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engine sprocket offset

Submitted by martin_main on Mon, 22/05/2017
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How can i tell which engine sprocket offset i currently have?

I need to get a new one with more teeth, but am unsure as to whether it's central or has an offset one way or the other. Obviously I have nothing to compare it to.

thanks for any help

Martin

Submitted by richard_payne on Mon, 22/05/2017

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What year and model is it ?

Although it seems complicated, they didn't change that often. My understanding from the parts lists is that SV and OHV engines used the same offset from at least 1935 to 1947 inclusive.

OHC were different.

Submitted by martin_main on Mon, 22/05/2017

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I've heard mention of Norton George, but, up until now, have generally been involved with Phelon & Moores heavyweights, so he and the unapproachables were paid no heed. Thanks for the introduction Ian, and the interesting article. The sprocket dilemma, as such, is ended. I currently have the middle option, and know how to measure them.

Richard, my recent aquisition is a little complicated as its made from a mixture of models, and the research and learning is proving enjoyable.

To sum up, its a 53 big four frame with return spring girders. A laydown box from a model 7 with longer mainshaft, a 1950 500T engine and TT mag. An absolute cracker of a 289 carb for a T100 that Martin Bratby worked his magic on, and a mid thirties CS1 oil tank kindly identified by Richard Cornish.

Thanks again

Martin

Submitted by richard_cornish on Mon, 22/05/2017

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Just to add fuel to the fire, my understanding of the sprocket offset is..... the sprocket with the offseton the outsideis the early type for engines with only 1 drive-side main bearing. The engines were redesigned in 1933 and fitted with 2 drive-side main bearings which was to be the same layout until production ceased in 1963. The sprocket with the offset nearest the crankcase was used from 1933 until 1958 with any gearbox differences corrected by the gearbox main-shaft and/or gearbox sprocket. The third type with the central offset(?) was usedto accommodatethe alternator that was fitted in 1959. I cannot be sure this is 100% accurate so any other info would be good.

Regards, Richard.

Submitted by martin_main on Tue, 23/05/2017

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any gearbox differences corrected by the gearbox main-shaft and/or gearbox sprocket.

So there are variations in offset for gearbox spkts? I need a new one of these also, so would need to consider any diffferences before buying.

Thanks, Martin

Submitted by richard_cornish on Tue, 23/05/2017

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Previously martin_main wrote:

any gearbox differences corrected by the gearbox main-shaft and/or gearbox sprocket.

So there are variations in offset for gearbox spkts? I need a new one of these also, so would need to consider any diffferences before buying.

Thanks, Martin

Hi again Martin,

If you have a model 7 gearbox, chain alignment will be something you will need to check with a straight-edge. You will be best to get the standard gearbox sprocket as there are different sleeve gears for different gearboxes. I don't know how this will line up with the back wheel as the twin engine wasn't used in the rigid frame or how the clutch will line up with the engine and I guess there will be problems with the primary chain-case clearance. It might pay you to look out for a singles gearbox and sell the model 7 one.

Regards, Richard.

 


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