3 Speed Super spark plugs | Scotts Owners Club

This topic has 7 replies, 5 voices, and was last updated 11 months ago by André Martins.

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1st March 2022 at 8:27 am #33278

gammakeith
Participant

Dear all,

Our Three Speed Super came with a pair of AC 87 18mm plugs. They are quite old so at the very least we would like a spare set of plugs for the bike. Our later Scott also needs 18mm plugs and the Champion D21 plugs on it have been great since the engine rebuild. Alas, there is simply not enough space under the radiator on the Super to fit the Champion plugs as the insulator is longer. Does anyone have any suggestion as to what might fit and work well on a short stroke engine?

Many thanks!

Keith and Chris

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2nd March 2022 at 12:28 pm #33283

gammakeith
Participant

OK. I went for an unconventional solution and shortened two Champion D21 plugs. They are a very “soft” plug so ran lovely and clean in our Flyer for several hundred miles without a hitch. Thus, they should be ideal for the Super. I don’t see why shortening them would impact their performance but it is going to be easy enough to find out. Anyone else done this? If not, I wonder if its worth adding the details in Yowl.

Keith

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2nd March 2022 at 9:47 pm #33285

SwedScott
Participant

Hi Keith

Neat job to shorten the insulator without cracking the whole thing! Watch out for overheating although because of the diminished surface area. But according to a one of my old books AC 87 could be a bit softer than Champion D21. Say could, as different old lists translate heat ranges differently. I did rummage through my little stock of NOS plugs as I remembered that I have a pair of AC’s. Unfortunately they turned out to be AC 84 wich is three steps cooler, so to say.

I suppose that you have a radiator with neck, otherwise with a tunnel radiator it’s possible to raise the radiator. This by making an offset upper bracket and substitute the lower screw with a rubber cushion that the radiator rests on. By doing this I even got place for proper plug caps.

By the way, I don’t think there are any other plugs shorter than the the AC ones. Can’t remember seeing any.

Best regards and good luck!

Leif

3rd March 2022 at 7:50 am #33286

gammakeith
Participant

Thanks Leif!

I’m not too worried about overheating as I would have thought a water-cooled head would keep things reasonably in control. An aircooled engine may well be different though! The D21 plugs have a very long tapered electrode with a thin and pointed porcelain insulator so they “look” like a very soft plug. The AC87 “look” much more conventional and harder. I don’t know if that means anything though 🙂

Keith

3rd March 2022 at 11:35 am #33287

SwedScott
Participant

Hi Keith

As the saying goes, it’s hard to judge a book by its cover. The pair of AC 84 also have an uncommonly long centre electrode. I also have one AC 84-M (the marine version with nickel plated body) this one have a much more conventional centre electrode! Must have something to do with the internal construction, I think.

Best regards

Leif M

8th March 2022 at 11:06 am #33295

BRIAN MARSHALL
Participant

Hi all, Just a quick reminder that it is our Eastern Section March meeting on Sunday 13th, at our usual venue, The Buttercross pub/restaurant in the Market Place, Bingham, Nottinghamshire. 10 am to 12 noon in the Snug. Doors open at 8am, so why not come early and have an inexpensive breakfast ? Pay for your first tea or coffee, then refills are FREE all day ! All welcome….

10th March 2022 at 9:14 pm #33302

george millar
Participant

Hi everyone

Just to let people know that we have a Scott stand at The Scottish Motorbike Show, held at Ingliston, Edinburgh, this Saturday Sunday 12th and 13th March with fourteen Scotts on show.

So if you are attending the show please drop by and say hello.

Best regards George Millar

2nd May 2022 at 10:08 pm #33436

André Martins
Participant

Hi everyone.

I think I talked to Keith during the Stafford show, isn’t that rigth?

I believe that you told me about those Champion D21 you were using and so I am now trying to find some and give it a try on my 1947 Squirrel.

The thing is that Champion D21 plugs seem to be quite hard to find in here (though I still have to check in all the agricultural machinery shops). However, Champion D16 are sold in some motorcycle shops.

By that said, as anybody tried both Champion D16 and D21? If so, what differences did you notice?
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