Stolen this of of the old site, thought it was valuable information. Credit to darkangel

Parts you need:

Top end gasket set (base gasket, head gasket, exhaust gasket)
Piston kit (Piston, rings, expander ring, circlips, piston rod)
Top end bearing

Remove sidepanels.
Remove seat.
Remove fuel tank
Remove front exhaust and throw away old copper exhaust gasket
Drain coolant (open the coolant drain plug, located to the right of the oil drain plug on the right side of the engine case, open the radiator cap.
Disconnect spark plug
Observe the top of the cylinder head, there is a large pipe that leads from the radiator onto it, its very obvious, use a screwdriver to undo the binder on it and slide it off.
Observe the head again, you should see that there are some metal supports that bolt from the frame onto the cylinder head, undo and remove these supports (i used a breaker bar and a wrench, one each side, they can be tight)
On the northeast side of the head there is a temperature sensor, remove the connector that goes into it, it should just pull out.
There should be two hoses still attatched to the cylinder head that lead to the carb, squeeze the binders (i used pliers) and slide them down the hose, then pull the hoses off.
There should now be nothing attatched to the cylinder head, carefully loosen the bolts on the head one by one in a diagonal motion (good idea to check out the haynes manual for this).
Now try and pull the cylinder head off, if you find the powervalve servo is in the way, remove it, if you cant remove it undo the rear panel on it so that the motor and wires are showing, this will give you just enough room to slide the head and barrel off. At this point you need to remove the pullies attatched to the powervalve anyway.
Throw old head gasket away.
Carefully loosen the bolts holding the barrel on at its base, and once undone gently tap the barrel free using a mallet or the rubber end of a screwdriver and a hammer (but be VERY careful, take your time and be very gentle not to dent the barrel)
Before removing the barrel observe the position of the piston and using the kickstart (or if you dont have one you could try putting the bike in gear and pushing it slowly) make the piston reach the lowest position in its cycle to help the barrel removal.
Take your time and get someone to help you, you need to slide the barrel up and free from the piston and anything else that is holding it in its place like the protruding threads, have someone hold the barrel up while you observe where you need to move it next to get it out of the bike, it can be quite tricky.
shove a high quality rag (make sure its clean and does not flake apart) into the bottom of the engine to stop dirt getting in, it doesn't have to be a rag aslong as it doesn't flake apart and leave bits inside the engine it should work.
take some time now to carefully peel off any gasket left on the cylinder head, barrel and base of the engine, DO NOT USE A KNIFE or anything that will scratch the surface, use your fingernail, scratches cause an air leak and this will cause you to lose coolant. While you are at it try and clean away some of the carbon in the combustion area inside the cylinder head.
Using some needle nose pliers pull the circlips out of the old piston, and push the piston rod out, if it wont move, brace the piston and con rod with one hand and get a large screwdriver and you should be able to carefully push it out, just dont hit it with a hammer.
Remove the piston and the small end bearing out of the top of the conrod.
Put the base gasket on, it should only match up with the holes one way round, some base gaskets are no properly cut to the water channels, if your gasket covers up one of the channels i suggest cutting it out so water moves freely.
Prepare the new piston, possition the piston so that the arrow is facing forwards and put the circlip in the left side, it should go in with a bit of force. Get the piston rings out and put the expander ring in the lower groove, the rings are supposed to be different sizes but often its not obvious, slide a side of the ring in the upper and lower ring grove to see if it fits better in one, if it fits equally well in both then you'll have to check out the haynes manual to see if it says anything about tapered edges. Put the rings on and now coat the piston, rings, piston rod and new small end bearing with 2-stroke oil.
Put the new small end bearing into the conrod and slide the piston over it, the arrow should be facing forward, push the 2-stroke covered piston rod through the right hand side of the piston, it might need a little force to get it all the way in. If everything looks good then put the remaining circlip on.
With the piston still at the bottom of its stroke, remove the rag from inside the engine and get someone to help you. Apply a thin coating of 2-stroke on the cylinder walls to help lubrication, then you need to hold the piston rings tightly so that they are as flush with the piston as possible and are in their correct places, get the friend to maneuver the barrel onto the piston while you make sure its going in stright and not damaging the rings. Try not to twist the barrel when the piston is inside as the rings can catch.
If the barrel is now firmly on and the piston moves inside it properly then you have done the hardest part, all you need to do now is follow the steps in reverse, start by bolting the barrel on and putting your new head gasket on, hold it on the little tab that says "UP" and dont forget to put your new exhaust gasket in.

When everything is back together:

make sure all parts of the cooling system and the engine are done up tight!

(Do this step each time you start the bike till its run in)
Start the engine like you would normally (choke?), do not let it idle, keep varying the revs never letting it sit at a constant speed, touch the top of the radiator, once this is hot get on the bike and take it for a ride for 15-20 mins, after the ride let the bike cool for a MINIMUM of an hour, make sure the bike is stone cold before starting it again.

Before you go out remember to keep looking at the thermometer!!!! if anything has gone wrong its an early warning! Anything up to half way across the bar is safe.

1'st ride: Gear 1 and 2, never going past 1/2 revs, do not lug the engine, vary the revs and use lots of engine breaking, no hard acceleration.

2'nd ride: Gear 1,2,3 and 4 not going past 3/4 revs, do not lug the engine, vary the revs and use lots of engine breaking, no hard acceleration.

3'rd ride: Use all the gears, but go easy and do not redline the bike

4'th ride: Go for a longer ride 30-45 mins, all the gears, no red-lining.

5'th ride: Your bike is run in! just let it cool down properly one last time.

If you have had a full engine rebuild I don't suggest using this run in method, its likely too harsh on the new bottom end.
Be creative.
I suggest following the steps steps up untill the '2nd ride', then you should continue babying the bike for a few hundred miles gradually increasing the gears and rev range you use.

First 100 miles : DO NOT RAG THE BIKE, just get on some back roads and keep the revs and speed low, use lots of engine breaking.

First 200 miles : DO NOT RAG THE BIKE, you can start rolling on the throttle to 7 or 8K, no hard acceleration though, and dont stay at high revs

First 300 miles : DO NOT RAG THE BIKE, your good to use the whole rev range in all the gears now, just try stay away from motorways, dual carriageways e.c.t

First 400 miles : The bike is run in, i wouldn't suggest you race the bike till the 600 mile mark however

Problems you might have with your bike:

Leaking oil:

First identify where the oil is leaking from, the most common thing to leak is transmission fluid.

you MUST top up any oil that you lose while riding the bike, if you know you are leaking oil then check it frequently or fix it asap, if you run out of transmission oil the worst case is a siezed or destroyed clutch, the best case are ruined bearings, its not good to let this happen.

Leaking from the gear lever: Buy new oil seal, you should be able to remove it and get a new one on with minimal dissasembly.

Leaking from the engine case gasket: Buy new engine case gasket, drain oil, remove kick starter and any obstructions, remove casing and put a new gasket on, its an easy fix. leak may cause milky oil.

Leaking from the crankcases: Game over man! its time to take the engine to bits or send it to a mechanic, however allot of the time it may appear to be leaking from the crankcase when it actually isn't, may also cause milky oil.

Can you use regular 4 stroke 15w40 oil as transmission oil? YES, but the clutch may slip more

Coolant Loss

Check your radiator, if the level decreases over a period of time you have a problem.
Just filling it back up will work for a while, i managed to go 400 miles on the motorway with a blown headgasket, two bottles of de-ionized water in my bag from halfords i had to stop and 'water' my bike every 15 miles...it wasn't fun.

My bike lost coolant WTF? it wasn't leaking! : Head gasket, base gasket, crankcase leak or a leak somewhere else, its unlikely for coolant to dissapear and leave nothing visible

My bike leaks coolant from near the sidestand area : Thats the pressure valve, head gasket failure

My rad/ expansion bottle is full of black stuff : head gasket failure imminent, your gasket is flaking apart, or you rebuilt the engine and didn't flush the system.

My expansion bottle blew out : head gasket

My bike is overheating but not losing coolant : thermostat, running very lean, radiator damaged or not working properly, coolant system blocked

I just rebuilt my bike and its leaking coolant again! : you didn't clean the surfaces of the cylinder head and barrel properly (likely), or the head is warped (unlikely) or cracked (very unlikely), or base gasket

My cylinder head/barrel is warped! : Rebore/skim
My cylinder head/barrel is cracked! : depends where the crack is and how big, if its inside the water jacket you better start looking for a new barrel/head, if its just a little chip on the surface then a skim will sort it

Engine noises

A rattle at varying rpm ranges, for example 1k-1.4k no rattle 1.5k rattle 1.6k-2k no rattle and so on, you can hear it very well in neutral if you rev the engine and then let the revs drop down to idle : Either you need a new top end rebuild, you need new piston rings or your piston-barrel clearance is too high and you need a rebore

Vibration through footpegs or a loud rumble : Get the bottom end looked at, probably big end bearings

DT125R FORUM

ConnorDTR