Make a spark tester from an automotive cylinder head?

[ajgraz]

I've got a 1979 Mercury 70hp 3cyl. Want to check spark safely. I don't have a spark tester.

I was thinking, what if I took an old 3-bank cylinder head from a GM 3800 v6, grounded it to the OB powerhead with a big grounding strap, installed 3 automotive spark plugs with the ground electrodes cut off (to make a nice gap) into the head, hooked all three OB coil wires up (assuming they are long enough) and cranked the starter.

Any reason this wouldn't work as a spark tester? Any danger here to any ignition components? Or to life and limb?

 

[j_martin]

Re: Make a spark tester from an automotive cylinder head?

I've got a 1979 Mercury 70hp 3cyl. Want to check spark safely. I don't have a spark tester.

I was thinking, what if I took an old 3-bank cylinder head from a GM 3800 v6, grounded it to the OB powerhead with a big grounding strap, installed 3 automotive spark plugs with the ground electrodes cut off (to make a nice gap) into the head, hooked all three OB coil wires up (assuming they are long enough) and cranked the starter.

Any reason this wouldn't work as a spark tester? Any danger here to any ignition components? Or to life and limb?

If you're going to do that, just ground the shell of a spark plug to the engine. You don't need a cylinder head.

The problem is, though, that a spark needs to be able to jump 7/16" in free air in order to fire reliably at compression in an engine. Air is an insulator. Compressed air is a better insulator.

So go get a ten buck spark gap tester and do it right. If yer real cheep, you could make one with a couple blocks of dry wood, some screws and bailing wire or paper clips.

hope it helps
John

 

[ajgraz]

Re: Make a spark tester from an automotive cylinder head?

Yeah, OK, I see your point that it doesn't give me a 7/16" gap to jump.

Already got a junk head; guess I was just trying to save $10

 

[sschefer]

Re: Make a spark tester from an automotive cylinder head?

Yeah, OK, I see your point that it doesn't give me a 7/16" gap to jump.

Already got a junk head; guess I was just trying to save $10

Use a jumper cable to get your ground. An old set of steel core plug wires of the Studebaker pickup in the front yard should work as extensions. (red neck humour, I have a 53 Ford with a service body in my yard somewhere).

Bend the tang on the plug to open it up to 1/4". Opening up to 7/16 is something I've seen said a lot but it puts a heck of a load on the coils and so I don't go that far. You can if you want.

 

[ajgraz]

Re: Make a spark tester from an automotive cylinder head?

Hm, now that sounds like a vote to go ahead with my cockamamie idea. Now I'm confused, should I listen to the Captain or the Commander?

 

[sschefer]

Re: Make a spark tester from an automotive cylinder head?

Hm, now that sounds like a vote to go ahead with my cockamamie idea. Now I'm confused, should I listen to the Captain or the Commander?

John's method is the most correct. Your idea will work. I paid nearly 100.00 for my CDI spark gap tester. It'll do 8 cylinders at once and is the cat's meow. Before that I had a rig with spark plugs and #2 copper battery cable. It worked but I got bit a lot.

 

[cannonford57]

Re: Make a spark tester from an automotive cylinder head?

always the option of the local pawn shop and a 5 dollar timing light,,,just a thought.better than toting a car head around.

 

[CharlieB]

Re: Make a spark tester from an automotive cylinder head?

There is enough weight in my not-so-little-any-more to-go service bag, I for SURE do not want to and ANY cast iron, much less a multi-cyl head!

 

[ajgraz]

Re: Make a spark tester from an automotive cylinder head?

I've got a timing light; but that won't tell me whether I've got a nice blue spark or a weak-looking yellow/orange spark.

And I'd only be toting the head from the garage to the side yard, I wouldn't be putting it in the toolbox

 

[jeffnick]

Re: Make a spark tester from an automotive cylinder head?

I like the idea. Check all plugs at once the fun way. I wouldn't mess with the gaps - if it's a question of firing under load more sophisticated equipment would be needed anyway.

 

[CharlieB]

Re: Make a spark tester from an automotive cylinder head?

Using an adjustable spark gap tester set to a 7/16 inch gap is a very reliable load test of the gnition system module AND the low speed voltage of the stator.

The voltage required to bridge the 7/16 gap is very near that required to bridge the .032 to .035 gap of the spark plug while under compression.

The major problem with a spark gap tester is that it is not able to load test the high speed windings of the stator.

This is the advantage that the DVA Meter has for testing a stator, and is,or should be, a required tool in your tool kit.

 

[ajgraz]

Re: Make a spark tester from an automotive cylinder head?

Just got my DVA tester, will perform battery of tests this weekend. I just wanted ta show the kids some purty blue sparks! (I am part redneck.)