RC Tanks Australia Forum

At the time of writing I've just had the unfortunate experience of a new Heng Long KV-1 arriving with broken idler wheel mounts. The retailer offered me a discount on a metal hull, or a free replacement plastic hull, but neither are in stock. A good time to buy some aftermarket track tensioners? Maybe, but they come from overseas, are out of stock, and don't fit the metal upgrade wheels I bought.

Options left, try to negotiate for a full replacement or refund (losing a lot of money in shipping even if successful) or try to fix it. I decided to try for a repair.

The attached picture shows how my KV-1 arrived after several days on the road. The mounting lugs for the idler wheels and track tensioners were broken away from the hull. This had occurred on both sides.

First stage of the repair:

I removed the tracks, idler wheels, track tensioners, and the speaker (speaker out for some more working room).

With this done I glued the broken off mounts back on using thick CA glue (slower drying "super glue" with good gap filling ability).

Stage 2 of the repair:

From the inside of the hull I used a fine drill bit to drill back into the idler mounting lugs. I then used some epoxy to glue some thin, but very stiff, wire into these holes. My intention here was to restore some of the structural integrity when a twisting force is applied to the lugs, such as when doing up the idler wheel screws.

Stage 3 of the repair:

I used some sandpaper to roughen up the inside of the hull for a better glue join. I then pushed the flat ends of a slighly undersized drill bits into the holes through the lugs, from the outside. With careful placement, and removal, of the drill bits I was able to use a miniscule amout of Blue Tack to form a thin skin over the end of the holes. The purpose of this was to stop excess glue flowing down the holes in the next stage.

Stage 4 of the repair:

Next I used epoxy to glue a piece of 1.5mm plywood inside the hull, I also placed a layer of lightweight fibreglass between the plywood and the hull. The plywood and fibreglass were clamped in place while the epoxy cured. My intent in adding this plate inside the hull is to spread the load over a larger area. My WWII tank now has a composite hull.

With the epoxy cured, I used a small (1.5mm) drill bit from the outside of the hull and drilled through to the inside. This gave me the centre of where to drill a hole from the inside. I didn't want to use larger dill bits from the outside so as not to upset the thread that had already been cut into the idler mounting lug by the screws that hold on the wheels. I then drilled, just through the ply and fibreglass, from the inside back toward the mounting lugs. If I recall correctly it was 2.5mm drill bit for the top holes and 3mm for the bottom holes.

With the holes drilled and cleaned up I then put a few drops of thin CA glue (thin superglue) onto the plywood. This soaks into the wood and strengthens it when dry. The attached picture shows the repair at this stage. Also note the screw in the bottom of the hull. I'll be discussing it in the next stage.

Repair stage 5:

I went through the selection of screws I have on hand looking for something very slightly undersized for the hole through the mounting lug for the idler wheel. By undersize, I was looking for something where there was enough thread to grip, but not a firm fit for the hole. The intention here is to put a screw from the inside of the hull, toward the outside, so there is a few mm of metal inside the plastic mounting lug reinforcing it through the area it was previously broken.

Keep in mind that the screws for the idler wheel have to come back in from the other direction. If you select something too long, you won't be able to get the screws for the idler wheels back in.

After going through my collection I settled on screws like the one shown in the stage 4 picture. The threaded shaft on these screws is 8mm long long and they have a broad head (good for spreading the load). These screws actually came from the housing of a broken VCR player so it's unlikely you'll have them lying around. Some other ideas that came to mind while I was looking for something suitable were screws from a computer case (of which there were several likely candidates in my collection) or even cutting a flat headed nail off at an appropriate length.

With the screws selected I used a toothpick to put a small amout of epoxy at the opening of the holes through the idler wheel mounting lugs (on the inside of the hull only). Coated the thread of the screws with a thin layer of epoxy, then screwed them into place. Metal reinforcing of the broken idler wheel mounts completed.

Repair stage 6:

I moved on to the plastic track tensioners next. Out of the box they are not glued to the tank and are primarily cosmetic. I decided that they could be used to help spread the load placed on the idler mounting lugs and glued them in place with epoxy.

There is a small mounting lug for the track tensioner, above the lug for the idler wheels, that is hollow. There is also a hole through the track tensioner where it sits over this lug. It is possible to put a screw or bolt through the track tensioner and this lug into the hull.

Here's were the new owner tip comes in: Others have also found the idler wheel mount to be a weak point in the Heng Long KV-1, even when they arrived intact after shipping. A tip I found in other forums was, at the bare minimum, to strengthen your KV-1 by putting a screw or bolt through the track tensioner and the mounting lug.

I decided to take this on board and put a bolt through the track tensioner, the mounting lug, and into my reinforcement inside the hull. The bolts I had on hand were undersized for the hole so I coated them with epoxy before putting them in place. The repair now looks like the attached picture.

Repair stage 7:

Now that I have 2 sets of metal reinforcing in place, I decided to use some of the surface area of the plastic track tensioner to spread the load. I mixed some 30 minute epoxy up with micro balloons. If you haven't heard of them before, "micro balloons" are an additive you can get to mix with epoxy or resin so it is lighter, sands easier, and fills gaps better. You should be able to find micro balloons at a good hobby shop. If stuck, I've heard you may also be able to use talcum powder as a replacement.

I used the micro balloons and epoxy mixture to fill the gap between the plastic track tensioner and the hull, I also made a reinforcing fillet around where the idler wheel mounting lug joins to the hull. It may trade off a bit of scale detail for strength, but you won't see much of this area when the tracks are in place anyway.

Finally, stage 8:

The tidy up. I decided to paint over the repairs to make them look a little more tidy. A full repaint of the model will follow in future so an exact colour match wasn't important. After painting the rest of the the platic track tensioners were put back into place and the speaker remounted. Job done.

Have I tried it - as yet no. However I have put some force on the mounts by hand and they appear quite strong.

Have I over engineered it? I hope so....

That is exactly how I got my KV years ago. Good to see Heng Long have addressed the QC issue NOT!

Dave you'll have to remind me to show you the glue for ABS plastic. It is called Tetra something but just shortened to Tetra. It welds the ABS plastic together it also will 'weld' in styrene. Sometimes CA glue will not do the job and you need something like Tetra.

Cheers
KG

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