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capacity change 175cc-125cc
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tom a
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December 14, 2011 - 9:41 pm
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I want to change my 175 to a 125 as I can't ride a bigger bike. Any tips on how I can do this.

Thanks,

Tom.

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Sprung Chicken
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December 14, 2011 - 10:06 pm
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Hi Tom, at one time T&G Motorcycles sold a 175 - 125 reduction kit.This is no
longer available. I'm sure 125 barrel and head can't be fitted to your engine as the barrel studs
are wider apart,ulness someone else knows better...

That leaves possibly 3 options:
Buy a complete D1 engine

If it is possible,have your barrel sleeved down and buy a D1 head

Get through your test.

Dougie;Glasgow ;Scotland.
1955 D1, 1970 B175

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lazybandittattoo
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December 15, 2011 - 12:35 am
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I really think the restrictive motorcycle laws in this country are pants.

Most current 125s are faster than Our 175 Bantams. you'd be breaking the letter of the law but not, I think, the spirit of it.

I agree though. You don't want to make a bantam any less powerful than it already is. Crack on and pass your test and best of luck with it.

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regal raptor
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December 15, 2011 - 8:24 am
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Quote from lazybandittattoo on December 15, 2011, 00:35
I really think the restrictive motorcycle laws in this country are pants.

Most current 125s are faster than Our 175 Bantams. you'd be breaking the letter of the law but not, I think, the spirit of it.

I agree though. You don't want to make a bantam any less powerful than it already is. Crack on and pass your test and best of luck with it.

what do you mean most 125 faster........all are faster i think LOL

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bantsam
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December 15, 2011 - 11:12 am
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In 1985 I put a 125 engine in a D14, it was the Lucas generator type and was simple enough to wire up with the D14. The bike seem a bit gutless after riding the full bore D14, it's a little bit heavier than a D1. It looked alright, I have a photo somewhere, I'll see if I can find it and scan it. I sold the bike later on as a"learner bike" and 14 years later it turned up at an autojumble for sale which I bought. The original 175 unit came with it which I duly put back in.

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regal raptor
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December 15, 2011 - 11:24 am
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Quote from bantsam on December 15, 2011, 11:12
In 1985 I put a 125 engine in a D14, it was the Lucas generator type and was simple enough to wire up with the D14. The bike seem a bit gutless after riding the full bore D14, it's a little bit heavier than a D1. It looked alright, I have a photo somewhere, I'll see if I can find it and scan it. I sold the bike later on as a"learner bike" and 14 years later it turned up at an autojumble for sale which I bought. The original 175 unit came with it which I duly put back in.

lovely little story that one mate.........thanks for sharing

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Paul Davies
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December 15, 2011 - 12:56 pm
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Quote from regal raptor on December 15, 2011, 08:24
what do you mean most 125 faster........all are faster i think LOL

I think its probably fair to say also that all modern 125s are also heavier, have better suspension & brakes than a D1.

It might be interesting to derive an overall comparison of power-weight ratio, handling & breaking efficiency for 125s, and see how the D1 stands up against the far eastern new-comers.

While a D1 is wildly underpowered, compared to a modern 125, the D1 is far lighter, but equally under braked - I wouldn't be surprised if the 2 ends of the spectrum were closer than many people might expect....

cheers

Paul

'55 D1 Custom (Note: my Avatar is a QR Code which links to my D1's Flickr photo set.)

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regal raptor
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December 15, 2011 - 1:13 pm
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Quote from ukpauldavies on December 15, 2011, 12:56

Quote from regal raptor on December 15, 2011, 08:24
what do you mean most 125 faster........all are faster i think LOL

I think its probably fair to say also that all modern 125s are also heavier, have better suspension & brakes than a D1.

It might be interesting to derive an overall comparison of power-weight ratio, handling & breaking efficiency for 125s, and see how the D1 stands up against the far eastern new-comers.

While a D1 is wildly underpowered, compared to a modern 125, the D1 is far lighter, but equally under braked - I wouldn't be surprised if the 2 ends of the spectrum were closer than many people might expect....

cheers

Paul

i have just bought a 50cc (2t) scooter for my son ........ it is water cooled and got all the electrical equipment on (electric start etc) it will near enough touch 50 mph faster down hill like you say its far heavier than a d1 ........... its a case of things improve ! if the bantam were never born ( dkw or what ever) these modern bikes would not exist i suppose

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Jubilee
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December 15, 2011 - 3:53 pm
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Hello,

I have a D1 and a Honda C90 (123cc and 85cc respectively) Both have three speed gearboxes and the Honda is by far the best for acceleration, top speed and fuel economy. 46mph/52 mph and 90 mpg/145mpg. The modern motorcycle 125 cc bikes will all be nearly three times as powerful and I would thing there is no comparison really. The early Honda CB125 twin produced 16 bhp in unrestricted form to the D1's 4.5 bhp.
I also rode a D1 to work in the 1960's and several Honda C50's during the 1970's. These were more evenly matched in terms of speed but the C50 was better on fuel and also provided better protection fom the weather. Running costs were cheaper with lower insurance and no two stroke oil to add to the fuel.

Regards,

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Peter Savage
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December 15, 2011 - 8:33 pm
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I have done some work on a sleeved down 175 bantam.It was a four speed model with a sleeved down barrel to make it a 125. The bike was very nice but would not start or run properly.
The problem was that it still had the 26mm concentric fitted with the standard settings.
After getting the carburettor to run better the lack of power was very noticeable. I was given the barrel which has a new piston and has only covered about 5 miles if you are interested but I think fitting a 125cc engine would perhaps be the way I would go if your heart is really set on having a smaller engined machined of BSA manufacture.

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theprodigyuuk
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December 15, 2011 - 9:03 pm
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Yes pass your test or.......
A little bird told me its possible to re register one as a 125 and because of its year it wont need a vosa check
Not that I recomend doing that of course

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DoubleDiamond
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December 15, 2011 - 10:12 pm
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I did have a T&G 175 to 125 conversion so that the Mrs could pass her test. However being on the rather 'heavy' side (me that is!) I found that it lacked power so changed it back.

To help Mrs DD this time I bought a nearly new AJS 125. OK it is made in China but it does meet all the modern requirements to enable a straightforward pass of the latest bike test. And it is perhaps as close as you can get to a home grown learner legal bike. Its a shame that Triumph don't seem interested as I'm sure a premium quality 125 from a good manufacturer would sell well and hold its value too.

Just my thoughts.

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regal raptor
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December 15, 2011 - 10:31 pm
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Quote from DoubleDiamond on December 15, 2011, 22:12
I did have a T&G 175 to 125 conversion so that the Mrs could pass her test. However being on the rather 'heavy' side (me that is!) I found that it lacked power so changed it back.

To help Mrs DD this time I bought a nearly new AJS 125. OK it is made in China but it does meet all the modern requirements to enable a straightforward pass of the latest bike test. And it is perhaps as close as you can get to a home grown learner legal bike. Its a shame that Triumph don't seem interested as I'm sure a premium quality 125 from a good manufacturer would sell well and hold its value too.

Just my thoughts.

yes this is true.......its fact that small capacity bikes out sell the big stuff........and im all for it !

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bantsam
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December 16, 2011 - 1:30 pm
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Photo as promised

http://i1081.photobucket.com/albums/j360/bantsam/Scan10.jpg
http://i1081.photobucket.com/albums/j360/bantsam/Scan3.jpg

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tom a
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December 16, 2011 - 8:55 pm
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Quote from Peter Savage on December 15, 2011, 20:33
I was given the barrel which has a new piston and has only covered about 5 miles if you are interested but I think fitting a 125cc engine would perhaps be the way I would go if your heart is really set on having a smaller engined machined of BSA manufacture.

Sounds interesting, if you are offering the barrel and piston, how much would you be looking for them?

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vintage keith
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December 16, 2017 - 10:35 am
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Old topic, but I thought I would add some personal experience.

Some years back, I bought a B175 for the wife to learn to ride on. We found a good T&G barrel/piston at an autojumble, thankfully it came with a set of instructions (that I've since lost). The advice was to go back to a plain little end bush, rather than the needle roller version, fit a smaller engine sprocket (can't recall no. of teeth), and retain the B175 head (so that the stud spacings were correct). The standard B175 carb was also retained, but different jets were fitted. A nominal size was given in the instructions, but I bought that and one size up/down to make sure I got the sweetest running. As I'm writing this, I think the air slide cutaway was also changed.....

I seem to recall the T&G conversion was aimed at the racing fraternity, so the ports were gas flowed. This resulted in road performance being fairly similar to the standard B175, if you played an energetic tune on the 4 speed box. I would imagine the performance would have been a bit better again if a true 125 head was converted to suit the B175 stud spacings, but never tried it. It's probably better for light weight riders rather than medium or plus size (but that's true for all Bantams).

In truth, we didn't run it as a 125 for very long, as the wife couldn't get on with the Bantam (she preferred a Yamaha RXS100 and now rides a 250 Virago), so I eventually turned it back to original. It recently departed (complete with conversion kit) to a 17 year old who has an interest and hopefully will cut his teeth on the 125/175 Bantam.

Now I really need my son to get on with the 2 x D1 projects in the shed.

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