Articles - Proto Max Couplers
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These Proto Max couplers by Walthers (product ID 920-6000) are HO-scale couplers that come 20 pair to a package. They are also sold 4 pair to a package. If Walthers doesn't have them, check out (S-scale-only online retailer, who carries them as well). I bought them to see if they would be of any use to me in S-scale. When they arrived, I measured them and found them to be just a hair smaller than true S-scale couplers. My research and model measurements show the following:

Coupler Face Height:
Prototype = 11"
Proto Max = 10"
S-Helper Service = 11.5"
Kadee S = 12"

Coupler Width:
Prototype = ?
Proto Max = 15.5"
S-Helper Service = 17.5"
Kadee S = 18"

Subjectively, when I compare these three model couplers, I find the Proto Max HO-scale coupler to be of a more pleasing size. These are now my fleet standard.
Proto Max Couplers
The photo on the right shows the four parts that make up one coupler. The metal centering spring fits in the main body at the bottom of the photo. Then the coupler is installed, and the lid closes the whole assembly. The parts need to be held together at all times until the mounting screws are installed, because the lid doesn't snap onto the body.

There are several things I like about this coupler. First, the entire head is made of diecast metal, not plastic. It looks like the real thing. Also, because of that, it should be able to stand up to the forces of S-scale equipment.

The other thing I like, which is a personal taste, is that they are painted black. I know that the brown couplers are supposed to look like rusted couplers, but to my eye they don't. I also know that Kadee and SHS both sell couplers in colors other than brown. There's a movement in N-scale also to use more brown couplers, but I was already not liking those, which was before I switched to S, so for me this is not an S-specific issue.

The final issue which made me decide in favor of this Proto Max coupler is that they are easy to uncouple. My preferred method for uncoupling is to insert a round, pointy BBQ stick (large toothpick-like) in between the couplers, twist, and the couplers come apart. That worked like a charm in N-scale when I was using Micro-Trains Z-scale couplers. When I switched to S and started using the Kadee and S Helper Service couplers that come with the rolling stock, I couldn't get them to work. Now that I have installed the Proto Max couplers, I can once again use my favorite uncoupling tool!

Of all the praise I have for the Proto Max coupler, there are a few things I don't like. There is some play between the back of the coupler and the coupler's face, which means that you can actually run out the slack on a train (that doesn't apply to the Kadee and SHS couplers). I don't think that this causes the jo-jo effect that a train full of Kadee couplers produces, but it can cause some motion. I think, however, that because of this spacing, uncoupling is easier, so, I am more than happy to trade that off. The trip pin is way too short for S-scale equipment. However, I never use magnetic uncouplers; I prefer to manually uncouple. Therefore the trip pins are cut off on my couplers, and so this is a non-issue. It might be an issue for you if you want to use these.

Finally, the biggest problem I have found with these couplers is that the mounting holes on their draft gear box are just a hair too far apart to be an exact replacement of "normal" S-scale couplers in SHS rolling stock. I can only get the center and either one of the outside screws installed to mount the coupler to the car. However, if you use one of the side holes, your coupler will off-center with respect to the car, so I just use the center hole only for the screw, and then add a single, small drop of superglue on the back of the coupler draft box where it attached to the underside of the car's body.

Once installed on two cars, they still leave a 3-foot gap between the cars, which is prototypically correct. I have had no operational issues with them being used in a full-length freight train on our club layout, so they are plenty strong for S-scale use.
Proto Max Couplers
Here's a better angle view of the couplers.
Proto Max Couplers
This photo shows the absolute tightest curve on my layout, and the couplers seem to be able to handle it just fine. One final comment about comparing these Proto Max couplers to Sergent couplers. The S-scale Sergent couplers look like the real thing and are correctly sized. The Proto Max coupler has about the same 3D volume space as occupied by a Sergent S-scale coupler. Their shapes are different, but they are about the same size.
Proto Max Couplers