What effect does changing the spark plug gap have?
Thread Starter
|
Road Captain
Joined: Oct 2009
Posts: 744
Likes: 8
From: Pluto
What effect does changing the spark plug gap have?
I gapped my plugs at .40 (split the difference from the manuals suggestions of .38 - .43) but I was wondering what the effect is if I would gap at .38 or the other end at .43?
Outstanding HDF Member
Joined: Jan 2010
Posts: 2,916
Likes: 31
From: earth
A tighter gap is typically used in a higher compression motor, I think the tighter gap makes a smaller but more powerful spark to resist flame out, or whatever the tech term is. I would say experiment with it, try the .043 and .038, see if you feel any difference, either way you're not going to hurt anything. I've been messing with indexing, side gapping, and colder heat ranges, My opinion the side gap does feel like it makes a stronger running motor, but it does shorten the life of the plugs. I've always been an advocate of indexing, it positions the ground strap so it doesn't shield the spark from the air/fuel charge.
Outstanding HDF Member
Joined: Jul 2010
Posts: 2,727
Likes: 6
A tighter gap is typically used in a higher compression motor, I think the tighter gap makes a smaller but more powerful spark to resist flame out, or whatever the tech term is. I would say experiment with it, try the .043 and .038, see if you feel any difference, either way you're not going to hurt anything. I've been messing with indexing, side gapping, and colder heat ranges, My opinion the side gap does feel like it makes a stronger running motor, but it does shorten the life of the plugs. I've always been an advocate of indexing, it positions the ground strap so it doesn't shield the spark from the air/fuel charge.
When indexing, how can you tell where the fuel comes into the cylinder so you know which way to point the plug?
Tourer
Joined: Nov 2007
Posts: 442
Likes: 4
Before installing plug put a mark on the ceramic part of the plug showing where the open end of the plug is, meaning the opposite of the ground strap. Then you may need some indexing washers that come in different thicknesses. Screw in the spark plug all the way and look at where the mark is. You want to line it up to the opening of the intake manifold on each side. Once you can get the mark lined up with the opening, your good to go.
Tourer
Joined: Jan 2011
Posts: 335
Likes: 0
From: Leesburg, FL
Spark plug gap
Have heard of people gapping at .035 instead of recommended .038-.043 (.040) supposedly to reduce pinging?? Does that seem right??
Most mechanics gap to lesser number to make plug last longer as it wears. If you change plugs often (they ARE pretty cheap) no problem.
Experts- what do you say??????
Postman
Most mechanics gap to lesser number to make plug last longer as it wears. If you change plugs often (they ARE pretty cheap) no problem.
Experts- what do you say??????
Postman
Outstanding HDF Member
Joined: Jan 2010
Posts: 2,916
Likes: 31
From: earth
My indexing method:
1. Draw a line on the plug insulator, in line with the ground strap
2. Dry run the plugs to minimum torque value and note the position
3. Add required thickness copper washer to rotate the open gap facing the intake valve
The alignment doesn't need to be perfect but thats what I always shoot for. If you need just a little more rotation, try the upper limit torque value, if you still need a little more, loosen and retighten, you will gain a little. Sometimes you can get lucky by swapping plugs around and playing with the torque and retightening, and don't need any shims. The indexed plug provides optimum spark exposure to the incomming AF charge to promote faster, stronger flame propogation, and the incomming AF charge helps keep the plug gap cleaner for longer. The only problem I experienced was finding 12mm copper shims, I ended up getting some close to what I needed and then drilling them out with a custom fixture. I think there are some kits on the market but they are relatively expensive for what you get. Possibly more research would yeild some shims that would work, but McMaster Carr and Maryland Metric didn't have them.
jjnoble, with my method, on the rear cylinder, align the mark between 4-5 'oclock as viewed from the side, and the front cylinder align between 7-8 'oclock, that'll get the gaps pointed about right at the valve stem, dead center.
A lot of people call this snake oil but I gaurantee if you index plugs in a 6 or 8 cylinder vehicle, you will feel the power increase, you may not feel it on a 2 cyl motorcycle but I'm sure it helps a little.
1. Draw a line on the plug insulator, in line with the ground strap
2. Dry run the plugs to minimum torque value and note the position
3. Add required thickness copper washer to rotate the open gap facing the intake valve
The alignment doesn't need to be perfect but thats what I always shoot for. If you need just a little more rotation, try the upper limit torque value, if you still need a little more, loosen and retighten, you will gain a little. Sometimes you can get lucky by swapping plugs around and playing with the torque and retightening, and don't need any shims. The indexed plug provides optimum spark exposure to the incomming AF charge to promote faster, stronger flame propogation, and the incomming AF charge helps keep the plug gap cleaner for longer. The only problem I experienced was finding 12mm copper shims, I ended up getting some close to what I needed and then drilling them out with a custom fixture. I think there are some kits on the market but they are relatively expensive for what you get. Possibly more research would yeild some shims that would work, but McMaster Carr and Maryland Metric didn't have them.
jjnoble, with my method, on the rear cylinder, align the mark between 4-5 'oclock as viewed from the side, and the front cylinder align between 7-8 'oclock, that'll get the gaps pointed about right at the valve stem, dead center.
A lot of people call this snake oil but I gaurantee if you index plugs in a 6 or 8 cylinder vehicle, you will feel the power increase, you may not feel it on a 2 cyl motorcycle but I'm sure it helps a little.
Trending Topics
Road Warrior
Joined: May 2008
Posts: 1,802
Likes: 2
From: Salt Lake City, UT. Dushore, Pa.
My indexing method:
. The only problem I experienced was finding 12mm copper shims, I ended up getting some close to what I needed and then drilling them out with a custom fixture. I think there are some kits on the market but they are relatively expensive for what you get. Possibly more research would yeild some shims that would work, but McMaster Carr and Maryland Metric didn't have them.
.
. The only problem I experienced was finding 12mm copper shims, I ended up getting some close to what I needed and then drilling them out with a custom fixture. I think there are some kits on the market but they are relatively expensive for what you get. Possibly more research would yeild some shims that would work, but McMaster Carr and Maryland Metric didn't have them.
.
http://youroilshop.com/plugs.htm
HD Forum Stories
The Best of Harley-Davidson for Lifelong Riders
7 Surprising Harley-Davidson Products that Are Not Motorcycles
Verdad Gallardo
8 Best Harley-Davidson Motorcycles Ever
Pouria Savadkouei
10 Worst Harley-Davidson Motorcycles Ever
Pouria Savadkouei
Killer Custom's Jail Break Is The Breakout That Refused to Blend In
Verdad Gallardo
Crazy Bunderbike Build Looks Amazing, But Is It Impossible to Ride?
Verdad Gallardo
Harley-Davidson Reveals Super Cool Cafe Racer Concept
Verdad Gallardo
Engraved Rebellion: Inside Bundnerbike's Glam Rock II
Verdad Gallardo
10 Motorcycles You Should Never Buy
Joe Kucinski
10 Things Harley-Davidson Needs to Fix in 2026
Verdad Gallardo
Outstanding HDF Member
Joined: Jan 2010
Posts: 2,916
Likes: 31
From: earth
Negligible, would not even consider the impact on CR when indexing. I haven't done the volume math, but raising the plug .010-.030" is not going to upset the perfectly balaned tune in your motor. Off hand I would say max possible shimming, roughly 3/4 turn, would still be less than 0.1cc, you'll get more volume difference than that from the front and rear cylinder heating and expanding at different rates, if it was that big a concern builders would cc the combustion chambers with indexed plugs. If someone else machined and built your motor, chances are you already have a greater variation in CR between the two cylinders, than you will from indexing.