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This is my first post so hopefully I don't get beat up too much or sound like a complete idiot. I've searched and found a decent amount of info so far but haven't seen any posts regarding this for quite a while so I figured it wouldn't hurt to start a new thread that can hopefully consolidate and update any info on the subject.

I bought my first bike, a 2016 390 Duke with ~7800 miles on the clock this past week knowing that it had an overheating problem with a bit of suspicion about what the root cause may be but not knowing for sure. I got it for a healthy discount from what I see them priced at in numerous places online so I am OK with the fact that it's going to start out as a bit of a project. I had hoped to get by for a bit with what the previous owner had described as an overheating problem that he said he fixed before I picked it up but after getting to ride it only 10-12 miles it's obvious I need to do some work before I ride it any further.

My first couple short rides were good and everything seemed OK but then I took it on a longer ride a couple days ago and by the time I got home the temp gauge was at a full 10 bars, right on the edge of over heating. I then noticed I had coolant all over my right leg and it was smeared all down the right side of the bike. Upon further inspection it looked like the coolant was spewing from the radiator cap (1.4 bar cap so it looks like someone has already gone down this road) and then the final test was open the overflow reservoir and sure enough we have bubbles.

So a blown head gasket it is, based on the reading I have done about the symptoms on here and the RC390 forum. Even though I have enough projects to keep me busy for the next 5 years already I am going to take on doing the top end rebuild myself. Going through all the different discussions about this issue and the fix in the past I have just a few questions for anyone that has dealt with it themselves.

Assuming that this may be caused by the issues of flatness with the head I plan to either go the emery cloth route or see if a machinist friend can skim the head flat for me but I had also been looking at the updated P/N head that may help with this issue. Has anyone actually installed this new P/N head and if so was it flat or did it need to be worked over as well?

I'd like to try to get away with just smoothing out the head and re-installing without tearing apart the cylinder if possible but how many of you have had issues with the piston and needed to replace it? Or would it just be advisable to replace it since it sounds like the bike has overheated (not sure how hot) in the past and may have questionable rings?

Assuming I do have to take some material off the head and possibly the cylinder (I'm hoping to avoid this) and I use a new OEM gasket, is there any real concern about an increase in compression ratio or any clearance issues?

Last but not least, is there anything else I'm missing that would be good to know going into this?

Thanks for making it to the end of my rambling story. For the sake of any other gluttons for punishment like myself I will try to take pictures and come back here with info and any tips or tricks or anything else I come across as I try to dig myself out of this mess I got myself into.
 

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Welcome, JeffRoyJenkins,

Sorry to hear about your overheating issue.
First off, no I have so far not done the cylinder head job myself.

  • as you mentioned this happened before on this bike, I would first determine the cause of overheating.
  • check the flatness of the cylinder head first before having it machined (might not be necessary).
  • some compression increase should not be a problem as you can actually buy higher compression kits.
 

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Since I am assuming the bike already had the head gasket issue when I got it I am not sure if there was a previous overheating issue that caused the head gasket to blow or if the overheating only started as a result of the head gasket failing which was inevitable if the head does indeed have a machining issue. I am hoping I can get the top end rebuild done and using some quality coolant and oil it will run just fine without overheating but until the head gasket is fixed I won't know if that is the only cause of it or not.

At this point I am actually leaning towards just buying a new head with the new part number since I have no idea how hot the previous owner got it and if I could be dealing with a warped head issue as well. I'm just curious to find out if the machining issues have been corrected on the new ones. 🤞
 

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After getting distracted by other projects for a bit I was finally able to get the bike torn down. Suspicions all appear to be correct, blown head gasket at pretty much the exact same location between the cooling jacket and exhaust side of the cylinder. The head looks like it has a small gap at the same location which likely caused the issue in the first place.

Piston looks like it is in good shape so that is good news as all I should need to replace are gaskets since I am going to try to just smooth out the head instead of buying a new one.

One thing I did notice though is the exhaust cam shaft appears to be off by a tooth, anyone that has done their valves noticed this?

Valve clearance was in spec but most right at the bottom end of the tolerance range so I will likely swap out the shims to move it to the other end of the tolerance based on the thinking that the gap will get smaller over time as the valves wear.






 

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I don't get it, does that head gasket block most of the coolant holes?
Based on the photo it looks like he had a little bit of a gap. It also shows exactly where the pressure from the cylinder broke into the cooling galleries.
 

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I don't get it, does that head gasket block most of the coolant holes?
The way it is setup the coolant flows through just the one small hole in the gasket at the rear and the larger coolant passage at the front of the head. The other holes you see on top of the cylinder block don't have corresponding passages in the head so any openings in the gasket would just be a potential point of failure. You can see how the holes matchup here.


I was watching this video earlier today and saw that on the 2017 and newer heads they actually opened up some more places for the coolant to pass through, likely because they split the forward passage into 2 sections. This is probably a result of the exact issue I have because most of the blown head gasket issues I have been able to research have been at the exact same location as mine.

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Based on the photo it looks like he had a little bit of a gap. It also shows exactly where the pressure from the cylinder broke into the cooling galleries.
Yep, just a quick check it looked like only about 0.04-0.05mm gap though, the manual specs a tolerance of 0.1mm so I was expecting it to be a little more of a gap. Given that most of the reports I have found of these blown head gaskets are all at the exact same location and all have flatness issues at this exact spot I am still pretty hopeful that if I can get this smoothed out and flat, a new gasket will hold a lot longer than this one did.
 
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