Setting Spark Plug Gap

It occurred to me that maybe some of our new mechanics out there were unaware of the easy and "proper" way to adjust spark plug gaps. (Bashing the ground strap on the bench to close the gap is not recommended.)

The "special tool" required isn't all that special; most of you already have one or more of them and don't even realize it. Virtually all "spark plug" feeler gauge sets have one attached.




Depending on the QC of whoever stamped out the tool, it's sometimes necessary to clean up the edges with a small file to prevent gouging the insulator.

 

I'll throw in my bit of XJ advise - the factory gap is 0.028- 0.032

Since XJ's are not the greatest at throwing a spark, I gap at 0.027 to help out the electrical system. The gaps get larger as they wear anyway.

Re-using plugs?? Dress the electrodes "square" with a small file before re-gapping. The spark jumps easier from the new, sharp edges ( by 25% or so)

 

On the 650/750/900s, the bikes with 14 mm plugs.

For the 550s the factory gap is 0.6~0.7mm (.024"~.028")

Don't forget we've got a lot of new 550 owners on the site.

 

(a year later) - - actually, flat feeler gauges aren't the way, they suggest the round feeler gauges are more accurate.

But back to my point - - the smaller end of the "gap range" will help out the inherent defects in a 30 year old electrical system.

And today's XJ's will have bad alternator brushes, bad ground straps, iffy batteries, crusty electrical connections thruout the harness, cracked coils, degraded secondary leads (the sparkplug wires) and cracked or fouled plugs.

anyway - the new guys should gap a bit "short". (friendly advise).
until the bike is fully sorted.

 

True. I use wire spark plug feelers myself. Good point, I didn't have a pic. Will remedy.

 

Yeah either the ramped disc or the circle of box shaped bent wires

 

Now wait - the ramped disc would be just as bad as the flat feelers - -

Filing the electrodes square is "the ticket" as they arc-off 25% easier.

 

Just bought new plugs for two of my bikes and a few were tight so I forced a feeler gauge in between to open the gap. It would be nice to have a feeler gauge just for plugs but of the three I have none were purposely built for that. I used to have one years ago but it disappeared. Also if the gap is wide I usually just give it a slight bump with the handle of a screwdriver.

But yeah it is better to do it the proper way. Especially when showing those new to the trade as having a feel for things sometimes requires time and experience. Like torquing bolts. The only time I put a torque wrench on a bolt is when tightening head bolts. The rest I go by feel. I do not recommend that method as if you have not been wrenching long you probably will not want to go through snapping bolts off like I did to develop the feel required. ''Oh I think it is tight enough. Well maybe just a smidgen more.''
SNAP.
Oops!

 

the book says a torque wrench is only about 25% accurate and a experienced wrist is 35%, so your not far off.
did you ever not believe your torque wrench and quit before it clicked?
i did

 

Does it really say that?

No I bought into what it told me. I have used the click kind and the old type with the pointer across the scale. Those were a trick to hit the mark at times.

And what's with these new torque settings on some auto engines? Changing the head gaskets on the 3.1 chevy and it says torque to 45 pounds, or something like that, and then an additional 95 degrees. I am like what?. What is wrong with saying 45 and then to 110 or whatever it comes out to?

 

These are "angle torque" specs. BMW has been doing this for 30 years or more. The idea, as I understand it, is the initial torque is what it takes to compress the gasket and the second spec is the specific bolt stretch the engineer wanted. Because the bolt has a known thread pitch, this is a very accurate way to specify bolt stretch. You might notice that the longer the bolt of a given diameter, the greater the number of degrees specified.

I am only an amateur mechanic, but that is my understanding. If some of the pros on here have a better explanation, please chime in.

 

Different bolts as well, the bolts have a stretch factor built in.

Normal tension wrenches measure twisting force. Clamping force is what is important. For example if one bolt has oil under the head, 100 foot pounds of turning force clamps the gasket well. If the bolt head is dry, 100 foot pounds of turning force gives less clamping force.

This new tech came about when technology caught up with us. Engineers have pressure sensors that take the place of a head gasket and shows actual clamping force, and now we have 'stretch' bolts and new tension ideas.

 

Another thing to add, with the ramped disc gauges, the hole that they are generally hanging from in the store is not only for display, but rather for bending the ground strap as well.

 

Along with these are really simple to use, as Fitz has already mentioned:

http://www.amazon.com/Lisle-67800-Spark ... roduct_top

 

The stretch bolts are called "torque to yield" and to be more on topic the best way to go are wire type feeler gauges. I just bought a new one for the xj as I didn't have one that went small enough for this ignition was around 3 dolllars at autozone.

 

For clarification . . .
It is acceptable, perhaps even encouraged, to set the spark plug gap below the manual's specification?
I have always targeted nominal (.030"), but putting a fresh set in this weekend.

Re-dressing questions:
It was mentioned earlier to file the old plug's electrodes square. Is that saying that you file the ground electrode so that it is parallel to the surface of the center electrode?
Would you also want to file the end of the ground electrode perpendicular to the surface of the center electrode? Creating more defined sharp corners for the spark to jump from?

If the above is true shouldn't new plugs be 'dressed up' to have sharp corners and thus making them even more efficient . . .?

 

The book specifies a range. The gap widens as they "wear." So if you set them toward the lower end of the range, they will stay within spec longer.

I have never in nearly 50 years working on not only motorcycles but vehicles of all types ever filed any part of a spark plug; and have never had any issues as a result. Honestly, that's a new one on me, can't help you there. I just make sure they're properly gapped and in they go. Then they get properly torqued.

And don't get me started on the new torque-to-yield "stretchy" bolts. The rear hub retaining bolts on my VW go to 133 ft/lb + 180 degrees. Not easy to accomplish.