I'm confused. The owner's manual for my '69 AT1 calls for a B-8E sparkplug. It appears they're almost unobtainable for purchase new. The sparkplug cross reference guide lists the Bosch W5CC as the only alternative plug to the B-8E. Should I order the Bosch as a spare or is everyone using the B-8ES these days? I'm not sure what the difference is between the B-8E and B-8ES. Does it matter?

B8ES is the correct plug... last letter indicates "standard electrode" I think... they also made a "V" (B8EV) which was a fine-wire gold pallidium electrode.

In the early days they might not have used the last letter or left it off because the B8E is the important part... S or V electrodes are both fine.

Now they have platinum and who knows what...

B8ES is a good plug... just don't buy a BR8ES... R is for resistor and your bike should have a resistor cap. Two resistors is not a good idea.

You got me thinking so I dug out one of my old books.

B= 14mm x 1.25 thread
8= Heat range (in NGK, higher number is a colder heat range plug)
E= 3/4" thread reach

S= "Super wide range electrode"

My 1964 MG1T had a B7H as original... I think I have the original plug someplace and it's listed in the owner's manual.

My Dad made me keep "maintenance records" which I also still have. Yamaha dealer sold me a B7HS in 1974 for $0.95. My notes also say the spare plug under the seat was a B7HZ (unless my little kid writing made an S into a Z... doubtful, but possible... I might still have that plug too!)

I have a CT2 that will spark with a BR8ES but not with a B8ES.. Does that mean I do not have a resistor cap???

Wouldn't stop it from sparking. My first guess is bad B8ES or if not that, something strange going on.

This makes sense. Great information. Thanks! BTW, the original NGK B-8E plugs are still in the bike (head has two holes as you know) and either one works fine. I really like the idea of being able to swap to the other plug if one fouls. I cleaned and regapped them but thought I should probably replace them with new for best performance. If nothing else, new plugs will allow me to "read" them to ensure I'm not running too lean or rich.

Might just have to save the originals as collector's items. LOL. They've certainty performed their purpose well at 4K plus miles and 51 years of service!

In Norcal, at the parts store we would ask for a B7 Hella Small.

Yes! Definitely keep those plugs! A true original plug has to be extremely rare. Picture would be great to see!

First production 1969 AT1 only had one plug hole in head. Yours must be a later production 69... Or someone could have swapped heads back in the day? The head with the second hole for a compression release** was installed at factory after engine number 107185.... about halfway through the model run.

End of 1969 model run was VIN under 120000... VIN higher that 120101 was a 1970... full VIN tables can be found in Tech Library here.

(I see the white tank bike in your avatar which should be a 1969 by the color)

**Officially, second hole was for an "engine braking" compression release... a fad in the early 70's. Most people installed a second plug.

My particular bike's VIN is 112778 so, based on your info, I'm going to assume the head is original. My idea that the plugs were designed for a quick changeover in the field came from this article:

cutfrommetal.com/why-we-love-it-yamaha-at1/

In it, the author, Gabriel Trench, wrote "A duel-plug head provided a back up for any fouled plugs..."

In any event, the holes are symmetrically opposed on either side of the compression chamber and the bike seems to run equally well on either plug. Nice feature to have in my view, even if it's not exactly a design feature. I suppose when my bike was made Yamaha may have anticipated a compression release as being needed in the near future. The AT1M spec was released the next year. Did the AT1M come stock with compression release?

Any performance reason not to leave both plugs installed? If one side were to be blocked off, which side would be preferred to run the plug in?

That sounds logical, doesn't it? Two plugs in the head gives you a spare to switch to?

It doesn't work that way in real life though.

The extra hole was for an accessory compression release that worked to provide a little bit of engine braking similar to a "Jake Brake" on a diesel but not nearly as effective.

They made a really cool loud "BRAAAAAAAAAAAP" noise though! And were nice to have on long steep downhills out on the trails. Drawbacks were they made a mess and could suck dirt into the engine. I grew up in the hotbed of motocross, Southern California, and while some riders used them, it became obvious very quickly that the fast guys didn't.

I had one as a kid. Threw it away after dirt got in it and it stuck open out on the trails.

As far as I know, Yamaha never installed a cylinder head compression release in any model from the factory. They did sell cylinder head compression releases as accessories and they did provide a plugged hole for one to make installation easy. In my opinion, the extra hole was a marketing strategy more than anything.

I also can't remember ever seeing any Yamaha literature recommending installing a spare spark plug in the head. The hole came plugged with a metal plug, not a spark plug.

Off hand, I can think of a minor and a major drawback to using the extra hole for a spare spark plug... minor one is extra spark plug adds to the head volume... on a small engine like a 125 even one cc more volume will make a decent difference in chamber volume... 10% plus or minus. (I don't know the added volume of a spark plug... next time I have my burette set up I will have to measure it)

Major drawback is relying on the second plug as a "spare"! Specifically as your only spare.

I grew up with these bikes when we had leaded fuel and two-stroke oils that were not as good as they are today... Fouling a plug was pretty darn common!

(I think fouling was mostly due to leaded fuels because when the mandatory switch to unleaded happened in about 1977, fouled plugs became far less common a problem for me... in both two and four strokes... used to have to change plugs in your car a lot too... these days they come out looking new with 100,000 miles on them)

The problem is that the spare plug is in the combustion chamber getting hit with all the same pressures and fuel and oil and contaminants... and it's getting just as much heat and just as hot as the firing plug! (The heat generated by the spark on the sparking plug is flea power compared to the heat of combustion in the chamber that both the firing and spare plug are dealing with.)

Sometimes a plug just fails. But if one truly does foul nine times out of ten the "spare" plug in the head fouls too!!!

I know this from a LOT of experience. What we used to do is put a fouled plug in the spare hole and keep the spare plug in our pocket or toolbox.

One thing that did work if you fouled a plug starting the bike or putting around too slow was to put the fouled plug in the spare hole, new plug in the other hole and ride the hell out of the bike (high RPM and speed). Then switch over the plug cap and if previously fouled plug worked, remove it and put a bad plug there again. Now you had an extra spare!

Yes, we had quite a system.

But we needed a system... spark plugs were precious and expensive... it's the most common thing that kept us from riding. Not only that they cost a whole dollar but we had to get someone to drive us to the motorcycle shop to buy a new one.

My advice is to always keep at least one spare spark plug that's NOT in the head.

As far as which hole to use for the main plug if they are symmetrical? "The easiest one"... that's what Yamaha did. Probably the one on the left on your bike. Parts lists aren't always perfect but usually the drawing will show you which hole the spark plug goes in and which hole the blank plug goes in.