plug tip using an M5 rivnut seated there, but the seat escapes me. Maybe it's pin taper 0. I have a 2 and a 3. I tried the 2; it felt close. What fun I had! I haven't produced with the Super Shop in years.

Got 'er up to 65 on MD 4 S to the Shoah for a day in the Sun this Sat. (giggle) What fun we had. Vitamin D megadose, never felt better, lasted until 3 AM Sunday morn.

Cheers mates, good to hear from all y'all, as we say Down South.

Douglas (Dana)

Reply to
The Dougster

Woops! That was Calif Bill with the link to just buy the damn thing. Right on!

Thanks to DoN for the detailed instructions.

Doug

Reply to
The Dougster

I ended up with a set of 10, 12, and 14 mm brass compression tester adapters, each with an o-ring seal. I turned down a brass Schraeder truck tire valve, the kind with brass body, two rubber washers and a nut, not the car kind with molded overlay. I put the valve in the adapter body with Loctite Red Bearing Mount. It's setting now.

This will give a more accurate result, eliminating nearly all dead space in the hose connecting adapter and gage. It remains to adapt to that hose.

I just need 2 more truck tire valves to finish the other two adapters.

The shop reported low compression, 30 psig, on my scooter engine. With no valves, just ports, the ring and bore are the only thing that can go wrong, unless somehow the injector leaks HP air. I've got rings and gaskets, so unless the cylinder needs boring, I'll be set to go soon.

I got the Colortune adapter, too.

Cheers!

Doug

Reply to
The Dougster

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