In a prior post I mentioned a bike some teenagers dragged home - they are tearing down the rest of the bike but have left the engine to me to figure out (I dont let them off the hook quite that easy, they are just as capable of learning as I am and I have never done a motorcycle engine before!).  Having said that, they are looking for guidance.  We did pull the cylinder/piston which isnt great, and split the case which showed the bearings are all shot.  Transmission and other gears are all in decent shape.  So it is a basic bottom end/top end rebuild (all the difficult important stuff).

Please offer your advice and experience on the following plan:

I am looking for a local shop to do the cylinder rebore.  If it will clean up is TBD, but once I find a shop in Northern or central MA, Southern NH, or possible SE CT I can take it in and inspect and determine where to go from there.  Alternatively we might just send it along to enduronut... in fact that may be the best approach anyway.  If the cylinder doesnt clean up then we will buy a used one and try again.

I plan to do the crank myself.  (I do have some machine shop experience from a prior life, but am not an expert in engine rebuilding)

Where do you recommend the purchase of components:

Main/crank bearings and seals?
Rod bearing/new rod?  (seems like we may as well replace the rod at the same time as everything else)

Piston, wrist pin, bearing and rings?  (if we send to enduronut we will just buy these from him to do at the same time)

Gasket kit?

If we can get this far we will then tackle the carburetor, currently soaking.

This bike has sat outside for years.  It really 'shouldnt' be a project since so much to do, but - you know how that goes...

Oh - and the not that uncommon pita - when they pulled the carb/exhaust, they broke a bolt on each.  So we need to get those out - worst case, should we consider helicoil?  (we are going to try some tricks to get them to back out - but sometimes these dont work although we will put it in the mill before we are done)

 We love pics, but anyway, i'm sure you will when ya can please. But--i've had 100% success rate on getting broken studs out but sitting a washer or plate over the stud or bolt & i arc weld it on using some good rod that welds real nice. Bought me a gasless mig but i'm useless with it--give me my old arc welder!!. The worst one i've had in 50 or more years is the pic i'll load of the 10mm cylinder studs on my old 360. Possibly loctited in but didn't look that way so one broke off & the others felt close to breaking before turning. As the pic shows i had a 4 false starts & damn near gave up but man it was a nice feeling to feel it move haha. Didn't fancy drilling it & high tensile too i guess. So--long story but that's what i do & they're always broken off flush or just below the surface but untill then every one has come out first try. Mostly 8mm & 6mm except this pic.

Some pics on the other posts, in new member section then also the twin shock (older section - I posted in the wrong place by accident).

Welding is an option on the one, the other is more difficult.  It can work though... an option to try still.  On one they stripped out the aluminum, hence the helicoil question

 

Ah right, found the post with pics. All good thanks.

heres some advice ive learned about rebuildin these old motors over the years, mostly learned the hard way
make sure you send the cylinder to be bored by a machinist that is familliar with 2 stroke engines, confirm they will chamfer the ports after boring.
Rebuilding the crank will take a lot of time to figure out how to true correctly for a first timer, when shops do it for pretty cheap, how much is your time worth?
The supply of parts particularly for these little dt125 engines has been flooded with bad chinese parts so you must be careful to only buy hard parts made in USA or Japan.
Make SURE you buy genuine made in japan main bearings, you can source them directly by size off the old numbers from the old bearing and buy Koyo or All Balls or Yamaha OEM bearings, even major bearing brands supply bearings from India and China now, so make sure when you get the bearing it says Japan on it and dont settle for anything less.
The same goes for the piston and rod Kit, dont buy a chinese piston made from the wrong alloy. Get an OEM Yamaha piston from Enduronut or a Pro-X Rod kit or other name brand piston.
Freezing the bearings and a little warmth from a propane torch goes a long way to install the bearings and crank
OEM Yamaha, Wiseco, Shindy made in japan quality top end bearings are available cheap.
Clean Clean Clean is the name of the game on reassembly. The transmission is a bit more resilient and forgiving against dirt, but the crankcase must be spotless. When you get the cylinder back from the machinist, you should be able to wipe a solvent soaked white paper towel in the bore without ANY black dirt or grit showing up on the paper. Clean the cylinder yourself with soap and water. You went through all the hard work to split cases and rebuild with good parts, dont let any small dirt ruin it. Use a good air filter and make sure the old muffler isnt full of junk to suck up into the fresh engine.

If you give me some time, I think I can scrounge up some good parts for you to use cheap...  it will be at least a week or so.

 

Thanks Akara for the insight

And thank you Mark for anything you might be able to find!

 

We are making some progress on this project, so adding some pics (who doesnt like pics).  The case is split and started cleaning them up, nothing overly exciting yet - much work to do.  



 

How'd ya get the that good considering how they were. Just rubbing--elbow grease.

Yes, pretty much.  'It depends', but a combination of wire brush on the drill, some sanding, some buffer/compound.  Aluminum polish.  Just tried a combination of things until we got tired of it.

They are not too bad but we are not done, I believe we can bring them up another level still.  More to come.

In the meantime they have the frame pretty much disassembled and ready to strip/refinish.  An update on that soon - they are going to town and its great to see them having at it.  Healthy activity for teenagers and has even taken a bite out of the time spent on video games...

They are looking to me for guidance on getting the engine rebuilt.  I am motivated because if we can pull this rebuild off they will remember it forever (and who knows, could end up reliving their own youth someday by rebuilding another one in 20-30 years).  

 Pressure!!!  (so I really appreciate all the guidance and great advice of this group!)

Trying to put together a rebuild parts list (and source to purchase from), so if anyone has such a list handy please PM me!  Rod kit, piston kit, gasket kit, bearing kit, etc... the whole shebang.  Still do not yet have the machine shop identified for boring the piston.