Note the greenish yellow material in the intake ports of MXA’s 2017 Yamaha YZ250F cylinder. That’s epoxy, and it should last for around 40 run hours before needing to be serviced again. 

By John Basher

A world exists beyond high-compression pistons, lighter rods, titanium valves, aggressive camshafts and factory parts. If you’re really looking for that added dose of performance, then epoxy is your answer. Not much is made about epoxy because, quite frankly, race teams don’t walk around the pits bragging about changing the size and shape of their ports. The factories hold details close to their chests. For that reason, it’s entirely possible that the likes of HRC Honda and Monster Energy Kawasaki don’t use epoxy in their engines. What we do know is that epoxy has been used by master engine tuners since two-strokes were the only gig in town.

While visiting JGRMX this past week to check on the progress of my 2017 Yamaha YZ250F project build, I noticed John “Bundy” Mitchell mixing two-part epoxy together. Like seeing an albino bison roaming the plain as a unicorn flew overhead, I witnessed something magical. Bundy, a former member of factory Suzuki, Pro Circuit Kawasaki, Yamaha of Troy and factory Honda, as well as a title-winning mechanic with Nathan Ramsey, was applying epoxy to the ’17 YZ250F intake ports. In order to learn more, I asked the JGRMX retail manager about the hidden art of epoxy.

Bundy, what kind of epoxy do you use?
In the old days, and probably still now, a lot of race teams use a product called Belzona. I don’t know much about the product, but it was a reliable product not that long ago. At JGRMX we currently use a salt water epoxy that’s made for repairing steel bolt holes. It is a chemical reaction that can cure in salt water. We don’t have to use heat to cure it. There are a lot of good epoxies out there now. You’ll be fine as long as you have something that’s chemically resistant and adheres well.

Where did you learn how to epoxy engines?
You always hear about it when you’re a privateer guy. People talk about it like it’s some big deal, kind of like a works part. It’s basically nothing more than the ability to change the shape or size, or both, of the port. Whether it’s a two-stroke or four-stroke, it gives you the ability to change the port. There’s obviously a lot that can go into that. The Japanese are really good at doing it.

Are there drawbacks?
Yes. I think that 99 percent of the time, if you change the port too much, then you’re going backwards in performance. There are certain instances where you can improve performance with a little bit of epoxy. It’s not something that gets sold to the public a whole lot, because it comes down to maintenance and price. Applying epoxy to the ports is time consuming, which results in costing more money. No matter what epoxy you use or how well you prepare the piece that you’re using the epoxy on, it is going to lift, chip or flake after a certain amount of time. In a race team scenario, where everything is getting torn down every week, it’s not that big of a deal. For the average consumer, where you could potentially sell a guy an engine and never see it again, it’s not the best scenario. Let’s face it; generally speaking, if you sell a guy an engine and you’re never going to see it again, then the engine probably doesn’t need to be at ten-tenths, which is trying to get all you possibly can out of the engine.

“IT COMES DOWN TO DYNO NUMBERS. AGAIN, IT’S PLAYING AROUND WITH THE PORTS IN ORDER TO GET THE HIGHEST FLOW AND HORSEPOWER NUMBERS POSSIBLE. THERE ARE CERTAIN TRICKS YOU LEARN ALONG THE WAY, LIKE INCREASING BOTTOM- OR TOP-END POWER.”

How often did you change the size and shape of the ports during your tenure at Honda?
I didn’t do much epoxy work while I was at factory Honda. Looking back, I really didn’t use epoxy much at any of the teams I worked for. We didn’t even use much back in the Pro Circuit days. We only used a little bit on the intake. Using epoxy is really best for testing, because you can try to make performance gains, and if it doesn’t pan out then it’s not a loss. Epoxy can be a helpful tool, but for 99 percent of the population it’s not necessary. If you aren’t being really thorough with your testing, then it can cause just as many problems as it can help.

Describe the process of applying epoxy in the engine.
It’s just like clay. You mix it up to start the chemical reaction. I make little balls of epoxy and form them into specific shapes. You can do all kinds of cool stuff. It’s possible to change the shape of your bowl or your bottom turn, make the port smaller or whatever else in a four-stroke application. That’s an important part of the process, because you need to mix the epoxy really well. It needs to be consistent, so you have to spend the time mixing the two parts properly. You also need to prepare the surface that you’re applying the epoxy to. The surface needs to be really dry and clean, with no oil residue, and it also has to be roughed up with sand paper so that the epoxy has something to grab ahold of. After that I lay in the epoxy. In a situation where I frequently use epoxy, such as a Yamaha YZ450F head, I use a mold so that I’m consistent with the amount and shape of the epoxy. Then I let it dry. You can work on the epoxy after eight to ten hours, but a full cure is 24 hours. Heat isn’t needed. Like I said, it will set up under salt water. Then it’s just like grinding aluminum. You could technically fill the entire port up with epoxy and then make your own shape and size.

What do riders notice most when you change the intake ports on a four-stroke?
It comes down to dyno numbers. Again, it’s playing around with the ports in order to get the highest flow and horsepower numbers possible. There are certain tricks you learn along the way, like increasing bottom- or top-end power. When we would go testing at the factory, we wouldn’t tell the rider that one engine had epoxy in it, while the other didn’t. Instead, we would give the rider a Spec A, and then a Spec B. After riding, we would ask him what engine package was better.  

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