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Morning all, I have just acquired an 89 dt125 with a 3MD bottom end, the piston is domed (should it be flat) so not sure if this is correct as there is also a spare 4GW bottom en with it aswell, It has come with a 3MB barrell and a 3BN head, it had 3 gaskets on the base of the barrel, I am not sure if the innards of the 2 bottom ends habve been changed or whether it would make a difference, the 3MB barrell also has 3BN stamped inside on the powervalve ports.
Question is, is there any experts out there that can point me in the direction of what i can do with this lot of parts and can i make a working engine out of them all, they look like they will all go together certainly but not sure if it would last, just unsure as to why there was 3 gaskets on the base of the barrell, possibly for clearance.
I am the 23rd owner on the logbook so god knows whats happened to it. LOL!
Many thanks guys. Look forward to some help.
Nicky.
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@89dt125nick Hi there Nicky,
Yes certainly.
3MB Barrel with 3BN head is standard for the year. The 3BN head is a "restrictive" head and can be swapped out for a 3MB with no further modifications required.
Early models ran a 3BN barrel which is slightly larger bore which would result in a shorter stroke in order to run the same displacement. So running it the other way round would result in the deck height being too tall.
In other words, you don't need three base gaskets. And removing those gaskets shouldn't be a problem. If the original barrel is a 3BN then the stroke will fall short. Adding gaskets would exacerbate this. It's my understanding that it's only the very early '88 models that ran that barrel. So an '89 should be a 3MB.
But either way, swapping the rod out for a 3MB and 3MB barrel would be the way to go.
The piston should be domed not flat.
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4GW is TDR from memory - handy for an electric start conversion if you could be bothered!
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Hi Calum, many thanks for the info, i have now assembled the engine with a new gasket set and it turns over fine. Just one thing my powervalve has no servos or anything, just two covers on either side, with a 10mm bolt in the middle of one, the powervalve is in there, and looks to be wide open. Does this move, or is it meant to have a servo, i cant really find any accurate info. I also have a wiring question but i will do a new thread for that. Many thanks for your help.
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@89dt125nick hi nick, I used to have a 88 model, the early ones didn't have a servo as this was the learner legal restriction. Later ones had working powervalve but restricted elsewhere. If you remove the powervalve cover on the right hand side of the engine you'll see a small metal dowel which locks the valve in place, you can only move it 180 degrees due to the dowel so either fully open or fully closed. In short open for high end power and closed for low end power so each one is a sacrifice of the other. You can easily retro fit a servo for the best if both worlds but for the older models as you have I believe you need the 3 wire servo as opposed to the later 5 wire.
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@markus-w It's my understanding that at the time of order, if you wanted a servo, you were required to provide your motorcycle license.
The 80's was brutal to the motorbike scene with fast bikes being released to kids with no licenses, resulting in catastrophic accidents. RD250 smokers with virtually no braking capabilities spelled disaster.
Always Originate, Never Pirate!
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C Calum moved this topic from Welcome New Owners! on
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@Calum yes that's my understanding also. And yes about the 80's learner bikes, i think they were right to reduce the power/capacity of learner bikes and introduce the CBT. Imagine a 17 year old with no training or experience loose on the road on a 250 2 stroke?
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Thanks guys, info much appreciated.
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You can get a servo to bolt on easy enough. I did it with my bike and it makes all the difference, low end usibility and power at 7000rpm..
It has to be the 3 wire version for 88- 90 something bikes though...
They are usually expensive but if you keep looking they do turn up. I just brought an good useable, original 3mb barrel, piston, power valve, pristine 3mb head and 3 wire servo with cables, rubber cover and pulley & housing for £130! 🤪
