rebuilding an '15 RC390 motor

projek7r

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I have a 2015 project bike in stock form with a blown motor. Not sure the extent yet but looking at the forums, it looks like the common preventative measures for these older motors is

  1. Resurfacing the head as there are known issues there (warped head issues)
  2. 1.6 Bar rad cap and SPAL fan (overheating)
  3. PVC tuning (richen up fuel)
When I tear it down and find its a top end problem, i'm thinking of starting with the above but a couple of questions:

  • Is the above a good place to start or could I update the head to a newer model 2018+? Cant find info if anyone has successfully done this to resolve any mating issues.
  • If the damage is beyond the head, source a newer 2018+ motor - Does anyone know if its actually plug and play or will I require a full harness and dash conversion?
Gonna start looking at online parts microfiche to start comparing but any advise is appreciated!
 

1JZSupra

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projek7r

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The heads are pretty much the exact same. I recently have completed the blown head gasket work myself I can honestly say isn't that bad, but I can tell you that even the 2nd gen experiences the same head gasket failures as well. I installed a "high" compression head gasket, new head bolts, new valves, and valve stem seals along with some other stuff.

https://www.ktmduke390forum.com/threads/1jzs-build-documentation-thread.47254/page-2#post-424127
Awesome, this is going to be sooo helpful. Thank you very much for sharing this info. I will need all the help I can get.
 

Dangeruss471

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I would recommend getting in touch with Matt at Formula390 or Paxton at Gray Area about installing a fire ring, I've heard that this basically eliminates the head gasket issue. I've actually got my cylinder on the way to Matt right now for this. I see that Paxton has a new cylinder with fire ring and piston ready to go on his website incase you need to start fresh.

https://grayareaktm.store/products/austrian-oem-cylinder-with-fire-ring

As far as the engine being plug and play, it is not, but it's not far from it. I know the shift drum/gear pos sensor on the 17+ is different and therefore will not connect to the 15-16 harness. I think the stator may be different as well, but not 100% sure.
 

projek7r

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Thank you
I would recommend getting in touch with Matt at Formula390 or Paxton at Gray Area about installing a fire ring, I've heard that this basically eliminates the head gasket issue. I've actually got my cylinder on the way to Matt right now for this. I see that Paxton has a new cylinder with fire ring and piston ready to go on his website incase you need to start fresh.

https://grayareaktm.store/products/austrian-oem-cylinder-with-fire-ring

As far as the engine being plug and play, it is not, but it's not far from it. I know the shift drum/gear pos sensor on the 17+ is different and therefore will not connect to the 15-16 harness. I think the stator may be different as well, but not 100% sure.
Wow thanks - with all my research the past week (got it a week ago and started consuming and taking down all the notes I could) I havent read about this fire ring. Will reach out to them. Thanks for this!!!!
 

kostean

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Hi. Can also recommend replacing OEM head bolts with hardened head studs and nuts. Got last these from Spears and they are really made of a strong material and have not managed to get the old design head leaking after by going up to 70nm/52lb torque on nuts. Spears studs need to be machined though has had too little thread at the case side and excessive on top, after though the studs use up all the case thread drilled and more of a question of how much one dares to torque the nuts on top.

Noticed though at the 2019 engine i have for spare the OEM head bolts at least looked different so maybe no need for newer engines, but the 2015 year had bolts were made out of cheese.
 
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projek7r

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Hi. Can also recommend replacing OEM head bolts with hardened head studs and nuts. Got last these from Spears and they are really made of a strong material and have not managed to get the old design head leaking after by going up to 70nm/52lb torque on nuts. Spears studs need to be machined though has had too little thread at the case side and excessive on top, after though the studs use up all the case thread drilled and more of a question of how much one dares to torque the nuts on top.

Noticed though at the 2019 engine i have for spare the OEM head bolts at least looked different so maybe no need for newer engines, but the 2015 year had bolts were made out of cheese.
This is great info - wow so, so far its been good for you? At first I was really bummed about the potential of having to fix it and do it all over again in a short period of time due to all the posts I read but all these recommendations is making me see some light at the end of the tunnel. I will do all of this and report back and hope this helps others out who are just starting on their RC390's like me.
 

kostean

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This is great info - wow so, so far its been good for you? At first I was really bummed about the potential of having to fix it and do it all over again in a short period of time due to all the posts I read but all these recommendations is making me see some light at the end of the tunnel. I will do all of this and report back and hope this helps others out who are just starting on their RC390's like me.

Have not managed to blow any head gaskets after going for head studs and increased torque, but can't say for sure is it really due to that or just because the fact the engine's top end has been that frequently opened up for inspection that I have managed to replace the gaskets before they have blown (this is track bike engine so lives most of it's life at WOT).
 

projek7r

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Thanks for the clarification, may I ask how often you checked for inspection? This bike would be used similarly to yours, not a street bike.
 

TVann

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Blew the head. Skimmed and replaced gasket with KTM hi-comp gasket. Blew head again in 60 miles. Now rebuilding with fire ring and spears head bolts. Pray for me.
 

kostean

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Did you get the head decked?
Head decking might not also be an absolute solution for the old style head with no bridge at the cooling gallery. I have seen a decked head that were flat as a mirror after machining on getting out of speck at the exhaust cooling bridge weakest area after already some months, and could only conclude that due to head design or bad material at early model heads, that area just warps.
If not having already new design head, just save yourself from the hassle and go for the fire ring cylinder.

Despite all the negative publicity 390 engine reliability has got, we have raced for years the heck out of few 2015 CUP models at constant WOT at fast circuits and there ain't anything else probable to blow up at these thumper engines as the head gasket (well all right, blew also one flywheel to pieces but that damn thing were so out of balance that had 1/4 of radius drilled full of holes already at the factory).
 

projek7r

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Well I finally got around to removing the fairings and draining the oil on the bike and it started to piss green chocolate. Guess its more than just top end at this point, gonna source a newer motor and for preventative measure, install a fire ring cylinder with newer Spears studs from all the above recommendations.
 
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