Complete Engine Rebuild Guide
PLEASE NOTE THESE ARE BASICALLY MY NOTES FOR THE TIME BEING BUT MAY AS WELL GO ON HERE IN CASE ITS USEFUL. AS I SAY BELOW, ILL GET ROUND TO TYPING IT UP NICELY WITH PHOTOS/VIDEOS ONCE IM DONE
So I'm about to undertake my first full DT125R engine rebuild. I suppose its testament to the little powerhouses that I've never had to do it before but this one has main bearings that sound like 2 cheese grates shagging.
So I started with this video as a guide. I'm sure the Haynes manual would also be useful but I find videos a lot easier to follow. I will try to make my own with commentary and more close ups etc but this one is now split so too late.
Tips along the way.
The alluminium to lock the gears was very useful. I used a bit from a CPU heatsink from a computer that i chopped up with an angle grinder. What a mess that made!
I couldn't get enough leverage on the nut with a breaker bar and resorted to an impact gun. I've got a Clarke mains powered one. They are about £60 and worth their weight in gold!
Before you start, make sure you have pots and labels and a notepad to make notes of things you'll need to buy. Stuff comes off in excitement and its possible to forget woodruff keys. But if you keep them in pots you'll notice if you miss something.
Order your parts before or as soon as you take it apart. You dont want to be putting it together and realise you forgot to order a tab washer or bearing. DONT RE USE THEM......ITS NOT EXACTLY QUICK TO TAKE APART AGAIN!
Change all the bearings and all the seals. If you think about the extra cost you'd be happy to pay it if it was an extra little insurance policy on the engine. Dont go cheap. Koyo bearings at a minimum and Athena gaskets at a minimum. Any other head gasket will pop!
If the barrel needs reboring passed 2mm then you may need the powervalve reworking. In this case it may even be worth finding a different barrel but personal choice.
make sure you have the right barrel. There are slight difference between 3bn and 3mb. If you can get a 3mb head its worth doing at the same time.
Whilst the piston and head are off, give the crown (top) of the piston and the combustion area of the head a polish with some metal polish and toothbrush then a bit of rag. It will stop carbon deposits and improve airflow. Its worthing doing whilst you have easy access to both parts.
Maybe one of the most important tips is to work out why you are rebuilding. If there is an issue, are you resolving it by rebuilding. If you dont know why the engine is being rebuilt then try and work it out first so you dont end up with the same situation after all your hard work.
I'm at the point where my cases are split and i need to remove the gearbox bearings so ill update once thats done and I'll keep updating this as i go along and eventually add photos etc to accompany the words.
note: It's worthing getting a bearing slide hammer set to remove the gearbox bearings.
Crank Rebuild £42 PJME
ConRod kit £43-55 PJME (depedning on Long or moto RRT)
Barrel Rebore £36 PJME
Athena Full Gasket Set £28 PJME
Mitaka Piston Kit £47 PJME
Athena Oil seal kit £25
Gearbox Bearings £22.50 Simply Bearings
Threebond 1084 case sealant £9 ebay
3 x tab washers(gen yam) £10
Note: needle roller bearings are generally not replaced. They dont seem to be available from bearing stockists so gen yam seems to be the only option. About £14 for the one ive checked.