What he said ^^^^^^
With 2 important warnings:
Buy a quality bump switch from somebody like OTC, with properly insulated clips that stay on and a UL rated 12v switch with THICK WIRES. The amp load on a cheap switch can burn up on first use with cheapo bump tools. This works without starting a fire or electrocuting you.
This unit is $58.00. That's a fair price for a tool you will not burn your rig up under normal use. You can remote start your fireworks display with it too!
The light turns red when you have power.
When adjusting your valves, it's indespensable. This thing loves the fan...
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The only reason I do not advise new guys to use a bump switch is the risk of electrical shock/damage, shorting out the starter fuse-able links and the B/S quality of the vast majority of the bump switches out there.
I recommend to new builders to use a D-Cell timing light... especially with any computer controlled 7 wire ignition module.
The D-cell light is isolated and does not pick up static from the electronics, which your Dad's old timing light is susceptible to.
This unit is $95.00. You will not get a ghost reading because it is isolated from the electrical system. Same situation as the bump switches. Get the right one for the proper money the first time.
If you read this and buy the cheapo knock-off version of this light, with the sub-standard pickup and wire gauge, non insulated wires and it grenades the first time you plug in.
You are forewarned, the cheapo electrical tools,
for this application, will ruin your day. You are paying for the very expensive black pickup in the picture.
You want the expensive pickup. The other's are a joke, when tested for accuracy.
You'll want to wrap electrical tape around the battery cap to keep from dumping them during use. Remember to take the batteries out when you are done using it, or they will corrode and ruin the light. This thing loves the fan too.
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******* Please Remember,
There is lots of DC current and 700-800 cold cranking amps, flowing through your battery hot lug to your starter.
You are going to want to preserve your soft fleshy bits, as they will be next to the GROUNDED frame, while attaching the clips for the Bump Switch.
You can easily weld your hand to the frame or ruin stuff down there if you are not positive, what you are clipping on to.
If this scares you, (and it should), try removing the ground wires from your battery, then attach your welding switch, then carefully put your ground back on the battery.
If you see lightning, remove the ground connection immediately.
Do NOT skip the ground removal step, when the clip falls off three or four times in a row.
You will think you can skip it, when you're angry they fell off.
Resist the stoopid urge to die.
You absolutely can stop your heart wiring a bump switch, if the DC voltage goes from one hand through your heart to your other hand on an active ground.
A properly installed Battery Disconnect switch on the ground side works well for this type of repeated work on your engine. The $95 300A rated one, (not the cheapo). Now you can switch your battery ground off when these situations arise.
"IF" you show up at the hospital with your fingers welded, your surgeon will know exactly what you did to yourself, before he even speaks to you in the ER.