Need Help Coolant disappearing from 2017 KTM RC 390 race bike

ReidMcT

Active Member
Premium Member
Elite Member
Site Supporter
Country flag
I appreciate your dilemma, john13. I would definitely plan on replacing both pump seals with the reportedly better seals from the 450 engine. You can't really 'tear down' the pump very far without damaging at least the outer seal.

You also might ask the machine shop how much material they had to remove to flatten the head; that could give you a clue as to whether the head was the culprit. It is a known trouble spot, as are the pump seals (to a lesser degree). I think the crankcase seam - while a potential leaky area - is much lower probability, based on reports in this forum.

Since I know my head had low spots and obvious leak pathways, I am going to leave my cases intact for now. If I were in your situation, I'd do the same. It is not so much work to R&R the engine, in comparison to the time (and risks) of complete engine disassembly and reassembly.
 

john13

Member
Country flag
I appreciate your dilemma, john13. I would definitely plan on replacing both pump seals with the reportedly better seals from the 450 engine. You can't really 'tear down' the pump very far without damaging at least the outer seal.

You also might ask the machine shop how much material they had to remove to flatten the head; that could give you a clue as to whether the head was the culprit. It is a known trouble spot, as are the pump seals (to a lesser degree). I think the crankcase seam - while a potential leaky area - is much lower probability, based on reports in this forum.

Since I know my head had low spots and obvious leak pathways, I am going to leave my cases intact for now. If I were in your situation, I'd do the same. It is not so much work to R&R the engine, in comparison to the time (and risks) of complete engine disassembly and reassembly.
Sounds like a good plan. I really don't want to split the case and I already ordered those upgraded water pump seals. Fingers crossed!
 

hvlmike

Member
Country flag
Had the same problem with my 2017. Turned out the water pump seals were put in backwards from the factory. They would leak at high rpms only.
 

Formula390

Supporting Vendor
Vendor
Country flag
You also might ask the machine shop how much material they had to remove to flatten the head; that could give you a clue as to whether the head was the culprit. It is a known trouble spot, as are the pump seals (to a lesser degree). I think the crankcase seam - while a potential leaky area - is much lower probability, based on reports in this forum.

Since I know my head had low spots and obvious leak pathways, I am going to leave my cases intact for now.

Snipped the reply above, but in my experience practically ALL of the new heads will have low spots on them. If you blue the head and skim pass it across some 600 grit wet dry on a surface plate (or thick glass, marble countertop, whatever you can that's as flat as a surface as you can find) you will see that the outside perimeter will be "flat" but the whole surface will not be. As a machinist, it's EASY to see what the problem was and that's heat build up during the machining operation at the factory. Heat build up will expand the metal and "grow" up. It is then decked off, so when the material cools, you end up with a low spot that shrinks away. This was MOSTLY fixed when the stopped making the heads and cylinders in India and moved manufacturing for them to Austria... but I've still come across some new heads straight out of the box which are STILL not "perfectly" flat and have low spots. I would check EVERY head, new or not, for flatness. Note: You can't remove much material from the head without risking a problem down the road.

The BEST fix I've seen for head gasket failures, is to put a fire ring on the cylinder. PacMan is really the one for full details, but this photo should hopefully help you see what a "Fire Ring" is if you aren't familiar with them:
RC390-Fire_Ring.jpg

Any machine shop half worth their salt should be able to put the groove in for you. I think it's something like a 0.012" deep groove, but I'd need to confirm with PacMan to be sure on groove depth and wire / o-ring thickness, as he's the one that initially started / pioneered doing this on 390 race motors. If I ever pop the shop mule motor, I'll be doing this for sure.

All that said, Head Gasket failure is sometimes the cause of coolant consumption, but as can be seen here, it's hardly THE cause every time. People are very quick to say "Head Gasket" when anything remotely goes weird on these motors, but it's not the ONLY cause by any regard.
 

Razor1

New Member
Country flag
I did the emery cloth resurfacing on both the head and cylinder. The head had the low spot around the typical place where the gaskets blow. It cleaned up really nice. The cylinder was worse than the head, took a long time to clean up. Low all the way around the bore, and a major low spot between the bore and the cam chain tunnel. I didn't measure the spots, just kept sanding until they cleaned up. Luckily, I never had any coolant loss or an outright head gasket failure after 8200 miles and a couple track days. I will try to attach some pictures, it was pretty eye opening. On a side note, my valve clearances were all still in spec, I hadn't checked them since 900 miles. At the 900 mile check, the exhaust valves were tight, the intakes in spec.
 

Attachments

  • 1.jpg
    1.jpg
    42.2 KB · Views: 31
  • 2.jpg
    2.jpg
    32.9 KB · Views: 28
  • 3.jpg
    3.jpg
    46.5 KB · Views: 28
  • 4.jpg
    4.jpg
    40.8 KB · Views: 27
Top