Top End Tear Down, Inspection, Rebuild, 20,000
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Hi guys first post here, long time lurking! I have a DT125X with just under 20,000 at the minute and am wanting to do a top end rebuild as had this bike 4 months now and covered 4000-5000miles, not sure if ever been rebuilt for some reason I'm assuming it's still on stock cylinder and piston at this mileage, possible?
I have a new head gasket en route and am going to measure cylinder soon, to confirm if stock.
I have already had the exhaust off and respayed whilst there I checked through the hole to piston and could see the piston had quiet significant blow-by about 2/3 down walls.. and I also felt previously I'd lost some acceleration and about 5mph top end not much
I am hoping I won't need to rebore and can get away with just honing if possible? I have the tool for honing although I've never used before, any good? Will this increase the cylinder diameter to the point I may aswell rebore? And also does it need recoating, heard stuff about nikosil?
Another question I priced up an OEM piston and all bearing, rings etc @ £130, or I can get a wossner piston kit for £80 or Namura for £50,which one?
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F360851111029
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Personally, my bike didn't have a rebore until 23k miles until the top end went and it scored the cylinder.
If you take the barrel off and run your finger nail around the inside, as long as this doesn't catch I think you can get aware with a hone, someone else might want to confirm this though..
As for gasket set, make sure it's either genuine or athena or it will just blow. I'f get a Wossner personally, I've not heard great stuff about Namura. Also I believe these are cylinders are lined with steel rather than nikosil. Nikosil barrels tend to be on newer competition bikes like the KTM 2 strokes ect?
Hope this helps, I'm sure somebody else can chip in and correct me if I'm wrong. (I most likely am wrong)
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Unless you know what you're doing, I suggest taking the barrel to a professional for honing.
The bores can become out of round and need to be precisely honed to accommodate for this. I'd let a professional with laser alignment tools decide what size piston to use.
I used to use Wossner, with great effect, but PJME put out a notice saying they suspect their stuff is now manufactured in China. Again, nothing wrong with that, but each to their own. I've switched to vertex or Wiesco. Well worth going forged for the price.
The overbore will be on the piston crown, if it has been overbored.
If it has, then you may need to modify the powervalve.
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I honed my stock cylinder and fitted a stock sized Mitaka piston and a new small end bearing.
Even used a cheap headgasket, didnt know otherwise at the time, and its still going strong 8000km later.The honing tool you link to is for brake cylinders, so the grinding plates might be a bit short.
You need to make sure they dont catch the ports, or flip out of the cylinder when youre moving them, as they will catch the egde/end of the cylinder, damaging the stones.Labor seems cheaper overseas, so it might be worth it to get the job outsourced, here prices are up and it takes forever because they have so much work at hand. So here its a combination of high prices, and you have to wait several weeks.
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@calum I’m pretty sure pjs stopped doing those kind of services because of the turnaround time and issues with quality but that may be for the rebore and plating if they do honing in house then it will most likely be an available service
