Advice on a Viper rebuild - RC Groups
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Jan 14, 2013, 05:26 PM
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Advice on a Viper rebuild

I have a Kyosho Viper I got in the early 90's when I was about 12.

I don't have a lot of experience and I'm looking for advice in working with plastic. I want to remove the radio box and original motor mount and prop shaft and replace it with a brushless motor with a new prop/shaft/rudder.

My first problem is how do I remove the plastic radio box and motor mount from the hull. Its glued in and I think it's ABS but I'm not entirely sure.

I don't need to reuse anything so my first thought it to cut it out and sand down the left over bits.

My next idea was to heat it up and see if that helps. I don't yet own a heat gun though.

Any help would be appreciated.
Last edited by viperidae; Jan 14, 2013 at 07:18 PM.
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Jan 16, 2013, 08:16 AM
Boaters are nice people.
Hi viperidae,

Welcome to this forum!

I have worked on a Viper some years ago and found it well motorized with a Speed 600 on 7 cells NiMH, or 2S Lipo (with a bigger prop to compensate for the lower Voltage).
The point is, almosy all Kyosho hulls have a rocker in the keelline, which allows it to run quick with the cheapest motor available, but also induces porpoising when you try to make it run faster.

Even with just a (hot) 600 motor on 7 cells NiMH (23000 rpm at the prop) the boat was hopping like a bunny, more out of the water than on it.

If you want to use a brushless motor, don't go overboard on the power, slightly more that a Speed 600 is enough (200-300W).

As the hull is vacuum formed, using heat, I would only mechanically remove the batterytray etc, using a heatgun (paintstripper) will cause the hull to deform (badly, don't ask...).

Use strips of ABS to assemble a new battery tray and a motormount.

MEK or Acetone will fuse ABS, slightly melting it, so don't use too much in one place, the hull will get soft and buckle (again, don't ask...).

Let me know if you have more questions.

Regards, Jan.
Jan 16, 2013, 02:10 PM
Registered User
I had one of these from when I was younger, I put a 550 brushed motor on 2s with a larger prop, whatever you do DON'T use expanding foam in the nose I did and the hull ended up in the trash!
Jan 16, 2013, 07:58 PM
Registered User
Thanks for the advice Jan and Matt.

My plans are to first run the original Le Mans 360ST motor on 2S Lipo and see how that goes. My theory is it should be a little better than the old 7.2V NIMH packs I've got.

The motor gets very hot on 7.2V so that's my reasoning for replacing the motor mount, to give it some water cooling.

Then I though if I'm going to replace the motor mount, why not re-do the prop shaft too. since the old one seems every so slightly bent.

If I'm going to replace the stuffing tube/prop shaft, why not go one step further and change it to surface drive... but that means putting the shaft out the back, not the bottom... why don't I just buy a new hull? (answer to that question is to avoid the "why did you buy another toy boat, you already have one!" response from my wife)

regarding the expanding foam: I was thinking of just cutting up some expanded polystyrene
Jan 17, 2013, 07:22 AM
Boaters are nice people.
Hi viperidae,

The original Le Mans 360ST is a car motor, as a rule of thumb, car motors are not the best solution in a boat, where the load is continuous, as long as you apply power.

Add to that the horrible shaft construction in the Kyosho boats of that era, where the stuffing tube has no bearings at both ends, but huggs the shaft over the entire length,adding considerable drag, it's no wonder the motor gets hot.
A (watercooled) Speed 600 race does a much better job, if you replace the shaft assembly with something 'appropriate', the boat will perform better and have longer runtimes.

Changing ihto a surface drive is only usefull if you like a roostertail; the hullshape doesn't benefit from the fact a surface drive makes most designes run faster (less under the hull= less drag), due to the rocker in the keelline; the hopping will stil be there, regardless of the type of propulsion you use.
Large trimtabs reduce the porpoising, but also cause drag, slowing you down.

Ignore the wife, I have over 25 boats, all different...

Regards, Jan.
Jan 24, 2013, 09:56 PM
Registered User
The problem I have with replacing the shaft is the molded plastic that sticks out from the bottom of the hull to hold the shaft is only just wide enough to accommodate the original set up of no bushings. All the shafts/stuffing tubes I've come across are ~8mm OD. The original is ~4mm.

So is it worth hacking it up and putting a new shaft with a submerged prop or should I bite the bullet, cut if off, patch it up and stick the shaft out the back? (or option 3: give up on the Viper and build a new wooden hull)
Jan 25, 2013, 05:40 AM
Registered User
build a new wooden hull...
Jan 25, 2013, 08:31 PM
Registered User

Viper Rehab

I redid a Great Planes Wildcat. While bearings are nice, these boats will give good speed without them. I used a water cooled mount for a pair of $15 2500kv out runners. Replaced the original stuffing boxes with 3/16 brass tubing and used HK 1/8 solid shafts and Graupner plastic props from LHS. Not a great idea to run surface props with these boats, and hulls are not designed for really high speeds. Grease the shaft after each days running with Molly grease and you'll get very little water in boat and it will be fast enough to enjoy without self-destructing.
Jan 26, 2013, 07:59 AM
Boaters are nice people.
Originally Posted by viperidae
The problem I have with replacing the shaft is the molded plastic that sticks out from the bottom of the hull to hold the shaft is only just wide enough to accommodate the original set up of no bushings. All the shafts/stuffing tubes I've come across are ~8mm OD. The original is ~4mm.
I know...

The way to work around that is to look for a piece of thin walled brass tubing, that will fit snug over the stock one.

I took out the stock stuffing tube using heat and a pair of pliers.
Go easy on the heat to avoid deforming the plastic.

Once the stock tube is out, cut off two pieces 10 mm long, deburr the ends and solder them in each end of the new brass tube (which you cut to length first).
Do this leaving half of those pieces sticking out (cut them to size later) and with the shaft inside, to have the bearings alligned properly.

Once the 'bearings' are cut to size, you'll have 5 mm long bearings on each side.

Shuff the exterior and glue it back in the boat.

Once the glue has set, take out the shaft, put in some thin grease and put the shaft back in, holding a fingertip on the inside, to avoid pushing all the grease into the hull.

Regards, Jan.
Jan 27, 2013, 02:37 PM
Registered User
Time to invest in a pair of calipers and a heat gun then. Unless I just shove the soldering iron inside the tube to heat it up...
Jan 28, 2013, 06:21 AM
Boaters are nice people.
Originally Posted by viperidae
Unless I just shove the soldering iron inside the tube to heat it up...
That works fine too, just keep an eye on the temperature, enough is enough.

Regards, Jan.
Jan 31, 2013, 05:59 PM
Registered User
Viper was a good boat, underpowered though. Worth a good shaft with bearings, it's not hard to retrofit. Be careful not to overpower it though, easily done now with modern power systems!
Jan 31, 2013, 07:49 PM
Registered User
Worth a good shaft with bearings, it's not hard to retrofit
I just don't know how to go about putting in a thicker shaft and tube. All I can find are 8mm OD shafts with bearings. There is only room for around 4mm.

Unless you're talking about Jan's idea of a slightly larger tube with some thin bushings soldered on the ends.
Feb 01, 2013, 09:33 AM
Registered User
I bought one for my son many yrs. ago and it never ran right . I think it was warped , it hopped and would spin out with anything more then a touch of rudder input .My son had fun with it so it was all good . But the problem is if you put to much motor in one it will twist and do all kinds of crazy maneuvers .
Feb 17, 2013, 04:55 PM
Registered User
So I've built a new stuffing tube and the "finger spinning the shaft" test reveals it spins *much* more freely.
Took ages to remove all the glue/junk/crap that held the old tube in place.
Completely gutted the inside.
Bought a new motor mount and a water cooling coil.

Need to glue the motor mount and tube in place, glue a new servo mount, find some kind of box for the radio/esc, some kind of battery mount.. a water pick up too.

My original plan was to buy a new rudder with an integrated water pickup but it's hard to find one that would fit. They all seem to be designed for surface drives and stick out quite far from the transom. It (use to) turns quite well in one direction but not in the other (im guessing something about the motor torque) so I don't want to risk getting a less effective rudder.

So basically looking for advice on a water pickup now. I'm thinking I'll stick a piece of brass off the transom with an angle cut in it.
Like this

It's a slow project, my time is being robbed by DIY home renovations.

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