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Discussion Starter · #22 ·
If it were too easy .. there be little merit and less fun>:D
Very true, dosent stop me hoping though ? just pooling the info at hopefully if anyone else does this all the info will be in one place. Ill get some pics up of all the parts shortly.
 

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Discussion Starter · #23 ·
Bit more info on this, i have read somewhere in this forum that the 390 front sprocket has different splines to the 125? Can someone confirm this for me please?

Also the rear sprocket althought still 45T has a 4 bolt mount the 390 has a 6 bolt. Does anyone foresee and issues with this as it could be costly to convert it for what it is?

Also there is a difference in cooling pipe on the head of the 390, it has a rubber coolant hose that runs from the head just by the throttle body to the pladtic T piece just under the rad. The 125 has a reduced metal pipe that runs half way and then connects to a shorter rubber hose. Ive order the 390 hose and fingers crossed it looks to just replace the metal one.
 

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Also the rear sprocket althought still 45T has a 4 bolt mount the 390 has a 6 bolt. Does anyone foresee and issues with this as it could be costly to convert it for what it is?
With the knowledge that KTM saw fit to increase the number of bolts to six, there is no question that if I were doing this conversion I would also want 6 bolts holding my sprocket on.

Motorcycle manufacturers normally have a good reason to spend money implementing such changes (after all it would have been easier and cheaper to just use the existing 125 4 bolt carrier).

In converting to a 390 engine you are looking to increase the forces acting on these sprocket bolts by a factor of roughly 3. Every time you open the throttle you apply a shear force to these bolts (literally the torque of the engine pulling on the chain and sprocket is trying to tear through the cross sectional area of the bolts).

In going from 4 bolts to 6, KTM saw fit to increase that C.S.A. by 50%, to greatly reduce the stress that is acting on each bolt.

My advice is you do likewise (i.e. use a 6 bolt sprocket carrier, I've no idea if that means you need to also change the back wheel)) otherwise, over time - and it's anyone's guess if and when issues would show up - you may be running the risk of sprocket bolts bending and ultimately failing - and you don't want to be riding a bike when that happens !!!!
 
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Discussion Starter · #25 ·
With the knowledge that KTM saw fit to increase the number of bolts to six, there is no question that if I were doing this conversion I would also want 6 bolts holding my sprocket on.

Motorcycle manufacturers normally have a good reason to spend money implementing such changes (after all it would have been easier and cheaper to just use the existing 125 4 bolt carrier).

In converting to a 390 engine you are looking to increase the forces acting on these sprocket bolts by a factor of roughly 3. Every time you open the throttle you apply a shear force to these bolts (literally the torque of the engine pulling on the chain and sprocket is trying to tear through the cross sectional area of the bolts).

In going from 4 bolts to 6, KTM saw fit to increase that C.S.A. by 50%, to greatly reduce the stress that is acting on each bolt.

My advice is you do likewise (i.e. use a 6 bolt sprocket carrier, I've no idea if that means you need to also change the back wheel)) otherwise, over time - and it's anyone's guess if and when issues would show up - you may be running the risk of sprocket bolts bending and ultimately failing - and you don't want to be riding a bike when that happens !!!!
Makes sense really i could cut corners and leave it on a four bolt but why risk my safety even more, most car drivers are out to kill me so should really do it properly. :nerd:

Anyways Ive taken a chance and order just the carrier and sprocket so we will see if its interchangeable? Again i don't see why not as the wheels seem to be identical apart from the colour?
 

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And while you are changing the sprocket carrier, you might want to take the opportunity to upgrade the cush drive rubbers. Apparently the OEM cush drive doesn't stand up too well to the 390's torque.

KTM - MNNTHBX
 
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Discussion Starter · #27 ·
And while you are changing the sprocket carrier, you might want to take the opportunity to upgrade the cush drive rubbers. Apparently the OEM cush drive doesn't stand up too well to the 390's torque.

KTM - MNNTHBX
Got some really nice bits of kit on there, dont know if they will ship to the uk though? My shopping list is just getting bigger and bigger ?
 

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I think they routinely ship internationally.
 

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Ill drop them an email see how much its likely to cost. Cherrs for the link ill break the credit card out����
Don't want to throw a spanner, but before you start stripping your credit card I think you should take a step back and see if you are exceeding the cost of getting a D390 of a similar vintage.

As exciting as this transplant sounds, you are going to end up changing quite a bit, including engine mounts and the various bushes to contain the extra vibes. Clutch plates, ABS system or not, wiring etc and not to forget the ECU. And you still don't know how good that used engine is.

Hope it's all within reasonable limits.
 

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Discussion Starter · #31 ·
Don't want to throw a spanner, but before you start stripping your credit card I think you should take a step back and see if you are exceeding the cost of getting a D390 of a similar vintage.

As exciting as this transplant sounds, you are going to end up changing quite a bit, including engine mounts and the various bushes to contain the extra vibes. Clutch plates, ABS system or not, wiring etc and not to forget the ECU. And you still don't know how good that used engine is.

Hope it's all within reasonable limits.

Cost is always at the forefront and yes its still cost effective even with repairs made to the Duke and upgrades I'm still coming in way under a second hand 390 cost. I even have spares that are sellable to recoup costs if need be including a second 125cc engine that came with the duke upon purchase. The engine was bought from a reputable breakers and has a full warranty. All the major parts have been purchased including the ECU at very reasonable prices. I've not gone at this gun Ho I have researched it as best I can as there is very little information out there. Wiring is the same KTM parts catalogue list them as the same part there are diagrams in previous posts. I will put up a full costing and list of parts used upon completion or a big sad face if it becomes too complicated.
 

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Rule #1 of car / bike modifications:
NEVER BUILD WHAT YOU CAN BUY.

It's expensive, you never get anything like the money back, you'll never be able to insure / sell it. The absolute best you can hope for is that your 125/390 goes as well as a proper 390 and doesn't fall apart. Why do it when it's not a rare model?

Now, if you said you were going to put a V4 in a Suzuki RGV frame, that would be different.
 

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Discussion Starter · #33 ·
Rule #1 of car / bike modifications:
NEVER BUILD WHAT YOU CAN BUY.

It's expensive, you never get anything like the money back, you'll never be able to insure / sell it. The absolute best you can hope for is that your 125/390 goes as well as a proper 390 and doesn't fall apart. Why do it when it's not a rare model?

Now, if you said you were going to put a V4 in a Suzuki RGV frame, that would be different.


WHY NOT?!


I Fail to see the logic in that statement? As already identified, cost at this moment in time is lower than buying a 390 outright, that's including serviced parts and upgrades.


Has this been done before?
Probably but not many and nothing that's been documented.


Can I sell/insure it?
That's easy yes to both, if I can convert a 1.2 car to a 2.0 and get insurance I'm sure that a 390cc engine into a 125cc frame (which is basically 90% the same bike) is not going to be an issue. Can I sell it? Not that I'm going to but yes the only one I have seen done sold for around £3k.


Why do it if its no a rare model?
As above WHY NOT if just the **** of it? Yes I could go and buy one, but where's the fun in that? The Knowledge alone that I've gained while doing this has been invaluable.


Hope that's answered a few things but most have already been address in the previous posts:D
 

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I have already issued several warnings about that individual " Diploman " I truly believe that the moderators here should post a " wallet warning " on his avatar >:D

@Powell9539 this is good advice, take it !

He's started fairly small on just some cush drive rubbers . Once the Diplodude starts persuading you of the benefits of suspension and brake upgrades etc. your economic arguments for this build could quickly start to unravel.

I suggest you ban him from this thread >:D
 

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Discussion Starter · #37 ·
@Powell9539 this is good advice, take it !

He's started fairly small on just some cush drive rubbers . Once the Diplodude starts persuading you of the benefits of suspension and brake upgrades etc. your economic arguments for this build could quickly start to unravel.

I suggest you ban him from this thread >:D

Thanks. I looked into this ordered them this morning $90 including shipping, seems the 125 ones may be slightly different but can confirm due do KTM discontinuing them accounting to the parts catalogue?


As for suspension and brakes they are on hold for awhile but I have been looking at uprated rear shocks that could be a possibility in the future, ill probably be trying to stuff a 690 into the frame by this point. :laugh:
 

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Discussion Starter · #38 ·
Ive been quiet lately got a lot on, anyways going to start this tomorrow so im hoping to have the engine in and running by tomorrow evening then that leaves me with with Thursday to iron out and issues. I am still awaiting deliver of the cushdrive its taken an absolute age to come, hoping it will be here tomorrow and it can be fitted Thursday. Hopefully ill be posting up with pics tomorrow of progress and hopefully alittle vid of engine running would be nice:)
 

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Discussion Starter · #39 ·
Well did go to pkan today. Only had 2 hours 9n the bike but it was basically a strip down. 2 engine bolts left and out comes the engine. It looks promising all connectors seem to be the same. So pkan for tomorrow is drop the engine. Refit the 390 and test fire. If all goes to plan re assemble. Problem thst i may have is still no cush drive.
 

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Well did go to pkan today. Only had 2 hours 9n the bike but it was basically a strip down. 2 engine bolts left and out comes the engine. It looks promising all connectors seem to be the same. So pkan for tomorrow is drop the engine. Refit the 390 and test fire. If all goes to plan re assemble. Problem thst i may have is still no cush drive.
Fingers crossed :)
 
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