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educate me about dominator/atlas engine sprockets #340007
10/26/10 7:44 pm
10/26/10 7:44 pm
Joined: Jan 2006
Posts: 39
Ambler PA
B
Bill H Offline OP
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Bill H  Offline OP
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Joined: Jan 2006
Posts: 39
Ambler PA
looking thru the parts book i see that their are alternative engine sprockets from 17 to 23 teeth. i was wondering why one would vary the engine sprocket?
would one vary the engine sprocket along with the gearbox sprocket?

thanks

bill
2002 mz skorpion
1972 norton commando
1971 triumpht100c
1967 norton 650ss

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Re: educate me about dominator/atlas engine sprockets [Re: Bill H] #340038
10/26/10 10:55 pm
10/26/10 10:55 pm
Joined: Sep 2006
Posts: 7
Ohio
R
rleisner Offline
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rleisner  Offline
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R
Joined: Sep 2006
Posts: 7
Ohio
Going to a larger crank sprocket on the primary rather than a larger gearbox sprocket is easier on the transmission.
Ron L

Re: educate me about dominator/atlas engine sprockets [Re: Bill H] #340046
10/26/10 11:58 pm
10/26/10 11:58 pm
Joined: Apr 2006
Posts: 1,397
New Zealand
J
johnm Offline
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johnm  Offline
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J
Joined: Apr 2006
Posts: 1,397
New Zealand
If you have a larger sprocket on the engine then you spin the gearbox faster. As Ron says this is easier on the gearbox because the torque is reduced at higher rpm.

This is not so much an issue with the 500 Dommie but is important with hotted up 750s which eat gearboxes. Not really surprising when the box was originally designed for 26 bhp not 60 plus.

Couple of things to watch. From personnal experience I have found some of the engine sprockets sold today have chain side clearance issues. Especially the ones with a smaller no of teeth. You may find the chain fouling the crankcase especially on the generator cases. The alternator cases are machined for clearance but this does reduce the crankcase strenght round the drive bearing which is not a good thing either!


To complete the story some of the gearbox sprockets will also have clearance issues on a featherbed frame.

There are no rules which say this always works. With the range of aftermarket parts you get today you just have to check case by case.

John

Last edited by johnm; 10/26/10 11:59 pm.
Re: educate me about dominator/atlas engine sprockets [Re: Bill H] #340159
10/27/10 6:22 pm
10/27/10 6:22 pm
Joined: Aug 2001
Posts: 1,263
Hamilton, Mass. USA
D
Dave Comeau Offline

Crew Chief
Dave Comeau  Offline

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Joined: Aug 2001
Posts: 1,263
Hamilton, Mass. USA
Sorry to only pass on rumors. Might have a financial impact otherwise.
I've heard the singles have different offset from the twins, though the taper is the same?
truth or fiction.....?


dynodave
BSA 3 1961-1963
Ducati 3 1992-2002
Norton many 1951-1975
87 Serv-Equip 100HP MC brake dynamometer,
Re: educate me about dominator/atlas engine sprockets [Re: Bill H] #340169
10/27/10 8:15 pm
10/27/10 8:15 pm
Joined: Apr 2006
Posts: 1,397
New Zealand
J
johnm Offline
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johnm  Offline
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J
Joined: Apr 2006
Posts: 1,397
New Zealand
Hi Dave,

Sorry I dont know about singles and twins but asked John Saywell at British spares in wellington and was told the following:

The tapers on twins and singles are different. The Dommie twin crank is 30mm in diameter, the single is 1 inch in diameter. The push rod single has at least two different tapers (possibly alternator and generator but he was not sure). The camshaft singles may have a different taper again but again of the top of his head he wasnt sure. He has had to go through the exercise of sorting a range of sprockets for singles so he is sure there are several types. British Spares have several in stock.

For the Dommie however there is only one type.

My personnal experience was in about 2003 when I bought a range of different teeth sprockets for the Dommie 500 twin from one of the main suppiers in the UK.

When I fitted one of the smaller ones (17 or 19?) the primary chain fouled. At first I though it was simply that the smaller diameter was the issue but on measuring I found the chain was sitting a couple of mm closer to the cases.

I rang them and they went and measured their sprockets and came back straight away saying one lot had been machined wrongly. They replaced it straight away no problem.

So no issue but there may be more out there so it pays to check.

Im now all belt drive so havnt looked at this in 8 years or so. I change the gearing for racing with a range of sprockets for the rear hub. I use an early Triumph rear hub which is much lighter than the standard Norton.

Regards

John

Last edited by johnm; 10/27/10 8:17 pm.
Re: educate me about dominator/atlas engine sprockets [Re: johnm] #340260
10/28/10 12:32 pm
10/28/10 12:32 pm
Joined: Aug 2001
Posts: 1,263
Hamilton, Mass. USA
D
Dave Comeau Offline

Crew Chief
Dave Comeau  Offline

Crew Chief
D
Joined: Aug 2001
Posts: 1,263
Hamilton, Mass. USA
Thanks for the research.
I knew the twins all had the same taper since I had tested the belt drive front pulley for a slimed down (commando) belt drive on my M7.
I had heard of different offsets so maybe the (bad/defective) sprockets from (XXX?) were the ones I heard of, thinking they were for singles and supplied by mistake for a twin.

Dave
Still looking for barrels for my 51 Model 7(w/generator)

Re: educate me about dominator/atlas engine sprockets [Re: Dave Comeau] #341766
11/06/10 10:49 pm
11/06/10 10:49 pm
Joined: Jul 2010
Posts: 44
australia
A
acotrel Offline
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acotrel  Offline
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Joined: Jul 2010
Posts: 44
australia
When I first raced my Seeley 828, it revved to 7,500 on the straight of our local circuit. So I took a couple of teeth off the rear sprocket to knock it's backside in. The bike went faster, and still revved to 7,500. I didn't realise how much torque those old motors have. The standard box is revolting when you're racing, the down shifts are never smooth. The close four speed box is much better, but a clutch start is too slow. When the bike becomes mobile, the CR box means it's in the right gear everywhere, and all gear changes are supersmooth. My bike is now in the process of getting two more gears!


AC

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