I have a 78 dt125 with about 4K miles on it that has been doing me well Until recently. Made about a 35 mile trip on backroads when moving on my last half mile bike dies and I’m stuck. Get it back to the house and there’s no compression , I can turn the kickstart with my hand very easily. Ripped the head off and found a little burn spot in the head gasket. Cylinder and piston still looked brand new. Bought a new headgasket a oem one and put it on still no compression. I’m going to flip it and see if I got it on the right side because the oem one had grooves. If that doesn’t solve my issue what’s my next step? Not sure where else I can lose compression at heard something about a crank seal? Thanks in advance !

Hard to say without pictures, did you pull the cylinder too? Hopefully it just needs new rings and a hone.

I would be surprised if flipping the head gasket helps, but while you have it off get a good straight edge and make sure the head isn't warped or damaged from the gasket leaking.

Would like to see pictures... and also doubt it's the gasket. Could be stuck rings. That doesn't happen as much today with the better oils and less gumming/carbon.

Crank seals are definitely important. But you could remove them completely and compression would still be good.

I may be a bit early on this, but try some liquid copper sealer or other on the gasket, replace head, kick over. Might be some kind of leak between the sleeve and cylinder casting or other. This would not be a fix as yet, but just a test for the gasket mating surfaces. Dunno.

Decided to put the tools up after my 24hours with no sleep day ended. Second look was there’s definitely a hole right in the center of my piston. What is the procedure for replacing this piston? And anyone have an oem spark plug part number and/or head bolt torque? (: thanks in advance

Yeah, definitely have to pull that piston and replace it.And Gaskets too.
Look in the Tech Library up above for the info that you seek from the service manual..
yamaha-enduros.com/index.php/workshop/se...als/78-dt125e-dt175e


a B8ES spark plug ( NGK ) is what we are using in the Greenie Thread here.
yamaha-enduros.com/index.php/forum/1977-...ard/9812-the-greenie

10mm bolts is torqued at 33 foot lbs.

Mix your gas at 32/1.

Crank seal have nothing to do with compression up top except without seals the fuel wouldn't get sent up to the top. Compression only builds after the rings pass the ports on the way up which is why the Japanese calculate a real low compression--like 7/1 instead of 11/1 [guessing] as they calculate it from ports closing point. Someone will correct me if i'm up the creek lol. BUT--Now your problems start because what was once a piston & now a hole has to go somewhere & now its in the crankcase & in the bearings. Idealy it needs a total dismantle. But otherwise try & flush it as best you can. There's stories up here on various ways to do that. Good luck with your venture & run a cold plug NGK-B9ES type of thing just to be safe. Make sure its got an air cleaner as its carburetted to that restriction of the filter. Ignition timing within spec & will be points ignition on yours [well think it is as 'is' down this part of the world].
Sorry Rick C above, didn't read yours before writing mine, just saw piston with hole. Also on mainly 175 of about that model they went to rings of some strange quality & used to get caught in the exhaust port when done a lot of miles. Rings were [whats the word] maleable & could just about tie them in a knot. 125's i don't recal the same issue but not getting the same beating the 175's got by kids trail riding them hard.

I'd like to add that you need to figure out why it happened or it will probably happen right away again.

Too lean (could be a bad crank seal), timing off (as RT325 said, should have points in 78), bad fuel... and yes, bottom end is contaminated with debris that needs to be removed... disassembly would be best.