Long-Term KTM RC390: Blown Head Gasket Repair

Our RC390 Project Bike Blows Its Top

RC390 engine repair, blown head gasket


The RC engine, apart on the bench in KTM USA’s immaculate shop in Temecula, CA. (Ryan Dungey’s 450 supercross engines were stacked five-deep on a shelf in the Factory Racing shop two doors down!) Mine was the first confirmed head-gasket failure that the factory techs had seen.©Motorcyclist

WRIST: Ari Henning
MSRP: (2015): $5,499
MILES: 2,966
MPG: n/a
MODS: KTM Performance gasket kit

Well, it happened. The RC390's head gasket blew. I had heard it's a problem with the little KTM and I know a racer who had it happen, but I figured that with over 3,000 miles and several track days and races on the clock I was in the clear.

I notified KTM of the issue and they requested that I disassemble the engine at their US headquarters in Temecula, CA, so the techs could take a look. Like me, KTM has heard rumors about head gaskets going south but they hadn't actually examined one yet so they were eager to see mine.

When we pulled the head off the breach in the gasket was evident. The KTM techs theorized that perhaps my bike had overheated during a race, warping the head and compromising the head-gasket seal.

Chad Wells, Service Manager at Commonwealth Motorcycles in Louisville, KY, agrees with the warped-head theory, but says my head may have come that way from the factory. Wells has been developing go-fast parts for the 390 since the day it came out, and he’s built a lot of reliable, high-horsepower RC and Duke engines. “I think the biggest contributor to failed gaskets is head-surface straightness,” says Wells. “On every engine I build I skim the head to ensure it’s straight and has the proper finish.”

RC390 blown head gasket


The site of the breach was clearly visible when we pulled the head. The RC390 uses a MLS (multi-layer steel) gasket with Viton-coated leaves. You can see where the thin rubber got blasted off the steel plates by the ingress of combustion pressure.©Motorcyclist

Wells says he's also heard that the cooling fan can fail, causing the engine to overheat in traffic and warp the head. KTM confirms that they've had warranty claims for radiator fans as well as temperature sensors, which are responsible for turning the fan on.

So what's an RC owner to do? First of all, check to make sure that your fan operates properly, and maybe swap your coolant for a quality liquid like Bel-Ray Moto Chill (belray.com). If you want to be proactive, have your head surfaced now before there's an issue, and have your bike's EFI mapped so it runs richer and cooler. If your engine does blow a gasket, take it to the dealership for repair, and ask that they surface the head and cylinder. KTM says that gaskets that fail within the warranty period will be repaired free of charge, even if the bike has been raced.

Project RC390 oil

Did You Order A Latte?

If you ever change your oil or peek in the site glass and what you see resembles a latte or a milkshake, something is definitely wrong. And it’s probably your head gasket.©Motorcyclist

As for our RC390, I had the cylinder head skimmed and then bolted everything back together using a gasket kit from Commonwealth Motorcycles. The $45 kit (available through ktmperformance.com) uses thinner Cometic gaskets that Wells says tighten squish to a closer-to-optimal clearance. Never heard the term before? Check out our MC Garage video on squish here.

Beyond tightening up the squish clearance, the gasket kit also throws cam timing off a bit (by reducing the cylinder height, which essentially lengthens the cam chain), so don’t be surprised if you have to turn the rotor about an eighth of an inch past the TDC line to get the timing marks on the cams to line up.

While I have the bike apart, I've also bolstered the radiator with a radiator guard to protect against punctures, and spun on a high-pressure radiator cap. You can read about those updates here.

Am I disappointed that the head gasket blew? Yeah, but at least it happened on the last lap of my last race of the weekend! Pulling the head did provide the opportunity to examine the engine's internals, which I'm always excited to do. The engine uses oil jets for the piston skirt and cam followers, and the ports are surprisingly smooth. And according to the KTM tech I worked alongside the piston is of the same design and manufacturer as on the MX bikes. So while the India-made engine (and the rest of the bike, for that matter) has a few chinks in its armor, it also has a level of sophistication not seen on other bikes in the class.