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trialsrider46

Help with the Conversion of a D7 Bantam engine to Trials

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Good day all, question for those in the know. I have obtained a 1964 D7 Bantam 3 speed engine in quite good condition. I have had a good look at it and think it could be worth converting to trials format. It already has in its favour a B175 barrel and head plus a modified clutch. However here are the first problems found which leads me to my questions. I would like to reduce Primary sprocket from 17 to 12 the same as my BB175. I have spoken to Rex Caunt Bantam specialist who informs me if I fit the smaller sprocket he supplies the primary chain will not line up. ? 2nd I would like to reduce the gear box sprocket ideally to 12 but the D7 has a seal housing which would not allow the drive chain to pass. ? Maybe if these can be sorted easy I will undertake the project. Or could I get away with leaving the primary at 17 if I can change the gear box and fit a much larger rear wheel sprocket. Possibly. 🤔
Thanks in advance Ken.

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Hi Ken,

I have done exactly what you are looking to do with my '65 D7. I reduced the primary drive socket down to 12 as you are wanting to do, I used one of Rex's sprockets. He quite correctly states that in its original state as purchased, the gears will not line up, I had to have it machined to fit, the sprocket will not go on far enough to allow the chain to line up or fit onto the taper of the crankshaft,  it needs the overhang on the back and also the front removed, then it will fit, and allow the nut to be fitted,.

You are then faced with what gear ratios to run. I started off after having a custom seal carrier (a lot thinner machined from billet aluminium), this enabled me to have a 12 front sprocket and 64 rear in 420 chain size. I found this far too low geared, so after going up to a 13 on the front and 60 on the rear, found it still too low. I am now running a 14 front and a 60 rear, however this is still a little lower geared than I would like and as you only have one usable gear, I am looking to either go bigger on the front or smaller on the back. If you go bigger than 14 on the front I dont think you have the chain clearance issue with original seal carrier. So a 15 front and 64 rear may work and remove the need for seal carrier alterations. I am probably now looking at 14 and 56 rear to have i usable gear.

 

Hope this of some help.

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58 minutes ago, gazza7965 said:

Hi Ken,

I have done exactly what you are looking to do with my '65 D7. I reduced the primary drive socket down to 12 as you are wanting to do, I used one of Rex's sprockets. He quite correctly states that in its original state as purchased, the gears will not line up, I had to have it machined to fit, the sprocket will not go on far enough to allow the chain to line up or fit onto the taper of the crankshaft,  it needs the overhang on the back and also the front removed, then it will fit, and allow the nut to be fitted,.

You are then faced with what gear ratios to run. I started off after having a custom seal carrier (a lot thinner machined from billet aluminium), this enabled me to have a 12 front sprocket and 64 rear in 420 chain size. I found this far too low geared, so after going up to a 13 on the front and 60 on the rear, found it still too low. I am now running a 14 front and a 60 rear, however this is still a little lower geared than I would like and as you only have one usable gear, I am looking to either go bigger on the front or smaller on the back. If you go bigger than 14 on the front I dont think you have the chain clearance issue with original seal carrier. So a 15 front and 64 rear may work and remove the need for seal carrier alterations. I am probably now looking at 14 and 56 rear to have i usable gear.

 

Hope this of some help.

Gazza, thanks for the reply. On my B175 I am running 12 Primary, 12 Gear Box and 58 Rear it works just great.  However back to the D7. The first bit about the 12 primary sounds easy enough. Basically you are saying remove both the shoulders of the 12 sprocket. Is this correct ? I understand your comment ref the gearbox interesting, I do not want to go to 15 Tooth so I would need to work on the thinner seal carrier. I would certainly try the 12/58 again and think it might just work. I want this D7 engine as a spare and if it would swap directly over without chain and sprocket change that would be the icing on the cake. I also run 420 chain with the engine moved over 7 mm. Look forward to your reply. Do you have a contact or could you help supply a modified seal carrier. Ken.

I am based in Suffolk so No big rocks etc to ride over. I would think for your eastern area location we ride very similar sections.

Edited by trialsrider46

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7 hours ago, section swept said:

Sounds complex, can you not achieve the same with just a larger rear sprocket?

Short answer maybe yes, but it is not my preferred course of action. If I can change the primary with little effort then the rear sprocket of say 64 might work on a 15 tooth gearbox. Last resort.   Or sell the D7 and look for a B175 problem solved. Thanks for the reply. Ken

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Ken,

You are correct, no big rocks around here either! You are also correct, just remove the shoulders on either side of the primary drive sprocket, this will enable the taper to fit on the shaft and give enough thread for the nut. With regard to the slimline seal carrier, i was not impressed with the cost the local engineering company I used wanted to charge me so that did not end well, so I cannot recommend anyone to make one. However, the design is very simple, it is basically a thicker version of the original plate (BSA part 90-133 I believe)  made from 6 or 7 mm thick aluminium plate and a 6 or 7 mm width seal to suit. I had the screws recessed into the plate so you could use the original screws and not have them protruding. 

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3 hours ago, gazza7965 said:

Ken,

You are correct, no big rocks around here either! You are also correct, just remove the shoulders on either side of the primary drive sprocket, this will enable the taper to fit on the shaft and give enough thread for the nut. With regard to the slimline seal carrier, i was not impressed with the cost the local engineering company I used wanted to charge me so that did not end well, so I cannot recommend anyone to make one. However, the design is very simple, it is basically a thicker version of the original plate (BSA part 90-133 I believe)  made from 6 or 7 mm thick aluminium plate and a 6 or 7 mm width seal to suit. I had the screws recessed into the plate so you could use the original screws and not have them protruding. 

Cheers Gazza, I will persevere with the conversion. Have you gone down the new electronic ignition or have you retained the standard points etc.  Ken.

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I ran a 14 tooth primary for a long while, but it does effect the clutch as it has to handle more torque. I would go down the route of the smaller gearbox sprocket. I don't like dinner plate rear sprockets as you get a torque reaction which lifts the rear wheel. One problem I had, was only solved by going for the 4-speeder as I broke the first gear a couple of times, it's splined onto it's shaft and cracks between the spline and gear tooth then soon after breaks in two. You have to have the later cases to match the gearbox but you can use the D7 crank and outer cases to hide the fact.

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10 minutes ago, teamferret said:

I ran a 14 tooth primary for a long while, but it does effect the clutch as it has to handle more torque. I would go down the route of the smaller gearbox sprocket. I don't like dinner plate rear sprockets as you get a torque reaction which lifts the rear wheel. One problem I had, was only solved by going for the 4-speeder as I broke the first gear a couple of times, it's splined onto it's shaft and cracks between the spline and gear tooth then soon after breaks in two. You have to have the later cases to match the gearbox but you can use the D7 crank and outer cases to hide the fact.

That is a new one, thanks for this. Gives me more to think about. Maybe it is worth rebuilding in standard road trim and selling. Thanks again Ken. 

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16 minutes ago, ruffmutt said:

Simple answer to your gearing problems trialsrider46, fit a Villiers engine, lol.

 

Here is the Greeves. Love the thing great to ride. Here is the Bantam ready to ride this weekend Also a great ride. :D5bc24759890cc_GreevesNowComplete1.thumb.jpg.74abc742f2e4dcfa9f89d3cc169172c1.jpg5bc247bee8a22_BSAAgreedValue.thumb.jpg.3095b9c1ca7c00098852cc0dacb7fde5.jpg

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